24. Nutrition and Metabolism

Energy, Nutrients, and the Biochemistry of Fuel

Learning Objectives

24.1 Nutrients: What the Body Actually Requires

A nutrient is any substance in food used by the body to promote growth, maintenance, or repair. There are six classes, and the division that matters is not chemical but arithmetical: three of them yield energy and three do not.

(Alcohol yields 7 kcal/g and belongs in no class — it is not a nutrient, because no body process requires it, but it is unavoidably a fuel.)

Note that fat carries more than twice the energy per gram of the other two. That is not arbitrary: fatty acid carbons are highly reduced — heavily loaded with hydrogen — and the energy released by oxidation is proportional to how much hydrogen there is to strip off. Carbohydrate carbons already carry oxygen and are therefore partly oxidized before you start. The 9-versus-4 ratio is a statement about chemistry, and it is exactly why the body stores long-term energy as fat: storing 100,000 kcal as glycogen, which also binds about 3 g of water per gram, would add well over 60 kg of body mass.

Essential and nonessential

Essential does not mean important. It means the body cannot synthesize it, so it must be eaten. Nonessential nutrients are no less necessary — you simply make them yourself.

Nine essential amino acids: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine. A complete protein supplies all nine in adequate proportion (animal proteins, soy, quinoa). Most single plant proteins are limiting in one or two — grains in lysine, legumes in methionine — which is why grain-and-legume combinations recur independently in cuisines across the world. They need not be eaten in the same meal; the body maintains a free amino acid pool over roughly a day.

Two essential fatty acids: linoleic acid (18:2, omega-6) and α-linolenic acid (18:3, omega-3). Humans lack the desaturases to place a double bond beyond carbon 9. From these two, the body makes arachidonic acid (and hence the prostaglandins of Chapter 23) and, inefficiently, EPA and DHA.

Fibre deserves separate mention because it is a carbohydrate that yields almost no energy and is nonetheless required. Insoluble fibre adds bulk and accelerates transit; soluble fibre forms gels that slow gastric emptying, blunt the postprandial glucose rise, and bind bile acids — which forces the liver to consume cholesterol making replacements. Recommended intake is 25–38 g/day; typical intake in the United States is about 15 g.

Dietary reference intakes

The AMDRs (acceptable macronutrient distribution ranges) are 45–65% of energy from carbohydrate, 20–35% from fat, and 10–35% from protein — ranges wide enough to accommodate most of the world's traditional diets, which is the point.

What the evidence actually supports

Nutrition is the area of physiology most burdened by confident claims, so it is worth stating plainly what survives repeated testing:

• Total energy balance governs body mass over time. No macronutrient composition repeals the first law of thermodynamics, though composition strongly affects satiety, adherence, and metabolic outcomes.
• Food matrix matters as much as nutrient content. Whole fruit and fruit juice have similar sugar content and different effects, because fibre, structure, and the rate of gastric emptying determine the glucose and insulin trajectory.
• Dietary patterns predict outcomes better than single nutrients. Patterns emphasizing vegetables, fruit, legumes, whole grains, nuts, fish, and unsaturated fats are associated with lower cardiovascular and all-cause mortality across many populations and designs.
• Replacement, not removal, is what matters. "Reduce saturated fat" is meaningless until you say what replaces it. Replaced by polyunsaturated fat, cardiovascular risk falls; replaced by refined carbohydrate, it does not.
• Sodium, added sugar, and alcohol have dose-related harms with reasonable evidence, and most people consume more of all three than they believe.

24.2 Vitamins and Minerals: Learn the Logic, Then the List

Why solubility determines everything

Two properties follow from whether a vitamin dissolves in fat or in water, and they explain nearly all of vitamin physiology.

Fat-soluble vitamins (A, D, E, K):

• are absorbed with dietary fat, in micelles, requiring bile — so any cause of fat malabsorption causes deficiency in all four (Chapter 23);
• travel in lipoproteins and are stored in liver and adipose tissue;
• are not readily excreted, because the kidney filters plasma water and lipophilic molecules bound to carrier proteins are neither freely filtered nor easily lost;
• therefore accumulate, and therefore can be toxic.

Water-soluble vitamins (B complex, C):

• dissolve in plasma, are freely filtered at the glomerulus, and are excreted when in excess;
• are not stored in quantity, so intake must be regular and deficiency develops in weeks to months;
• are rarely toxic, because excess leaves in the urine.

Two exceptions prove the rule. Vitamin B₁₂ behaves like a fat-soluble vitamin: it is protein-bound, requires intrinsic factor, recirculates enterohepatically, and is stored in the liver in a 3–5 year supply — which is why pernicious anemia takes years to appear. And vitamin B₆ and niacin have genuine toxicity at supplement doses (sensory neuropathy and flushing/hepatotoxicity respectively), because their excretion capacity can be exceeded.

Minerals

Major minerals (required at more than 100 mg/day): calcium, phosphorus, potassium, sulfur, sodium, chloride, magnesium. Trace minerals (less than 100 mg/day): iron, zinc, copper, iodine, selenium, manganese, fluoride, chromium, molybdenum, cobalt.

Their functions cluster into three: structural (calcium and phosphorus in hydroxyapatite), electrochemical (Na⁺, K⁺, Cl⁻, Ca²⁺ — the ionic basis of every action potential and contraction in this book), and catalytic (iron in heme and cytochromes; zinc in over 300 enzymes; iodine in thyroid hormone; selenium in glutathione peroxidase).

Two are worth remembering as absorption problems rather than intake problems. Iron is absorbed in the duodenum at a rate set by hepcidin, and heme iron from meat is absorbed 3–5 times more efficiently than non-heme iron from plants — which is why Amara's iron deficiency from Chapter 23 will not be corrected quickly by diet alone. Calcium absorption depends on calcitriol and falls sharply with age.

24.3 The Vocabulary of Metabolism

Metabolism is the sum of all chemical reactions in the body. It has two directions:

• Anabolism builds larger molecules from smaller ones. It consumes energy (endergonic) and usually proceeds by dehydration synthesis. Glycogen from glucose, protein from amino acids, triglyceride from fatty acids.
• Catabolism breaks larger molecules into smaller ones, releasing energy (exergonic), usually by hydrolysis and oxidation.

The two run simultaneously in the same cell. What changes between the fed and fasted state is not whether they occur but which direction has the upper hand — and that balance is set almost entirely by hormones.

Oxidation, reduction, and the electron carriers

Oxidation is loss of electrons; reduction is gain. In biological systems electrons usually travel as hydrogen atoms (an electron plus a proton), so a molecule that loses hydrogen has been oxidized and one that gains hydrogen has been reduced. The two always happen together — there is no free electron in solution — hence redox reaction.

Catabolism is a controlled oxidation. Its purpose is to strip hydrogen (and thus electrons) from fuel molecules and hand them to carriers:

• NAD⁺ (from niacin) accepts two electrons and one proton: NAD⁺ + 2H → NADH + H⁺.
• FAD (from riboflavin) accepts two electrons and two protons: FAD + 2H → FADH₂.

These carriers are the currency of the whole system. A reduced carrier is a packet of transferable energy, and the entire architecture of aerobic metabolism exists to walk those electrons down a chain to oxygen while capturing the released energy as ATP.

ATP

ATP is the universal energy currency because its terminal phosphoanhydride bonds release a large, usable quantity of energy on hydrolysis — about 7.3 kcal/mol under standard conditions and closer to 12 kcal/mol under the concentrations actually found in a cell.

Two facts about ATP are more instructive than the structure:

• The body's total ATP pool at any instant is only about 100 g. It is not a storage molecule; it is a transfer molecule, made and consumed within seconds.
• A resting adult turns over roughly 50–75 kg of ATP per day, and far more during exercise. Each molecule is recycled hundreds of times daily.

Energy is transferred to work by phosphorylation — the transfer of a phosphate group from ATP to another molecule, which raises that molecule's energy and changes its conformation. There are two ways to make ATP:

• Substrate-level phosphorylation: a phosphate is transferred directly from a high-energy substrate to ADP. Fast, oxygen-independent, and low-yield. It occurs in glycolysis and once in the citric acid cycle.
• Oxidative phosphorylation: energy from electron transfer is used to pump protons across a membrane, and the resulting gradient drives ATP synthase. Slower to start, oxygen-dependent, and responsible for about 90% of the ATP you make.

24.4 Carbohydrate Metabolism

Glycolysis

Glycolysis splits one 6-carbon glucose into two 3-carbon pyruvates. It occurs in the cytosol, requires no oxygen, and is present in essentially every cell — including red blood cells, which have no mitochondria at all and depend on it entirely.

It has two halves. The energy investment phase spends 2 ATP: hexokinase (glucokinase in the liver) phosphorylates glucose to glucose-6-phosphate, trapping it in the cell, and phosphofructokinase-1 performs the committed, rate-limiting, allosterically regulated step. The energy payoff phase produces 4 ATP by substrate-level phosphorylation and 2 NADH.

Net per glucose: 2 ATP, 2 NADH, 2 pyruvate.

The fate of pyruvate

Four possibilities, and which one occurs tells you almost everything about the cell's situation:

1. → Acetyl-CoA, by the pyruvate dehydrogenase complex, in the mitochondrion, when oxygen is available. This step also produces NADH and releases CO₂, and it is irreversible — a fact with enormous consequences, developed in §24.5.
2. → Lactate, by lactate dehydrogenase, when NADH accumulates faster than mitochondria can reoxidize it. The purpose is not to make lactate; the purpose is to regenerate NAD⁺ so that glycolysis can continue. Lactate is not a waste product and not the cause of muscle soreness — it is a fuel, shuttled to heart, other muscle fibres, and the liver, where the Cori cycle converts it back to glucose.
3. → Alanine, by transamination, for transport of both carbon and nitrogen to the liver.
4. → Oxaloacetate, by pyruvate carboxylase, replenishing the citric acid cycle and feeding gluconeogenesis.

The citric acid cycle

In the mitochondrial matrix, acetyl-CoA (2 carbons) condenses with oxaloacetate (4 carbons) to form citrate (6 carbons). Eight reactions later, oxaloacetate is regenerated and two carbons have left as CO₂ — the CO₂ you exhale.

Per acetyl-CoA: 3 NADH, 1 FADH₂, 1 ATP (via GTP), 2 CO₂. Per glucose, double it.

The cycle produces almost no ATP directly. Its output is reduced electron carriers, and it is also the metabolic crossroads at which carbohydrate, fat, and protein converge — which is why Figure 24.1 is drawn the way it is.

Oxidative phosphorylation and chemiosmosis

The electron transport chain is four complexes plus two mobile carriers embedded in the inner mitochondrial membrane. NADH delivers electrons at complex I, FADH₂ at complex II; both pass them along a chain of carriers of progressively greater electron affinity, ending at complex IV, where four electrons, four protons, and one O₂ combine to make two H₂O. This is the only place oxygen is used in the entire body, and it is why you breathe.

As electrons move, complexes I, III, and IV pump protons from matrix into intermembrane space. Because the inner membrane is impermeable to protons, this creates an electrochemical gradient — the proton-motive force, roughly 200 mV. Protons can return to the matrix through only one route: ATP synthase, a rotary molecular motor. Their flow spins it, and the rotation drives the condensation of ADP and phosphate into ATP. This coupling of an electrochemical gradient to ATP synthesis is chemiosmosis — Peter Mitchell's idea, rejected for a decade and then awarded a Nobel Prize.

Its yield: about 2.5 ATP per NADH and 1.5 per FADH₂.

  ATP ACCOUNTING FROM ONE MOLECULE OF GLUCOSE
  ══════════════════════════════════════════════════════════════════════

  STAGE            LOCATION        DIRECT ATP    CARRIERS   ATP FROM
                                   (substrate-              CARRIERS
                                    level)                  (oxidative)
  ──────────────────────────────────────────────────────────────────────
  GLYCOLYSIS       cytosol         -2 invested   2 NADH*    2 x 1.5 = 3
  glucose→2 pyruvate               +4 produced              (or 2x2.5=5)
                                   ────────────
                                   NET  +2

  PYRUVATE →       mitochondrial       0         2 NADH     2 x 2.5 = 5
  ACETYL-CoA (x2)  matrix                        (+2 CO2)

  CITRIC ACID      mitochondrial      +2         6 NADH     6 x 2.5 = 15
  CYCLE (x2)       matrix           (via GTP)    2 FADH2    2 x 1.5 = 3
                                                 (+4 CO2)
  ──────────────────────────────────────────────────────────────────────
  TOTAL                              +4                     +26  (or 28)
                                            ═══════════════════════════
                                            GRAND TOTAL: 30-32 ATP

  * Cytosolic NADH cannot cross the inner membrane. It is handed over by
    a SHUTTLE, and which shuttle a tissue uses changes the yield:
      GLYCEROL PHOSPHATE shuttle (muscle, brain) → delivers as FADH2 → 1.5
      MALATE-ASPARTATE   shuttle (liver, heart)  → delivers as NADH  → 2.5
    This is the entire reason textbooks disagree about "36 vs 38 vs 32".

  ── EFFICIENCY ────────────────────────────────────────────────────────
     Complete oxidation of glucose releases      686 kcal/mol
     Captured as ATP: 32 x ~7.3 kcal/mol    =   ~234 kcal/mol
     EFFICIENCY  =  234 / 686  =  ~34%
     The other ~66% is released as HEAT — which is not waste. It is
     precisely what holds your core at 37 C (see 21.9).

  ── ANAEROBIC COMPARISON ──────────────────────────────────────────────
     glucose → 2 lactate    yields  2 ATP     (6% of the aerobic yield)
     ...but it is ~100x FASTER per unit time, needs no oxygen, and needs
     no mitochondria. Speed bought at the price of efficiency.

Storing and releasing glucose

Glycogenesis — glucose → glucose-6-phosphate → glucose-1-phosphate → UDP-glucose → glycogen, by glycogen synthase. Stimulated by insulin. Glycogen is a highly branched polymer, and the branching is functional: each branch end is a site where an enzyme can work, so a branched molecule can be dismantled at hundreds of points simultaneously.

Glycogenolysis — glycogen phosphorylase cleaves glucose-1-phosphate from the branch ends. Stimulated by glucagon (liver) and epinephrine (liver and muscle).

Gluconeogenesis — synthesis of glucose from non-carbohydrate precursors: lactate (Cori cycle), glycerol from triglyceride, and glucogenic amino acids, especially alanine and glutamine. It occurs in the liver (~90%) and renal cortex (~10%), costs about 6 ATP equivalents per glucose, and requires four enzymes that bypass the three irreversible steps of glycolysis: pyruvate carboxylase, PEP carboxykinase, fructose-1,6-bisphosphatase, and glucose-6-phosphatase.

Why liver glycogen serves the blood and muscle glycogen does not

Glycogenolysis in both tissues yields glucose-6-phosphate. A phosphorylated sugar carries a charge and cannot cross the plasma membrane — that is the whole reason hexokinase phosphorylates glucose on entry, to trap it.

The liver possesses glucose-6-phosphatase, which removes the phosphate and produces free glucose that diffuses out through GLUT2 into the blood. The liver is therefore an exporter: it maintains blood glucose for the rest of the body, especially the brain.

Skeletal muscle lacks glucose-6-phosphatase entirely. Its glucose-6-phosphate cannot leave; it can only enter glycolysis within that muscle fibre. Muscle glycogen is a private fuel depot for the cell that owns it, and it contributes nothing directly to blood glucose.

This is not a design flaw. Muscle mass is about 40% of body weight and can consume glucose at enormous rates; a muscle able to export glucose would compete with the brain for it during exercise. The distribution of a single enzyme enforces a priority ordering between organs. Note also that muscle contributes to blood glucose indirectly, by exporting lactate and alanine to the liver for gluconeogenesis — the Cori and glucose–alanine cycles. The carbon returns; it just takes the long way round, and the liver pays the ATP bill.

24.5 Lipid Metabolism

Beta-oxidation

A fatty acid is activated in the cytosol to fatty acyl-CoA at a cost of 2 ATP equivalents, then carried into the mitochondrion by the carnitine shuttle. Carnitine palmitoyltransferase-1 (CPT-1) on the outer membrane is the rate-limiting step, and it is inhibited by malonyl-CoA, the first committed intermediate of fatty acid synthesis. That single inhibition is a beautiful piece of regulation: when the cell is building fat, the molecule it builds first physically closes the door to fat breakdown. The two opposing pathways cannot run at once.

Inside the matrix, beta-oxidation removes two carbons at a time in a repeating four-step cycle, each turn yielding 1 FADH₂, 1 NADH, and 1 acetyl-CoA.

Palmitate (C16) requires seven cycles: 8 acetyl-CoA, 7 NADH, 7 FADH₂. Feeding those into the citric acid cycle and electron transport chain gives roughly 106 ATP net — against 30–32 for glucose. That number is the 9-kcal-per-gram figure from §24.1, arrived at from the opposite direction.

Ketogenesis

Acetyl-CoA can enter the citric acid cycle only by condensing with oxaloacetate. In prolonged fasting, starvation, or uncontrolled diabetes, oxaloacetate is continuously siphoned off into gluconeogenesis, while beta-oxidation floods the liver with acetyl-CoA. Supply exceeds the capacity to burn it. The liver condenses the excess into ketone bodies: acetoacetate, β-hydroxybutyrate, and acetone.

This is not a pathological accident — it is a strategic conversion. Ketones are water-soluble, travel freely in blood, and — crucially — cross the blood–brain barrier, which fatty acids essentially cannot. The brain, which cannot oxidize fatty acids and normally consumes about 120 g of glucose a day, adapts over three to four days and by three weeks derives 60–70% of its energy from ketones, cutting its glucose requirement to roughly 40 g/day.

That adaptation is why humans survive prolonged starvation. Meeting 120 g of glucose a day entirely by gluconeogenesis from amino acids would consume roughly 200 g of muscle protein daily, and death would follow in about three weeks. Ketogenesis is what buys the extra two months.

Normal blood ketones are below 0.5 mmol/L; nutritional ketosis runs 0.5–3 mmol/L; diabetic ketoacidosis runs 10–25 mmol/L. The difference is not the pathway but the absence of the brake: insulin normally restrains lipolysis, and without any insulin at all the pathway has no upper limit.

Lipogenesis and lipolysis

Lipogenesis converts surplus acetyl-CoA — from carbohydrate as readily as from fat — into fatty acids, via cytosolic acetyl-CoA carboxylase (which makes malonyl-CoA) and fatty acid synthase. It occurs mainly in liver and adipose tissue and is stimulated by insulin.

Lipolysis hydrolyzes stored triglyceride to glycerol and three free fatty acids, by adipose triglyceride lipase and hormone-sensitive lipase. It is activated by glucagon, epinephrine, cortisol, and growth hormone, and inhibited powerfully by insulin — a detail that becomes the whole story in §24.7 and in the Case File.

And here is the asymmetry that governs human metabolism: carbohydrate can become fat, but fat cannot become carbohydrate. The reason is one irreversible reaction. Pyruvate dehydrogenase converts pyruvate to acetyl-CoA and releases a carbon as CO₂; there is no enzyme that runs it backwards. Once carbon is in acetyl-CoA, its only fates are oxidation to CO₂, storage as fat, or conversion to ketones. The two carbons that enter the citric acid cycle as acetyl-CoA are matched by two that leave as CO₂, so no net carbon is available to build glucose.

The only exceptions are marginal: the glycerol backbone of a triglyceride (about 5% of its mass) is glucogenic, and odd-chain fatty acids yield one propionyl-CoA. Fat, essentially, is a one-way store. This is why a person with no carbohydrate intake must make glucose from protein, and why "the body burns fat for the brain" is only true through the intermediary of ketones.

The lipoproteins

Lipids are insoluble in plasma, so they travel wrapped in a shell of phospholipid, free cholesterol, and apolipoproteins, which serve as address labels. Four classes matter, and they differ in density because they differ in the ratio of (light) triglyceride to (heavy) protein.

  THE FOUR LIPOPROTEINS — CARGO, ROUTE, AND ADDRESS LABEL
  ══════════════════════════════════════════════════════════════════════

  ┌─ CHYLOMICRON ── largest (75-1200 nm), least dense ──────────────────┐
  │ MADE BY:  intestinal enterocyte      LABEL: apoB-48                 │
  │ CARGO:    DIETARY triglyceride       ~90% TAG                       │
  │ ROUTE:    lacteal → thoracic duct → subclavian vein → BYPASSES the  │
  │           liver on the way out → capillaries of MUSCLE and ADIPOSE  │
  │           → lipoprotein lipase (LPL) strips TAG → remnant → LIVER   │
  └─────────────────────────────────────────────────────────────────────┘
                    "dietary fat, delivered outward"
  ┌─ VLDL ── very low density ──────────────────────────────────────────┐
  │ MADE BY:  LIVER                      LABEL: apoB-100                │
  │ CARGO:    ENDOGENOUS triglyceride the liver made or received        │
  │ ROUTE:    liver → blood → LPL in muscle and adipose strips TAG      │
  │           → becomes IDL → becomes LDL as more TAG is removed        │
  └─────────────────────────────────────────────────────────────────────┘
                    "the liver's own fat, delivered outward"
  ┌─ LDL ── low density ────────────────────────────────────────────────┐
  │ MADE FROM: VLDL after TAG removal    LABEL: apoB-100                │
  │ CARGO:    ~50% CHOLESTEROL — the delivery vehicle for cholesterol   │
  │ ROUTE:    blood → any cell with an LDL RECEPTOR → clathrin-coated   │
  │           pit → endosome → cholesterol released                     │
  │ WHY "BAD": when LDL exceeds what receptors clear, it lodges in the  │
  │           arterial intima, is OXIDIZED, and is taken up by          │
  │           macrophage SCAVENGER receptors — which are NOT            │
  │           down-regulated by cholesterol. Macrophage → FOAM CELL     │
  │           → fatty streak → ATHEROSCLEROTIC PLAQUE (Ch. 19)          │
  └─────────────────────────────────────────────────────────────────────┘
  ┌─ HDL ── high density, smallest, most protein ───────────────────────┐
  │ MADE BY:  liver and intestine        LABEL: apoA-I                  │
  │ CARGO:    starts nearly empty; PICKS UP cholesterol                 │
  │ ROUTE:    tissues (incl. FOAM CELLS in plaque) → ABCA1/ABCG1        │
  │           efflux → esterified by LCAT → carried to LIVER via SR-B1  │
  │           → excreted in BILE.  "REVERSE cholesterol transport"      │
  │ WHY "GOOD": it is the ONLY route by which cholesterol leaves the    │
  │           body. Also antioxidant and anti-inflammatory.             │
  └─────────────────────────────────────────────────────────────────────┘

   DIRECTION IS THE WHOLE STORY:
     chylomicron, VLDL, LDL  ═══► OUTWARD  (liver/gut → tissues)
     HDL                     ◄═══ INWARD   (tissues → liver → bile)

   AMARA: TAG 244 (high VLDL) · HDL 38 (low) — and these two are LINKED:
   CETP swaps TAG from VLDL into HDL; TAG-rich HDL is cleared faster.
   High triglycerides CAUSE low HDL. One lesion, two numbers.

24.6 Protein Metabolism

Proteins are continuously synthesized and degraded; an adult turns over roughly 300 g of protein a day while eating perhaps 70 g. Most amino acids are recycled. When one is to be used as fuel, its nitrogen must be dealt with first, because nitrogen cannot be burned.

Transamination. An amino group is transferred from an amino acid to α-ketoglutarate, producing glutamate and the original amino acid's carbon skeleton as a keto acid. The enzymes are aminotransferases — ALT and AST, both requiring pyridoxal phosphate from vitamin B₆, and both familiar from liver function tests: they appear in blood when hepatocytes are damaged and release their contents.

Note what transamination does and does not do. It does not remove nitrogen from the body; it funnels nitrogen from twenty different amino acids onto one carrier molecule, glutamate. Twenty problems become one.

Oxidative deamination. In liver mitochondria, glutamate dehydrogenase strips the amino group off glutamate as free ammonia (NH₄⁺), regenerating α-ketoglutarate to collect more.

The urea cycle. Ammonia is extremely toxic — it depletes α-ketoglutarate, interferes with glutamate neurotransmission, and causes cerebral edema. The liver therefore converts it, in five enzymatic steps split between mitochondrion and cytosol, into urea: a small, neutral, water-soluble, non-toxic molecule that the kidney excretes. Each urea molecule disposes of two nitrogens — one from ammonia, one from aspartate — and costs about 4 ATP equivalents. A typical adult excretes about 30 g of urea daily, and its blood concentration is measured as BUN (7–20 mg/dL).

Two clinical inferences: when the liver fails, ammonia rises and produces hepatic encephalopathy; and when the kidney fails, urea rises — which is why the BUN is a measure of renal function even though it is manufactured by the liver.

The carbon skeletons. Once stripped of nitrogen, an amino acid's carbon skeleton enters metabolism at one of seven points. Glucogenic amino acids yield pyruvate or a citric acid cycle intermediate and can become glucose. Ketogenic amino acids yield acetyl-CoA or acetoacetate and cannot. Only leucine and lysine are purely ketogenic; five are both; the rest are glucogenic.

Nitrogen balance compares intake with excretion. Positive balance — intake exceeding excretion — means net protein is being built: growth, pregnancy, recovery, resistance training. Negative balance means net breakdown: starvation, infection, burns, trauma, immobilization, and glucocorticoid excess.

Why protein is the fuel of last resort

Three reasons, all structural:

1. There is no storage protein. Every gram of protein in your body is doing a job — it is a muscle filament, an enzyme, an antibody, a transporter. Burning protein means dismantling working machinery. Fat and glycogen are inventory; protein is equipment.
2. The nitrogen is expensive. Disposing of it costs 4 ATP per urea molecule and obligates water loss in urine to excrete it.
3. The cost is functional and rapid. Losing 30–40% of body protein is generally fatal, and long before that, respiratory muscle weakness, immune failure, and impaired wound healing appear. In starvation the body works hard to avoid this — which is precisely what ketogenesis is for.

  THE METABOLIC MAP — THREE FUELS, ONE INTERSECTION
  ══════════════════════════════════════════════════════════════════════

  CARBOHYDRATE          PROTEIN                    LIPID
  ────────────          ───────                    ─────
   glycogen             body protein               triglyceride
      ▲│                    ▲│                        ▲│
      │▼ glycogenolysis     │▼ proteolysis            │▼ lipolysis
   GLUCOSE              AMINO ACIDS              GLYCEROL + FATTY ACIDS
      │                     │                        │         │
      │ glycolysis          │ TRANSAMINATION         │         │ BETA-
      │ (cytosol)           │ + deamination          │         │ OXIDATION
      │ 2 ATP, 2 NADH       │      │                 │         │ (matrix)
      ▼                     │      ▼                 │         ▼
  ══ PYRUVATE ══◄───────────┤   NH4+ ──► UREA        │    ┌─────────┐
      │        ◄────────────┘   (urea cycle,         │    │ 1 FADH2 │
      │  ▲                       liver, 4 ATP)       │    │ 1 NADH  │
      │  │ gluconeogenesis                           │    │ per cut │
      │  │ (liver, kidney)                           └──► └─────────┘
      │  │ from LACTATE, GLYCEROL,                         │
      │  │ GLUCOGENIC amino acids                          │
      ▼  │                                                 ▼
   ╔══════════════════════════════════════════════════════════════════╗
   ║                        A C E T Y L - C o A                       ║
   ║   ◄── the one-way door.  Pyruvate → acetyl-CoA is IRREVERSIBLE   ║
   ║       ∴ FAT CANNOT BECOME GLUCOSE.  Glucose CAN become fat.      ║
   ╚══════════════════════════════════════════════════════════════════╝
       │                    │                          │
       │ + oxaloacetate     │ lipogenesis              │ (if OAA is
       ▼                    ▼                          ▼  scarce)
   ┌────────────────┐   FATTY ACIDS              KETONE BODIES
   │  CITRIC ACID   │   → triglyceride           acetoacetate,
   │     CYCLE      │                            b-hydroxybutyrate
   │  3 NADH        │                            → CROSS the blood-
   │  1 FADH2       │                              brain barrier
   │  1 ATP         │                            → brain fuel in
   │  2 CO2         │                              starvation
   └───────┬────────┘
           │  all the NADH and FADH2 from EVERY pathway above
           ▼
   ┌──────────────────────────────────────────────────────────────────┐
   │  ELECTRON TRANSPORT CHAIN + CHEMIOSMOSIS  (inner mito. membrane) │
   │  NADH → 2.5 ATP · FADH2 → 1.5 ATP · O2 is the FINAL acceptor →   │
   │  H2O.  ~90% of all ATP you make.                                 │
   └──────────────────────────────────────────────────────────────────┘

24.7 The Absorptive and Postabsorptive States

Everything in the four previous sections is a capability. What determines which capabilities run at any moment is a hormonal switch with two positions.

The absorptive (fed) state occupies roughly the four hours after a meal, while nutrients are entering the blood from the gut. The dominant hormone is insulin, and the instruction is one word: store.

The postabsorptive (fasting) state occupies the rest of the time, including all of the night. The dominant hormones are glucagon, and — with escalating stress or duration — epinephrine, cortisol, and growth hormone. The instruction is: mobilize, and above all, defend blood glucose.

That last phrase is the key to the whole section. The regulated variable in the postabsorptive state is plasma glucose, held at 70–100 mg/dL, and the reason is the brain. The brain consumes about 120 g of glucose a day, cannot store any, cannot oxidize fatty acids, and takes up glucose by insulin-independent GLUT1 and GLUT3 transporters — meaning it takes what it needs regardless of hormonal signalling. Every other tissue's fuel arrangement in the fasting state is designed around leaving that glucose alone.

  THE TWO METABOLIC STATES — SAME ORGANS, OPPOSITE PROGRAMS
  ══════════════════════════════════════════════════════════════════════

  ABSORPTIVE (FED) STATE                POSTABSORPTIVE (FASTING) STATE
  ═══ INSULIN dominant ═══              ═══ GLUCAGON / EPI / CORTISOL ═══
  "STORE"                               "MOBILIZE — but spare glucose"

  GUT ──glucose,AA──► PORTAL VEIN       GUT: empty
       ──chylomicrons──► LYMPH
            │                                  ┌──────────────┐
            ▼                                  │    BRAIN     │ needs
  ┌───────────────────┐                        │  ~120 g/day  │ glucose
  │      LIVER        │                        │  GLUT1/GLUT3 │ ALWAYS
  │ glucose IN (GLUT2)│                        │  (insulin-   │
  │ ► GLYCOGENESIS    │                        │   INDEPENDENT)│
  │ ► LIPOGENESIS     │                        └──────▲───────┘
  │ ► protein synth.  │                               │ glucose
  │ ► VLDL export     │                        ┌──────┴────────────┐
  │ gluconeogenesis   │                        │      LIVER        │
  │        OFF        │                        │ ► GLYCOGENOLYSIS  │
  └─────────┬─────────┘                        │   (~100 g, gone   │
            │                                  │    in 12-24 h)    │
            ▼ glucose to periphery             │ ► GLUCONEOGENESIS │
  ┌───────────────────┐                        │   from lactate,   │
  │  SKELETAL MUSCLE  │                        │   glycerol, ALANINE│
  │ GLUT4 → membrane  │                        │ ► KETOGENESIS     │
  │ ► glycogen synth. │                        └───▲───────────▲───┘
  │   (~400 g store)  │                            │           │
  │ ► AA uptake,      │                     alanine│           │glycerol
  │   protein synth.  │                     lactate│           │+ FFA
  └───────────────────┘                     ┌──────┴─────┐  ┌──┴────────┐
  ┌───────────────────┐                     │   MUSCLE   │  │  ADIPOSE  │
  │     ADIPOSE       │                     │ burns FFA  │  │ LIPOLYSIS │
  │ GLUT4 → membrane  │                     │ NOT glucose│  │ ON (HSL   │
  │ ► LPL ACTIVATED   │                     │ exports    │  │ un-       │
  │   (grabs TAG from │                     │ alanine +  │  │ inhibited)│
  │   chylo + VLDL)   │                     │ lactate    │  │ → FFA +   │
  │ ► TAG storage     │                     │ proteolysis│  │   glycerol│
  │ ► HSL INHIBITED   │                     │ (LAST      │  │           │
  │   lipolysis OFF   │                     │  resort)   │  │           │
  └───────────────────┘                     └────────────┘  └───────────┘

  ══════════════════════════════════════════════════════════════════════
  INSULIN RESISTANCE (Amara): the switch STICKS IN THE FASTING POSITION
  ══════════════════════════════════════════════════════════════════════
     LIVER   gluconeogenesis NOT switched off  → fasting glucose 212
     ADIPOSE lipolysis NOT switched off        → FFA flux to liver ↑
                                               → VLDL export ↑ → TAG 244
                                               → CETP → HDL 38
     MUSCLE  GLUT4 does NOT reach the membrane → post-meal glucose ↑↑
     BETA CELL compensates → fasting insulin 28 uU/mL (high!)
     RESULT: the POSTABSORPTIVE program running DURING the ABSORPTIVE
             state. Both programs at once. Hyperglycemia + hyperinsulinemia
             + hypertriglyceridemia are ONE lesion, not three.

The hormonal control map

One asymmetry is worth pausing on. One hormone lowers blood glucose; four raise it. That imbalance is not sloppiness — it is a statement of relative risk. Hypoglycemia kills within minutes by starving the brain; hyperglycemia kills over years. Evolution built redundancy where failure is fast.

The postabsorptive sequence, hour by hour

24.8 Energy Balance and Body Composition

Basal metabolic rate

BMR is the energy required to keep a resting, awake, fasted, thermoneutral person alive — the cost of ion pumping, protein turnover, breathing, and circulation. It is measured under strict conditions; the more practical resting metabolic rate (RMR) is measured under relaxed ones and runs a few percent higher. A typical adult woman's BMR is about 1,300–1,500 kcal/day and a man's about 1,600–1,800.

What actually determines it:

Total energy expenditure

TEE has four components:

• BMR — 60–75% of the total. The largest and least changeable.
• Thermic effect of food (TEF) — about 10%. The cost of digesting, absorbing, and storing what you eat. It is not equal across macronutrients: protein costs 20–30% of its own energy content, carbohydrate 5–10%, fat 0–3%. This is a real, if modest, part of why higher-protein diets aid weight management.
• Physical activity — the most variable component, 15–30% in most people and up to 50% or more in an athlete.
• NEAT (non-exercise activity thermogenesis) — fidgeting, posture, standing, walking to the printer. It can vary by 2,000 kcal/day between individuals of the same size, and it falls substantially during energy restriction, which is one of the quieter reasons weight loss slows.

  WHERE THE CALORIES GO — AND WHAT A RESPIRATORY QUOTIENT TELLS YOU
  ══════════════════════════════════════════════════════════════════════

  TOTAL ENERGY EXPENDITURE, sedentary adult ~2,000 kcal/day
  ┌──────────────────────────────────────────────────────────────┐
  │████████████████████████████████████│▒▒▒▒▒▒▒│░░░░░░░░░│▓▓▓▓▓▓▓│
  └──────────────────────────────────────────────────────────────┘
   BMR 60-75%                           ACTIVITY  NEAT      TEF
   ~1,300-1,500 kcal                    15-30%    variable  ~10%

   Within BMR itself:  liver 20% · brain 20% · muscle 20% (at REST)
                       heart 10% · kidney 8% · everything else 22%
   Note: brain = 2% of body mass, 20% of resting energy.

  SAME PERSON, TRAINING FOR A MARATHON  ~3,400 kcal/day
  ┌──────────────────────────────────────────────────────────────┐
  │██████████████████████│▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│░░░│▓▓▓▓│
  └──────────────────────────────────────────────────────────────┘
   BMR (barely changed)   ACTIVITY now the largest slice

  ── MEASURING IT ──────────────────────────────────────────────────────
  DIRECT calorimetry    person in a sealed chamber; measure HEAT.
                        Accurate, and almost never used — vast and slow.
  INDIRECT calorimetry  measure O2 consumed and CO2 produced.
                        Since ~all O2 use is oxidative phosphorylation,
                        O2 consumption IS energy expenditure.
                        ~4.7-5.0 kcal per litre of O2 consumed.

  ── RESPIRATORY EXCHANGE RATIO   RER = VCO2 / VO2 ─────────────────────

    0.70 ────────────────────────────────────────  100% FAT
      |    palmitate: C16H32O2 + 23 O2 → 16 CO2 + 16 H2O
      |    16/23 = 0.70   (fat is hydrogen-rich, needs EXTRA O2)
    0.80 ───────────────────────  mixed diet at rest / ~protein
      |
    0.85 ───────────────────────  moderate exercise, ~50% each
      |
    1.00 ────────────────────────────────────────  100% CARBOHYDRATE
      |    glucose: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O    6/6 = 1.00
      |
    >1.00  NOT a fuel reading. Excess CO2 from BICARBONATE buffering
           of lactic acid during hard exercise, or from hyperventilation.

The regulation of food intake

Body weight is not a passive arithmetic outcome; it is defended. The machinery sits mainly in the arcuate nucleus of the hypothalamus, which lies next to a leaky region of the blood–brain barrier so that it can sample circulating signals directly. It contains two antagonistic neuron populations:

• NPY/AgRP neurons — orexigenic: they increase appetite and reduce energy expenditure.
• POMC/CART neurons — anorexigenic: they suppress appetite and increase expenditure.

The signals that act on them:

Why weight is defended, and defended asymmetrically. Leptin was discovered in 1994 amid expectation that obesity would prove to be leptin deficiency and be curable with leptin. It is not: people with obesity have high leptin and are leptin resistant. The deeper insight is that leptin's evolved role is as a starvation signal, not a satiety signal. A falling leptin level is a powerful alarm that triggers hunger, reduces energy expenditure, suppresses reproduction and thyroid output, and does not relent. A rising leptin level is a weak signal that is easily overridden.

That asymmetry is why weight loss is defended against so much harder than weight gain, and it predicts the clinical observation that after a 10% weight loss, measured energy expenditure sits roughly 10–15% below what body composition predicts — adaptive thermogenesis — while hunger hormones move in the direction that promotes regain, and stay there for years.

Body composition

The essential limitation of BMI is that it measures neither of the two things that actually predict risk: how much of the mass is fat, and where the fat is. Amara's BMI of 29.3 places her below the obesity threshold, and yet her waist of 96 cm and her metabolic profile identify a substantially higher-risk phenotype than the number suggests. Visceral fat is metabolically distinct — more lipolytically active, drained directly into the portal vein so its free fatty acids reach the liver first, and a source of inflammatory cytokines. Two people with identical BMI and different fat distribution do not have the same disease risk, and no height-and-weight formula can tell them apart.

24.9 Body Temperature Regulation

Only about a third of the energy released by oxidizing fuel is captured as ATP (Figure 24.1). The other two-thirds appears as heat — and that heat is not a loss. It is the entire reason a human can maintain a core temperature of 37 °C in a 20 °C room, and thermoregulation is the management of a by-product that happens to be essential.

What follows is the metabolic view of that management: where the heat comes from, and the four physical routes by which it leaves. Chapter 25 takes the same subject as a system in its own right — the controller, its set point, and what happens at the edges of human thermal tolerance.

Why it must be defended. Enzyme activity is temperature-dependent: reaction rates roughly double for every 10 °C rise, and above about 41 °C proteins begin to denature and membranes to fail. Below about 34 °C, reactions slow enough that consciousness, cardiac conduction, and coagulation are impaired. The usable window is narrow, and the same thermoregulatory machinery defends both edges.

Core and shell. The body is not one temperature. The core — brain, thoracic and abdominal viscera — is held near 37.0 °C with a circadian swing of about 0.5–1.0 °C, lowest around 04:00 and highest in the late afternoon. (Amara's twenty years of night shift have been fighting that rhythm, which is one of the several reasons shift work is metabolically costly.) The shell — skin and subcutaneous tissue — is deliberately allowed to vary by many degrees, and manipulating its temperature by changing its blood flow is the body's principal thermoregulatory tool.

Heat production comes from BMR, muscle activity (voluntary and shivering), the thermic effect of food, thyroid hormone, and sympathetic/epinephrine stimulation. Shivering can raise heat production three- to fivefold.

Heat loss occurs by exactly four physical routes:

That last row is the one to remember. Once the environment is warmer than the skin, radiation, conduction, and convection all reverse and add heat. Above roughly 35 °C ambient, evaporation is the only cooling mechanism you have — which is why high humidity, by preventing evaporation, turns a survivable temperature into a lethal one, and why anything that blocks sweating is a heat emergency waiting for a hot day (Chapter 1's anticholinergic case).

  THE THERMOREGULATORY LOOP AND THE FOUR ROUTES OF HEAT LOSS
  ══════════════════════════════════════════════════════════════════════

                    ┌───────────────────────────────────┐
                    │  HYPOTHALAMUS — preoptic area     │  SET POINT
                    │  compares CORE temp to SET POINT  │  37.0 C
                    └────────┬─────────────────┬────────┘
              too COLD ◄─────┘                 └─────► too HOT
                    │                                 │
   ┌────────────────▼──────────────┐   ┌──────────────▼────────────────┐
   │ HEAT-PROMOTING RESPONSES      │   │ HEAT-LOSS RESPONSES           │
   │ • cutaneous VASOCONSTRICTION  │   │ • cutaneous VASODILATION      │
   │   (shell insulates core)      │   │   (up to 8 L/min to skin)     │
   │ • SHIVERING (3-5x heat prod.) │   │ • SWEATING (up to 1.5 L/h;    │
   │ • NONSHIVERING THERMOGENESIS  │   │   0.58 kcal per mL evaporated)│
   │   (brown fat, UCP1 — infants) │   │ • behaviour: shade, less      │
   │ • piloerection (vestigial)    │   │   clothing, stillness         │
   │ • behaviour: clothing, curl   │   │                               │
   │   up, seek warmth             │   │                               │
   └───────────────────────────────┘   └───────────────────────────────┘
              ▲                                        │
              │        CORE TEMPERATURE 37.0 C         ▼
              └────────────◄── negative feedback ──────┘
        sensed by: PERIPHERAL thermoreceptors (skin — early warning)
                   CENTRAL thermoreceptors (hypothalamus — the real vote)

  ── THE FOUR PHYSICAL ROUTES OUT ──────────────────────────────────────

      RADIATION  ~60%   ((( infrared ))) → cooler objects, NO contact
      CONVECTION ~15%   ~~~ moving air carries heated layer away ~~~
      CONDUCTION  ~3%   ▓▓ direct contact ▓▓  (x25 in WATER)
      EVAPORATION ~22%  ^^^ 0.58 kcal per mL of sweat EVAPORATED ^^^
                        ▲ only route that works when ambient > skin temp
                        ▲ blocked by HUMIDITY — sweat that drips is wasted

  ── FEVER: the set point MOVES ────────────────────────────────────────
     pyrogens (LPS; IL-1, IL-6, TNF) → PGE2 in preoptic area
       → SET POINT 37 → 39 C
       → body now reads itself as TOO COLD: chills, shivering,
         vasoconstriction — every heat-PROMOTING response, correctly run
       → PLATEAU at the new set point
       → pyrogen cleared or COX blocked (aspirin!) → set point back to 37
       → body now reads itself as TOO HOT: drenching sweat = the CRISIS
     Nothing in the loop is broken. Only the TARGET changed.

When regulation fails

Hyperthermia. Heat exhaustion is a circulatory failure with intact thermoregulation: massive sweating and cutaneous vasodilation deplete plasma volume, the patient is weak, nauseated, tachycardic, and sweating heavily, and core temperature is usually below 40 °C. Heat stroke is thermoregulatory failure itself: core temperature above 40 °C, central nervous system dysfunction (confusion, seizure, coma), and classically — though not always — hot, dry skin, because sweating has ceased. The distinction is the presence or absence of a working effector, and it determines whether the treatment is rest and fluid or immediate aggressive cooling.

Malignant hyperthermia is a different mechanism worth knowing: an inherited RYR1 channel abnormality in which certain anaesthetic agents cause uncontrolled calcium release from the sarcoplasmic reticulum. Muscle contracts continuously and hydrolyzes ATP at an enormous rate; the heat is generated in the muscle itself, so it is a failure of production control rather than of the thermostat.

Hypothermia. Mild (32–35 °C): shivering, vasoconstriction, confusion. Moderate (28–32 °C): shivering stops — a critical sign, because the body has lost its main heat-generating effector — with bradycardia, dilated pupils, and progressive obtundation. Severe (below 28 °C): ventricular fibrillation risk, apparent death. Two clinical corollaries: hypothermia is markedly protective of the brain, so resuscitation is continued far longer than usual — "not dead until warm and dead" — and rewarming must be gradual, since sudden peripheral vasodilation returns cold, acidotic blood to the heart and can drop core temperature further, the afterdrop.

24.10 Advanced Topic · Metabolic Syndrome, and Metabolism at Race Pace

Metabolic syndrome as a multi-organ disorder

Metabolic syndrome is diagnosed by any three of five criteria; Amara meets all five. But the criteria are a scoring convenience, not a mechanism. The mechanism is a coordinated failure of insulin signalling across several organs at once, each contributing its own abnormality:

Two ideas tie the table together. Lipotoxicity: adipose tissue has a finite safe storage capacity, and once exceeded, lipid is deposited where it does not belong — in liver, muscle, pancreas, and heart — where its metabolites directly impair insulin signalling. And selective hepatic insulin resistance explains what otherwise looks contradictory: how the same liver can be deaf to insulin's instruction to stop making glucose while obeying its instruction to make fat. The result is high glucose and high triglycerides at the same time, from one hormone and one organ.

Exercise metabolism, and what happened at mile 21

  THE SUBSTRATE CROSSOVER — FUEL MIX AGAINST EXERCISE INTENSITY
  ══════════════════════════════════════════════════════════════════════

  % of energy
  from each fuel
   100 ┤                                              ████████████
       │                                        ██████            CARBO-
    80 ┤                                  ██████                  HYDRATE
       │                            ██████
    60 ┤                     ▓▓▓▓▓▓█  ← CROSSOVER POINT ~60-65% VO2max
       │            ▓▓▓▓▓▓▓▓▓  ████       (where CHO overtakes fat)
    40 ┤     ▓▓▓▓▓▓▓      ████
       │▓▓▓▓▓         ████                                        FAT
    20 ┤          ████                    ▓▓▓▓▓▓▓
       │      ████                               ▓▓▓▓▓▓▓▓▓▓▓
     0 ┼──────┴────────┴────────┴────────┴────────┴────────┴──────
        REST   25%     40%      55%      70%      85%     100%
                        % of VO2max
                                 ▲
                          FATMAX ~45-65% VO2max:  the intensity at which
                          ABSOLUTE fat oxidation (g/min) is HIGHEST.
                          Note it is NOT the same as "highest % from fat",
                          which is at rest.

  ── WHY THE CROSSOVER HAPPENS ─────────────────────────────────────────
   1 Recruitment of TYPE II fibres, which are glycolytic
   2 EPINEPHRINE + Ca2+ + AMP all directly activate glycogen phosphorylase
   3 Rising glycolytic flux raises malonyl-CoA → INHIBITS CPT-1 →
     fatty acids cannot enter the mitochondrion
   4 Splanchnic vasoconstriction reduces FFA delivery from adipose
   5 Fat oxidation is simply SLOWER: max ~1.0-1.5 g/min vs
     carbohydrate ~4-5 g/min. Above a certain power, fat cannot keep up.

  ── THE ARITHMETIC OF THE WALL ────────────────────────────────────────
   TOTAL BODY GLYCOGEN                    ~500 g  ≈  2,000 kcal
     muscle ~400 g (private — no glucose-6-phosphatase)
     liver  ~100 g (exportable)
   MARATHON ENERGY COST  ≈ 1 kcal per kg per km
     58 kg runner x 42.2 km  ≈  2,450 kcal
     at ~80% VO2max, ~80% of that from CARBOHYDRATE ≈ 1,960 kcal

   ► The store is ~2,000 kcal.  The bill is ~1,960 kcal of carbohydrate.
   ► The marathon is roughly 20-25% LONGER than the glycogen store.
   ► 20-25% of 26.2 miles = 5-6 miles from the end = MILE 20-21.
   ► This is why "the wall" lands in the same place for almost everyone,
     regardless of talent: it is set by a ratio, not by fitness.

Chapter Summary

§24.1 Six nutrient classes; three yield energy — carbohydrate and protein at 4 kcal/g, fat at 9, alcohol at 7. Fat carries more because its carbons are more reduced. Essential nutrients are those the body cannot synthesize: nine amino acids and two fatty acids. The DRIs (EAR, RDA, AI, UL) serve different purposes, and the evidence supports dietary patterns and replacement rather than single-nutrient prescriptions.

§24.2 Fat-soluble vitamins (A, D, E, K) are absorbed with fat, stored, poorly excreted, and therefore potentially toxic; water-soluble vitamins are filtered and excreted, so they must be replenished and are rarely toxic. B₁₂ is the great exception, stored in the liver for 3–5 years. Minerals divide into major and trace and serve structural, electrochemical, and catalytic roles.

§24.3 Anabolism builds and consumes energy; catabolism breaks down and releases it. Catabolism is controlled oxidation: hydrogen and electrons are stripped from fuel onto NAD⁺ and FAD. ATP is a transfer molecule, not a store — a 100 g pool turned over 50–75 kg per day — made by fast, low-yield substrate-level phosphorylation and slow, high-yield oxidative phosphorylation.

§24.4 Glycolysis: cytosolic, anaerobic-capable, net 2 ATP + 2 NADH + 2 pyruvate. Pyruvate becomes acetyl-CoA (irreversibly), lactate, alanine, or oxaloacetate. The citric acid cycle yields 3 NADH, 1 FADH₂, 1 ATP, 2 CO₂ per acetyl-CoA. The electron transport chain and chemiosmosis convert those carriers to ATP at 2.5 and 1.5 apiece, for a total of 30–32 ATP per glucose at ~34% efficiency. Liver glycogen serves the blood because the liver has glucose-6-phosphatase; muscle glycogen serves only the muscle because it does not.

§24.5 Beta-oxidation yields ~106 ATP from palmitate. Ketone bodies are made when oxaloacetate is diverted to gluconeogenesis, and they are the only fat-derived fuel that crosses the blood–brain barrier. Because pyruvate → acetyl-CoA is irreversible, fat cannot become glucose. Four lipoproteins move lipid: chylomicron, VLDL, and LDL outward; HDL inward, in the only route by which cholesterol leaves the body.

§24.6 Transamination funnels twenty amino groups onto glutamate; oxidative deamination releases ammonia; the urea cycle detoxifies it at 4 ATP per urea. Carbon skeletons are glucogenic or ketogenic. Protein is the fuel of last resort because there is no storage protein, the nitrogen is expensive, and the cost is functional.

§24.7 Insulin runs the absorptive program (store); glucagon, epinephrine, cortisol, and growth hormone run the postabsorptive one (mobilize, and defend blood glucose for the brain). One hormone lowers glucose and four raise it, because hypoglycemia kills faster. In insulin resistance the switch sticks: the liver will not stop gluconeogenesis and adipose will not stop lipolysis, so the fasting program runs during the fed state.

§24.8 TEE = BMR (60–75%) + TEF (~10%) + activity + NEAT. BMR tracks fat-free mass above all. Indirect calorimetry measures expenditure from O₂ consumption; the RER reports fuel mix, 0.7 for fat and 1.0 for carbohydrate. Intake is regulated in the arcuate nucleus by leptin, insulin, and ghrelin, and weight is defended asymmetrically — falling leptin is a far stronger signal than rising leptin. BMI cannot see fat distribution, which is what predicts risk.

§24.9 Two-thirds of metabolic energy appears as heat, and thermoregulation manages it. The hypothalamic preoptic area compares core temperature against a set point and drives heat-promoting or heat-loss effectors. Heat leaves by radiation (~60%), convection (~15%), conduction (~3%), and evaporation (~22%) — and evaporation is the only route that works above skin temperature. Fever is a deliberate set-point elevation by PGE₂; hyperthermia and hypothermia are failures of the loop itself.

§24.10 Metabolic syndrome is a coordinated multi-organ insulin signalling failure with lipotoxicity and selective hepatic resistance at its centre. In exercise, fuel mix crosses over from fat to carbohydrate at ~60–65% of VO₂max, and because total glycogen (~2,000 kcal) falls about 20–25% short of a marathon's carbohydrate bill, "the wall" lands near mile 20–21 for almost everyone. Contraction translocates GLUT4 through an insulin-independent AMPK and Ca²⁺ pathway, which is why a single session lowers glucose in insulin-resistant muscle.

Case File 24 · Resolution

Question 1 — Why does the same glucose molecule produce a fasting glucose of 212 in Amara and 84 in Nia?

Because the glucose is identical and the switch that decides what happens to it is not.

In Nia, a meal raises portal glucose, the beta cell releases insulin, and the absorptive program runs (§24.7). Her liver takes up glucose, runs glycogenesis and lipogenesis, and — the critical step — switches off gluconeogenesis. Her muscle translocates GLUT4 to the membrane and takes up glucose for glycogen. Her adipose tissue translocates GLUT4, activates lipoprotein lipase to capture circulating triglyceride, and switches off hormone-sensitive lipase so lipolysis stops. Four hours later the meal is stored, insulin falls, and the postabsorptive program takes over cleanly. Fasting glucose 84; triglycerides 62; HDL 71.

In Amara, the signal is sent and not received. Her fasting insulin of 28 µU/mL proves the beta cell is working hard — this is a failure of the effector limb, not of sensing.

• Her liver does not switch off gluconeogenesis, so it exports glucose all night into a bloodstream that already has too much. That, not her breakfast, is why she wakes at 212 mg/dL. A fasting glucose is a measurement of hepatic glucose output.
• Her adipose tissue does not switch off lipolysis, so free fatty acids flood the portal vein continuously. The liver, which remains selectively insulin-sensitive for lipogenesis, re-esterifies them and exports them as VLDL — hence triglycerides of 244. CETP then exchanges that triglyceride into HDL particles, and triglyceride-rich HDL is cleared faster — hence an HDL of 38. Two abnormal numbers, one cause.
• Her muscle does not translocate GLUT4 efficiently, so post-meal glucose has nowhere to go and stays in the blood. Her HbA1c of 7.4% is the integrated record of that.

The single sentence to keep: in insulin resistance, the postabsorptive program runs during the absorptive state. Both programs are on at once. That is why she can be simultaneously hyperglycemic (fasting program in the liver), hyperinsulinemic (fed signal being shouted), and hypertriglyceridemic (fed program in the liver's lipogenic arm). It is not three diseases. It is one broken switch, read at three points.

Question 2 — What ran out at mile 21, and why there?

Glycogen — specifically the glycogen in her running muscles, supplemented by whatever her liver could still export.

The arithmetic is in Figure 24.7 and it is unforgiving. Total body glycogen is about 500 g, or roughly 2,000 kcal, of which ~400 g is in muscle (private, because muscle lacks glucose-6-phosphatase) and ~100 g in liver. A marathon costs approximately 1 kcal per kilogram per kilometre: for Nia at 58 kg over 42.2 km, about 2,450 kcal. At marathon race pace she is running at roughly 80% of VO₂max — well above the crossover point of 60–65% — so about 80% of that energy must come from carbohydrate: roughly 1,960 kcal.

Store: ~2,000 kcal. Bill: ~1,960 kcal. Plus whatever she drank — at perhaps 40–50 g/h over 3 hours 22 minutes, another 500–700 kcal. The margin is thin, and it is thin in the same way for everybody, because both terms scale with body mass: a heavier runner stores more glycogen and also spends more energy per kilometre. The marathon is roughly 20–25% longer than the glycogen store will carry, and 20–25% of 26.2 miles is five to six miles from the finish. Mile 20 to 21. The wall is set by a ratio, not by fitness, which is exactly why elite and recreational runners report it in the same place.

Why her legs and not her lungs? Because glycogen depletion is a substrate limitation, not an oxygen limitation. Her cardiovascular system was fine. The muscle simply could not generate ATP fast enough: fat oxidation caps out around 1.0–1.5 g/min against carbohydrate's 4–5 g/min, so once glycogen is gone the muscle is restricted to a fuel that cannot support the required power. The legs feel like concrete because force production has become rate-limited by ATP supply.

And why did her heart rate fall? Because heart rate tracks power output, and her power output had dropped. Running 8:40 pace instead of 7:35 requires less oxygen, so cardiac output falls to match. This is diagnostic, and it is the detail that distinguishes glycogen depletion from overheating or dehydration, both of which cause heart rate to rise at a given pace (cardiovascular drift). A falling heart rate with a falling pace, in a runner who is trying as hard as she can, means the limitation is inside the muscle.

Question 3 — How does exercise improve insulin sensitivity before any weight is lost?

Because muscle has two independent ways to put GLUT4 into its membrane, and Amara's disease has broken only one of them.

Pathway one is insulin: receptor → IRS-1 → PI3-kinase → Akt → inhibition of AS160 → GLUT4 translocation. In insulin resistance, lipid metabolites accumulated inside the muscle fibre — diacylglycerol and ceramide — interfere at the IRS-1/PI3K step. That pathway is impaired.

Pathway two is muscle contraction. Contraction raises intracellular Ca²⁺ and the AMP:ATP ratio, activating CaMKII and AMPK, which converge on the same AS160 target from a completely different direction and mobilize GLUT4 from a partly separate vesicle pool. This pathway does not involve the insulin receptor, IRS-1, or PI3-kinase at any point. It is insulin-independent, and in insulin-resistant muscle it is intact.

So a 40-minute walk moves glucose out of her blood and into her muscle using machinery her disease never touched. And the effect outlasts the walk by 24–72 hours, for two reasons: contraction depletes muscle glycogen, and depleted glycogen strongly activates glycogen synthase, creating a glucose sink that keeps drawing glucose in; and the phosphorylation state of AS160 remains altered for many hours afterwards.

That is the acute effect, and it requires no weight loss because it is not about fat mass at all. The chronic effect is different in kind: regular training increases total GLUT4 protein content, drives mitochondrial biogenesis via PGC-1α, increases capillary density, and depletes the intramyocellular lipid metabolites that were blocking pathway one in the first place — so training does not merely bypass the broken pathway, it repairs it.

The dosing follows from the mechanism. Because the acute effect decays over 24–72 hours, exercise must be performed most days rather than concentrated into one long weekend session. This is a prescription written in molecular biology, and it is the reason exercise is considered first-line therapy for type 2 diabetes rather than an adjunct to it.

Systems Integration Case File · Entry 24

Review

Level 1 · Recall

21.1 The net ATP yield of glycolysis, per glucose, is:

a) 2    b) 4    c) 30    d) 36

21.2 Which molecule is the final electron acceptor of the electron transport chain?

a) NAD⁺    b) pyruvate    c) oxygen    d) ATP synthase

21.3 Muscle glycogen cannot raise blood glucose because muscle lacks:

a) glycogen phosphorylase    b) glucose-6-phosphatase    c) hexokinase    d) GLUT4

21.4 Which lipoprotein carries out reverse cholesterol transport?

a) chylomicron    b) VLDL    c) LDL    d) HDL

21.5 A respiratory exchange ratio of 0.70 indicates that the subject is oxidizing predominantly:

a) carbohydrate    b) fat    c) protein    d) ketone bodies

21.6 Which route of heat loss remains effective when ambient temperature exceeds skin temperature?

a) radiation    b) conduction    c) convection    d) evaporation

21.7 The dominant hormone of the absorptive state is:

a) glucagon    b) cortisol    c) insulin    d) epinephrine

21.8 Which vitamin's deficiency causes megaloblastic anemia and irreversible spinal cord degeneration?

a) folate    b) B₁₂    c) B₁ (thiamine)    d) C

Level 2 · Comprehension

21.9 Explain, in terms of one specific reaction, why fatty acids cannot be converted into glucose.

21.10 Why does the body store long-term energy as fat rather than as glycogen, and what is the trade-off?

21.11 Fever and heat stroke both raise core temperature. Explain why one is a functioning homeostatic mechanism and the other is a failed one, and why the treatments differ.

21.12 Explain why a person who loses 10% of body weight has to eat less than a person of the same weight who never lost any, in order to maintain that weight.

Level 3 · Clinical Application

21.13 A patient with type 1 diabetes stops insulin for two days. Glucose is 520 mg/dL, pH 7.14, bicarbonate 8 mEq/L, and potassium 5.6 mEq/L. Explain each finding, and explain why the potassium will fall dangerously once treatment begins.

21.14 An 82-year-old is found at home in winter, confused, with a core temperature of 30 °C and no shivering. Explain the absent shivering, the confusion, and why rewarming must be gradual.

21.15 A 55-year-old man drinks heavily, eats little for three days, and arrives with a glucose of 38 mg/dL, a lactate of 4.5 mmol/L, and mild ketosis. Explain all three, and state the order in which you would treat.

Level 4 · Integration and Synthesis

21.16 Amara and Nia eat the same 700 kcal meal of rice, chicken, and vegetables. Trace that meal's fate in each body, organ by organ, over the following six hours, and identify every point at which the two diverge.

21.17 A drug is proposed that uncouples oxidative phosphorylation — it makes the inner mitochondrial membrane leaky to protons. Predict its effects on weight, temperature, and survival, and explain why such a drug was actually marketed in the 1930s and then banned.

21.18 Both of Amara's problems in Chapters 23 and 24 were made worse by a treatment given for a problem in an earlier chapter. Construct both chains, identify what they have in common as a class of clinical problem, and propose a general principle for managing it.

Concept Map to Complete

Copy this onto blank paper and fill in every bracket from memory before checking.

                      THE TWO METABOLIC STATES
                                │
        ┌───────────────────────┴────────────────────────┐
   ABSORPTIVE                                     POSTABSORPTIVE
   hormone: [ _______ ]                     hormones: [ ______ ],
   instruction: [ _____ ]                   [ ______ ], [ ______ ],
        │                                   [ ______ ]
        │                                   instruction: [ ________ ]
        │                                        │
   LIVER   glycogenesis ON                  LIVER   [ ____________ ] ON
           gluconeogenesis [ ___ ]                  gluconeogenesis [ __ ]
           lipogenesis ON                           [ ____________ ] ON
   MUSCLE  GLUT4 → [ ________ ]             MUSCLE  burns [ ______ ]
   ADIPOSE LPL [ ___ ]                              exports [ ______ ]
           HSL [ ___ ]                                 and [ ______ ]
                                            ADIPOSE [ ________ ] ON
        │
        └──────────────► DEFENDED VARIABLE: [ _____________ ]
                         because [ _______ ] cannot store it,
                         cannot make it, and cannot burn [ _____ ]

   INSULIN RESISTANCE = the switch stuck in the [ ____________ ] position
      liver fails to stop [ _______________ ]  → glucose [ ___ ]
      adipose fails to stop [ ____________ ]   → TAG [ ___ ] → HDL [ ___ ]
      muscle fails to move [ ______ ]          → post-meal glucose [ ___ ]

   TWO ROUTES TO GLUT4 IN THE MEMBRANE:
      1  insulin → IRS-1 → [ ______ ] → Akt → AS160   [ BROKEN / INTACT ]
      2  contraction → Ca2+ and [ ____ ] → AS160      [ BROKEN / INTACT ]

Lab / Self-Exploration

1. Measure your own thermoregulation. Take your oral temperature at 06:00, 12:00, 16:00, and 22:00 on one day. Plot the four values. You should see a swing of 0.5–1.0 °C with a trough in the early morning and a peak in late afternoon. Note that this is the rhythm night shift work inverts.
2. Demonstrate evaporative cooling. Wet the back of one hand and leave the other dry, then wave both. Then repeat in a steamy bathroom. The difference between the two settings, not the two hands, is why humidity kills.
3. Estimate your own TEE. Calculate BMR with the Mifflin–St Jeor equation (10 × weight in kg + 6.25 × height in cm − 5 × age + 5 for males, −161 for females), then multiply by an activity factor of 1.2 (sedentary) to 1.9 (very active). Compare the result with what you actually eat for three days, recorded honestly. Most people underestimate intake by 20–30%; notice whether you did.
4. Find the crossover in yourself. Walk briskly for ten minutes and note that you can hold a full conversation — you are below the crossover point and burning substantially fat. Then run hard for two minutes and notice that speech becomes clipped. The "talk test" is a crude but genuine proxy for the ventilatory threshold, which sits near the crossover.
5. Read a nutrition label as a physiologist. Take any packaged food and calculate the percentage of its energy from each macronutrient using 4/4/9 kcal per gram. Compare with the percentage-by-weight figures on the front of the package. The two usually differ dramatically, and understanding why is the whole point.
6. Trace one atom. Pick a glucose molecule in the bread you eat. Write out, from memory, every location its six carbons could occupy 24 hours later — in CO₂, in muscle glycogen, in hepatic glycogen, in a triglyceride in adipose tissue, in a nonessential amino acid, in lactate. Then say which of those it can never return from as glucose, and why.

Key Terms

Next: Chapter 25 · Body Temperature Regulation — where the two-thirds of fuel energy that this chapter accounted for as heat stops being a rounding error and becomes a quantity the body must actively balance, and where UCP1 turns out to be a thermostat rather than an inefficiency.