Chapter 13 — Vitamins: What Each One Does, Where to Get It, Who's Actually Deficient, and When Supplements Help vs. When They're Expensive Urine

The Hook: The man taking a multivitamin who was deficient anyway

Walt Prosser mentioned the tingling almost as an afterthought, on his way out.

Pins and needles in both feet, worse at night, coming on gradually over maybe eighteen months. He'd mentioned it to someone at some point and been told it was diabetic neuropathy — which is entirely reasonable, because he has had type 2 diabetes for eleven years and that is exactly what diabetic neuropathy looks like.

I asked how long he'd been on metformin.

Nine years.

Then I asked what was in the multivitamin — one of the nine bottles, taken faithfully every morning with his coffee.

He brought it in. Standard formulation. B12: about 6 micrograms.

We checked his B12. It was low.

Here is what had happened, and it's a genuinely common story that gets missed constantly.

Metformin interferes with vitamin B12 absorption. The mechanism involves calcium-dependent uptake of the B12–intrinsic factor complex in the ileum (Chapter 3 §3.4), and it is well enough established that the American Diabetes Association's Standards of Care recommends periodic B12 measurement in metformin-treated patients, particularly those with anemia or neuropathy.

Nine years of metformin. Nobody had checked.

And here's the part that makes this the right story to open Part III with:

He was taking a multivitamin the entire time.

Six micrograms of B12 a day is a perfectly sensible amount for a person absorbing normally. It is nowhere near enough to overcome metformin-induced malabsorption, where the therapeutic approach uses much higher oral doses or injections — because when absorption is the bottleneck, a physiologically-sized dose doesn't reach the bloodstream.

So Walt had the appearance of coverage and none of the substance. The bottle said B12. His blood said otherwise. And the symptom that should have prompted the test was being attributed to a disease he definitely has, which is the most dangerous kind of plausible explanation.

Two things were true at once, and they are the two halves of this entire part of the book:

1. Most of Walt's nine supplements were doing nothing, at a cost of $2,244 a year.
2. He had a genuine, symptomatic, correctable vitamin deficiency, and the supplement he was taking for it was the wrong dose delivered by the wrong route.

If you take Part III to be a debunking exercise, you will get Walt's case exactly backwards.

🏃 Fast Track: §13.3 (the threshold concept), §13.5 (who is actually deficient), and §13.6 (where supplementation genuinely works). Twenty-five minutes, and §13.6 is the one most readers don't expect.

🔬 Deep Dive: §13.2 (the DRI framework, which almost nobody understands and which dissolves half the arguments about "recommended amounts") and §13.7 (the isolated-nutrient graveyard) are where students and clinicians should spend time.

13.1 What a vitamin actually is

A vitamin is an organic compound your body needs in small amounts, cannot make in sufficient quantity, and must therefore obtain from food.

Every word in that definition is doing work. Small amounts — micrograms to milligrams, versus grams for macronutrients. Cannot make in sufficient quantity — which is why vitamin D is arguably not really a vitamin (you synthesize it in skin) and why vitamin C is a vitamin for us and not for most other mammals, who make their own.

There are thirteen. That's the complete list, it hasn't changed since the 1940s, and anything marketed as a vitamin that isn't on it — "vitamin B17," "vitamin O" — isn't one.

That table predicts most of what follows. Fat-soluble vitamins are the ones you can overdose on; water-soluble ones are mostly the ones that become expensive urine. B12 is the exception in both directions and is therefore the vitamin that catches people out — as it caught Walt.

13.2 The DRI framework, properly

This section is dry and it dissolves an enormous number of arguments, so it earns its place.

When someone says "the recommended amount," they are usually referring to one of four different numbers that mean different things.

Four things follow that people get wrong constantly.

The RDA is deliberately set high. It's designed to cover 97–98% of people, which means most individuals need less than the RDA. Eating below it on a given day is not a deficiency — it is usually just Tuesday. Nutrient intakes are assessed over weeks, not days.

An AI is a weaker number than an RDA, and several vitamins only have one — vitamin K, biotin, pantothenic acid, and choline among them. When a supplement label says "100% of your daily requirement" for one of those, the underlying figure carries substantially more uncertainty than the percentage implies.

The UL is not "the dose at which harm begins." It's a conservative ceiling with safety margins built in. Exceeding it isn't automatically dangerous, and staying under it isn't a guarantee — but it's the best available marker, and §13.10 covers where it matters.

And crucially: the percentages on a supplement label are % Daily Value, not % RDA. The Daily Value is a single labelling figure derived from the DRIs but simplified for a general population — it doesn't vary by age, sex, or pregnancy. So "100% DV" is not "100% of what you need," and for some nutrients in some groups the gap is substantial.

💡 Aha moment. Notice what the RDA's design does to the phrase "most people don't get enough."

That claim, ubiquitous in supplement marketing, is usually derived from comparing population intake distributions to the RDA — a figure explicitly set to exceed what 97–98% of people require. Comparing everyone to a number designed to cover almost everyone will always make a large fraction look inadequate, and it says almost nothing about how many people are actually deficient.

The correct comparator for population adequacy is the EAR, and assessments using it produce much smaller numbers. The most common statistical claim in the supplement industry is an artefact of using the wrong reference value — and it isn't even a lie, which is why it works.

13.3 🚪 The threshold: status is not intake

Here is the concept that governs the whole of Part III.

🚪 Threshold concept.

Nutrient intake — what goes in your mouth — and nutrient status — what's actually in your tissues and doing work — are different variables, and they can diverge in both directions. Supplementing someone whose status is already adequate almost never helps.

The before-and-after

📊 Diagram (described). Picture the curve that governs every nutrient in Part III. Horizontal axis: intake, from zero on the left. Vertical axis: health outcome, better upward.

At the far left, intake near zero, the curve is on the floor — frank deficiency. Scurvy. Beriberi. Rickets. This region is dramatic, historically enormous, and completely uncontroversial.

Then the curve climbs steeply. Small increases in intake produce large improvements. This is the region where a few pennies of a nutrient prevents a disease, and it is where essentially all of nutrition's greatest public health victories live.

Then — and this is the part that gets forgotten — the curve flattens. It reaches a plateau. More intake produces no further benefit, because the relevant processes are already saturated. This plateau is very wide, and for most nutrients, for most people in wealthy countries, it is where you already are.

And then, eventually, for several nutrients, the curve turns downward. Hypervitaminosis A. Vitamin D toxicity. B6 neuropathy. The vitamin E prostate signal.

Steep rise, long plateau, eventual fall. Now notice what the supplement industry's implicit model is: a straight line rising forever. Every "more is better" claim is a claim about a graph that doesn't exist for any nutrient at any dose.

And the threshold concept, restated on this picture: supplementation moves you rightward along the axis. If you're on the steep part, that's transformative. If you're on the plateau — where most readers are, for most nutrients — you move sideways at your own expense. And if you're already far right, you move toward the descent.

The only way to know which region you're in is to measure, which is why §13.11 exists and why "am I eating enough?" is the wrong question.

Walt's case, in these terms

Intake: adequate. Six micrograms of B12 daily, plus meat and dairy.

Status: low.

Why they diverged: metformin impaired absorption at the ileum. A normal intake produced an abnormal status, and the multivitamin created the appearance of coverage precisely because it addressed intake, which was never the problem.

The four ways status and intake diverge:

And the direction that matters for the supplement industry: you can also have high intake and no benefit, because status was already adequate and the surplus was excreted or stored uselessly. That's the expensive urine, and it's §13.7.

13.4 The thirteen, briefly

A working reference. Keep this; the detail is in Appendix A and the NIH Office of Dietary Supplements fact sheets are better than any table I can print.

Fat-soluble

Water-soluble

Two things to notice about that second table.

The deficiency diseases are dramatic and historically enormous. Scurvy, beriberi, pellagra, rickets — these killed and disabled at scale, and they are why nutrition science exists as a field. This is the part of nutrition where the evidence is strongest and least contested, and it's worth stating plainly before the chapter starts being sceptical about anything.

B12 comes from animal foods. This is not a controversial or ideological claim; it's a fact about where the vitamin is synthesized (by bacteria) and where it accumulates. It has one unavoidable consequence, and §13.6 states it as directly as I know how.

13.5 Who is actually deficient?

The honest list. This is much shorter than the supplement aisle implies and much longer than sceptics tend to admit.

Who is generally not deficient: a healthy adult in a wealthy country eating a varied diet including some animal foods, with normal absorption, adequate sun exposure, and no relevant medication.

Which is most people reading this, and it is why the multivitamin question in §13.9 comes out the way it does — and also why §13.6 matters so much, because most is not all, and the people who aren't in that category are frequently the ones who never get tested.

🔄 Check your understanding. Which of these people most needs a supplement, and which needs a blood test rather than a supplement?

(a) A 34-year-old office worker eating a mixed diet who feels tired. (b) A 29-year-old planning a pregnancy in three months. (c) A 71-year-old on metformin and omeprazole with tingling feet. (d) A 24-year-old who became vegan two years ago and feels fine.

Answer

(b) needs a supplement now, without testing — folic acid, starting before conception. §13.6 explains why this one doesn't wait for a test: the neural tube closes in the first few weeks, often before a pregnancy is recognized, so the window closes before testing would help.

(d) needs a supplement, unambiguously — B12, which is unavoidable on a vegan diet. "Feels fine" is not reassuring: hepatic B12 stores last years, so deficiency develops silently and the first symptoms may be neurological and irreversible.

(c) needs a blood test urgently, and probably treatment — this is Walt. Metformin and a proton pump inhibitor, both impairing B12, plus a symptom that fits. ⚠️ Attributing it to diabetic neuropathy without checking is the error in the chapter's opening.

(a) needs neither, yet. Tiredness is the least specific symptom in medicine, has a hundred causes — sleep, mood, iron, thyroid, apnea, life — and "take a multivitamin and see" is how a treatable cause gets a six-month delay. The answer here is a proper history, not a bottle.

13.6 Where supplementation genuinely works

This section exists because Part III would be dishonest without it, and because the failure mode of a sceptical book is to leave a reader believing that no supplement ever helps.

Some do. Here are the ones with the strongest evidence in this chapter.

🔬 Claim → Evidence → Verdict

The claim: "Women who might become pregnant should take folic acid before conception to prevent neural tube defects."

Where it comes from: Observational associations between low folate status and neural tube defects — spina bifida and anencephaly — followed by randomized trials.

What the evidence actually shows: This is one of the strongest results in the history of nutrition. The MRC Vitamin Study (1991) — a multicentre randomized trial — found that folic acid supplementation substantially reduced the recurrence of neural tube defects in women who had previously had an affected pregnancy. A separate randomized trial in Hungary found reduction in first occurrence. Mandatory folic acid fortification of flour, subsequently introduced in many countries, has been followed by measurable declines in neural tube defect rates at population level.

Randomized trial evidence, plus a population-level natural experiment, plus a clear mechanism (folate is required for DNA synthesis during rapid cell division, and the neural tube closes by around week four). Convergence, in the sense of Chapter 2 §2.10, about as complete as nutrition offers.

And the timing is the critical part: the neural tube closes before most people know they are pregnant. Starting after a positive test is frequently too late — which is why the recommendation is for anyone who could become pregnant, not for those who are.

📉 Evidence quality: Rung 6–8. Randomized trials, population fortification data, established mechanism.

Verdict: ✅ Well supported. Standard guidance is 400 µg/day for those who could become pregnant, starting before conception, with higher doses for those with a previous affected pregnancy or specific risk factors — a clinical decision. This is the clearest supplement recommendation in this book, and it is not close.

🔬 Claim → Evidence → Verdict

The claim: "Vegans need to supplement B12."

Where it comes from: B12 is synthesized by bacteria and accumulates in animal tissues. It is essentially absent from unfortified plant foods. Claims that it can be obtained from spirulina, nori, tempeh, or unwashed vegetables do not hold up — several of these contain B12 analogues that are detected by some assays but are not biologically active, and may even interfere with genuine B12.

What the evidence actually shows: This isn't really contested. B12 deficiency in unsupplemented long-term vegans is well documented. The consequences are serious: megaloblastic anemia, and — critically — neurological damage that can be irreversible if prolonged. Hepatic stores last years, which means the deficiency develops silently over a long period and "I feel fine" carries no information.

📉 Evidence quality: Established biochemistry, consistent clinical observation, unambiguous mechanism.

Verdict: ✅ Well supported, and non-negotiable. Anyone eating a vegan diet — or a diet with very little animal food — needs a reliable B12 source: a supplement or genuinely fortified foods, with periodic testing. This is the single most important practical point in this chapter for anyone who eats that way, and it is the one thing about plant-based diets on which there is no serious argument.

🔬 Claim → Evidence → Verdict

The claim: "Vitamin D supplementation helps people who are deficient."

Where it comes from: Rickets and osteomalacia are unambiguous deficiency diseases with an unambiguous cure. Vitamin D is required for calcium absorption; without it, bone mineralization fails.

What the evidence actually shows: In people who are genuinely deficient, supplementation corrects the deficiency and prevents and treats the associated bone disease. This is not in doubt and has not been in doubt for a century. Supplementation is standard for exclusively breastfed infants, and is recommended in many countries for populations with limited sun exposure.

What it does not show — and this is §13.8 — is benefit in people who are already replete. The two claims get merged constantly and they are completely different.

📉 Evidence quality: ✅ for deficiency correction; much weaker for supplementation in the replete.

Verdict: ✅ Well supported — for the deficient. Which is a large number of people, particularly at high latitudes in winter, in those with darker skin, in older adults, and in anyone with limited sun exposure. Walt's 25(OH)D was 22 ng/mL, which is why his vitamin D was one of only two supplements I kept (Chapter 11, Case Study 1).

Three ✅ verdicts in one section. They share a shape worth naming: each corrects a genuine deficiency, or prevents one in a group with a genuinely elevated requirement. None of them is a claim that a nutrient improves the health of someone who already had enough.

That distinction is the entire chapter.

13.7 The isolated-nutrient graveyard

Now the other half, and by now the pattern should be familiar enough that you can predict it.

The template, from Chapter 2's Case Study 1:

Observed: people eating food F have less disease D. Inferred: compound C in food F prevents disease D. Tested: compound C, isolated, at high dose, in a pill. Result: nothing, or harm.

Filled in, four times:

Four for four. And note the third row, because it's the cleanest surrogate-endpoint failure in the book: the intervention did exactly what it was designed to do to the marker, and nothing to the outcome (Chapter 2 §2.7).

🔬 Claim → Evidence → Verdict

The claim: "High-dose antioxidant vitamins protect against cancer and heart disease."

Where it comes from: A coherent and genuinely attractive hypothesis. Oxidative damage contributes to disease; antioxidants reduce oxidative damage; people eating antioxidant-rich diets have less disease. Mechanism, observational data, biomarkers — all aligned.

What the evidence actually shows: Repeatedly tested, repeatedly failed, and in several cases harmed. Beta-carotene increased lung cancer in smokers. High-dose vitamin E showed no cardiovascular benefit and a prostate cancer signal. Antioxidant combination trials have been consistently null.

There is even a plausible reason for the harm: oxidative signalling is not purely damage. Reactive oxygen species are involved in normal cellular signalling, including some adaptive responses to exercise, and blanket high-dose suppression may interfere with processes you want.

📉 Evidence quality: Multiple large RCTs, some stopped early for harm.

Verdict: ❌ Not supported, and for smokers taking beta-carotene, harmful. Meanwhile, eating fruit and vegetables remains associated with better outcomes — which is the point. The food works; the extracted compound doesn't.

🔬 Claim → Evidence → Verdict

The claim: "Vitamin C prevents colds."

Where it comes from: Linus Pauling — a genuinely brilliant chemist and double Nobel laureate — advocated high-dose vitamin C from the 1970s with enormous public influence. Vitamin C is genuinely involved in immune function.

What the evidence actually shows: Cochrane reviews of regular vitamin C supplementation find no meaningful reduction in the incidence of colds in the general population. There is a modest reduction in duration — on the order of several percent, which for a week-long cold is a matter of hours. Starting vitamin C after symptoms begin generally shows no benefit.

The interesting exception: in people undergoing extreme short-term physical stress — marathon runners, soldiers in subarctic conditions — supplementation has been associated with a meaningful reduction in cold incidence. A narrow, real, and rarely-mentioned finding.

📉 Evidence quality: Extensive RCT evidence, systematically reviewed.

Verdict: ❌ Not supported for preventing colds in the general population. 🟡 Unclear in extreme physical stress, where there is a genuine signal. A cautionary note about how far a Nobel-laureate's confidence can travel without evidence behind it.

🧩 Productive struggle. Five minutes before reading on.

Four times now — beta-carotene, vitamin E, folic acid for cardiovascular prevention, antioxidant combinations — a nutrient that looked protective in food failed or harmed as a pill.

Generate as many explanations as you can for why the food works and the extracted compound doesn't. Aim for at least four, and try to make them genuinely different from each other rather than restatements.

What I'd list

1. The compound was never the active ingredient. It was a marker of eating the food. This is Chapter 2's healthy-user bias at the molecular level: blood beta-carotene indicated vegetable intake, which indicated not smoking, exercising, and having money.

2. Dose. Food delivers milligrams in a matrix; supplements deliver tens or hundreds of times more as a bolus. Being on the plateau and pushing further right (see the diagram above) is not the same intervention at all — and for several of these it moved people toward the descent.

3. Isolation breaks a system. Nutrients in food arrive as families — carotenoids compete for absorption, tocopherol isoforms interact, folate comes with B12 and B6. Flooding one member of a family can suppress the others, which is documented for carotenoids and is a plausible mechanism for outright harm.

4. Displacement. A person eating vegetables is not eating something else. A person taking a capsule displaces nothing — the same argument Chapter 11 made about fiber supplements, and Chapter 9's substitution question in a different costume.

5. Oxidative signalling isn't purely damage. Reactive oxygen species participate in normal cellular signalling, including adaptive responses to exercise. Blanket high-dose suppression may interfere with processes you want — a genuinely plausible mechanism for harm rather than mere nullity.

6. The population was wrong. These trials largely recruited replete people. Whether the same compound helps someone genuinely deficient, at physiological doses, was mostly not tested — and it's a completely different claim.

If you got four, you have the pattern. Notice that explanations 1 and 6 mean the trials didn't refute the food finding at all — they refuted a specific inference from it, which is a narrower and more interesting result than "antioxidants don't work."

13.8 Vitamin D: the special case

Vitamin D deserves its own section because it is the one nutrient where the deficiency claim and the supplement claim have completely different evidential support, and the two are merged relentlessly.

What's solidly established:

• Vitamin D is required for calcium absorption and bone mineralization
• Deficiency causes rickets in children and osteomalacia in adults
• Cutaneous synthesis depends on UVB, which depends on latitude, season, time of day, skin pigmentation, clothing, sunscreen and age — so it is genuinely common to be deficient in winter at high latitudes
• Correcting deficiency works (§13.6, ✅)

Where it gets contested:

What counts as deficient? There is a genuine, unresolved disagreement. Broadly, the National Academies' framing treats serum 25(OH)D below about 20 ng/mL (50 nmol/L) as inadequate for bone health, while the Endocrine Society has used a higher threshold of about 30 ng/mL for sufficiency. Where you draw that line dramatically changes how many people are "deficient" — and the higher threshold, applied to whole populations, produces the headline that most people are deficient.

Does supplementation help people who aren't deficient? This is where the enthusiasm has collided with the trials. Large randomized trials — VITAL most prominently — have generally not found that vitamin D supplementation in generally replete populations reduces cardiovascular events or cancer incidence. Results for fracture prevention in community-dwelling adults have been mixed to disappointing.

And there's a reverse-causation problem (Chapter 2 §2.4) that deserves more attention than it gets: low vitamin D is associated with a very long list of diseases — but obesity sequesters vitamin D in adipose tissue, and illness reduces time outdoors. Low vitamin D may frequently be a marker of poor health rather than a cause of it.

🔬 Claim → Evidence → Verdict

The claim: "Almost everyone is vitamin D deficient and everyone should supplement."

Where it comes from: Genuinely high rates of low 25(OH)D in many populations — particularly at high latitudes, in winter, in people with darker skin, in older adults — plus a huge observational literature associating low vitamin D with almost every disease studied.

What the evidence actually shows: Deficiency is real and common in identifiable groups, and correcting it is ✅. But the "everyone" framing depends on the higher sufficiency threshold, and the associations with non-skeletal outcomes have largely not translated in randomized trials. Reverse causation is a serious candidate explanation for much of the observational signal.

📉 Evidence quality: Strong for deficiency correction. Large RCTs largely null for supplementation in the replete.

Verdict: 🟡 Unclear / it depends — and the "it depends" is on your actual status, which is measurable. Supplementation is well supported if you're deficient and poorly supported if you're not, which is §13.3's threshold in a single nutrient. A modest dose (commonly 1,000–2,000 IU) in winter at high latitude is cheap, low-risk, and reasonable. 5,000 IU indefinitely without testing — Walt's dose — is not.

13.9 The multivitamin question

The most common supplement in the world, so it deserves a direct answer.

🔬 Claim → Evidence → Verdict

The claim: "A daily multivitamin is good insurance — it can't hurt, and it covers any gaps in your diet."

Where it comes from: Intuitive, cheap, and low-risk. Most people's diets do fall short of the RDA for something, and a multivitamin appears to address that at trivial cost.

What the evidence actually shows: Large trials and systematic reviews of multivitamin supplementation in generally well-nourished populations have found little or no effect on mortality, cardiovascular disease, or cancer. Some analyses have found small effects on specific outcomes; the overall picture is close to null. Major preventive-services bodies have generally concluded that evidence is insufficient to recommend multivitamins for chronic disease prevention in healthy adults.

Three honest qualifications, because the flat "multivitamins are useless" line is also wrong:

1. They genuinely do prevent frank deficiency in people whose intake is poor — the food-insecure, people with very restricted diets, people with poor appetite. That's not nothing.
2. They are cheap and low-risk at standard doses, so the cost of being wrong is small — unlike most of Walt's cabinet.
3. They are the wrong tool for a specific deficiency, which is Walt's entire case. A physiological dose in a broad formulation will not overcome malabsorption.

📉 Evidence quality: Multiple large RCTs and systematic reviews; consistent near-null in well-nourished populations.

Verdict: 🟠 Probably false as "good insurance" for a well-nourished adult — it mostly produces expensive urine and, more importantly, the appearance of coverage. 🟢 Reasonable for people with genuinely poor or restricted intake. Never a substitute for testing when a specific deficiency is suspected.

13.10 When more is actively harmful

The UL exists for a reason, and fat-soluble vitamins are where it bites.

⚠️ When to see a professional. Vitamins are not automatically benign, and three situations warrant real caution.

Pregnancy: avoid high-dose preformed vitamin A (retinol) and liver; take folic acid; discuss everything else. Prenatal formulations are designed around this.

Warfarin: vitamin K intake affects anticoagulation. The rule is consistency, not avoidance — sudden large changes in leafy green intake are the problem.

Before any blood test: ⚠️ tell the requesting clinician about biotin, including in "hair, skin and nails" formulations. High-dose biotin can produce falsely high or falsely low results on a range of immunoassays including cardiac troponin and thyroid function — which is not a theoretical concern (Chapter 16).

And generally: bring the actual bottles, or photograph the labels, to your appointments. Almost nobody does, because supplements are mentally filed as food.

🔄 Check your understanding. A supplement label reads: "Vitamin B6 — 100 mg (5,882% Daily Value)." Work out what's wrong with this, using §13.2 and §13.10.

Answer

Three separate problems, in ascending order of seriousness.

1. The percentage is theatre. 5,882% of the Daily Value sounds like generosity and is a warning sign. For a water-soluble vitamin the surplus is largely excreted — expensive urine (§13.3) — so the number is advertising an amount you cannot use.

2. The reference value is the wrong one anyway. That's % Daily Value, a single simplified labelling figure, not % RDA for you specifically (§13.2). It doesn't vary by age, sex, or pregnancy.

3. And this is the one that matters: ⚠️ B6 is the clearest water-soluble toxicity there is. Sustained high supplemental intake can cause peripheral neuropathy — tingling, numbness, and gait problems — which is sometimes irreversible. The UL is far below 100 mg, and this dose is not a harmless excess; it's in the territory where the curve in §13.3's diagram has turned downward.

The bitter detail: high-dose B6 appears routinely in "nerve support," "energy," and "hair, skin and nails" formulations — meaning a person taking it for tingling feet may be taking a dose that causes tingling feet. That is not hypothetical; it is a recognized clinical presentation.

And notice how this compounds with Walt's story. A man with neuropathy, taking a multivitamin, whose actual problem was B12 — if he had escalated to a high-dose B-complex "for the nerves," he could have added a second cause of neuropathy on top of an untreated first one, while still never being tested.

13.11 What's worth testing

Testing resolves §13.3's threshold directly, so it's worth knowing what's useful.

The general rule: test when there is a reason to suspect deficiency, not as a screening exercise — and certainly not as a prelude to a supplement recommendation from whoever sold you the test (Chapter 1 §1.5's incentive question, applied to diagnostics).

13.12 What to actually do

1. Eat varied food, including some animal products or deliberate fortified alternatives. This handles almost everything for almost everyone, which is why §13.5's "not deficient" list is so long.

2. If you are in a §13.5 group, act on it specifically. Vegan → B12, without exception. Could become pregnant → folic acid, starting now. Over 65, or on metformin, or on long-term acid suppression → get B12 checked. Limited sun exposure → vitamin D.

3. Match the intervention to the actual problem. Walt's multivitamin addressed intake when the problem was absorption. The right question is not "am I eating enough?" but "is my status adequate?" — and for the vitamins that matter, that's measurable.

4. Don't take high-dose isolated antioxidants. Four trials, four failures, two of them harmful.

5. Respect the UL for fat-soluble vitamins and B6. And tell your clinician about biotin.

6. Buy third-party certified products where it matters. Chapter 16 covers why, under DSHEA.

🧾 Cost check. The genuinely worthwhile ones are cheap. Folic acid: ~$0.03/day. B12: ~$0.05/day. Vitamin D 1,000–2,000 IU: ~$0.04/day. All three together, for someone who needed all three, come to about $44 a year.** Walt's cabinet was **$2,244, of which the two supplements worth keeping cost about $70.

What we don't know

We don't know the optimal 25(OH)D threshold, and the disagreement between expert bodies is genuine rather than a communication failure. It matters practically — it's the difference between "most people are fine" and "most people are deficient."

We also don't know whether the observational associations between low vitamin D and a long list of diseases reflect any causal contribution at all, or are entirely explained by reverse causation and confounding. The trials suggest mostly the latter; they don't settle it.

And we don't know how much of the isolated-nutrient failure is about dose, about isolation, or about the wrong population. Whether antioxidants might help someone genuinely deficient, at physiological doses, in the right context, remains open — and is a very different claim from the one that was tested.

Spaced Review

Answer before reading on.

1. (Chapter 2) The beta-carotene story appeared in Chapter 2 and again here. What was the general template, and how many times has this book now filled it in?

Observed in food → inferred to a compound → tested as a pill → nothing, or harm. Filled in for beta-carotene, vitamin E, folic acid/homocysteine, antioxidant combinations — and in Chapter 6 for L-carnitine and exogenous ketones, and in Chapter 11 for fiber supplements. Seven times.

2. (Chapter 3) Why does metformin cause B12 deficiency, anatomically?

B12 absorption requires intrinsic factor (made in the stomach) and an intact ileum (Ch 3 §3.4). Metformin interferes with the calcium-dependent uptake of the B12–intrinsic factor complex at the ileum. Two separate requirements in two separate places — which is why B12 has so many independent routes to deficiency.

3. (Chapter 11) Walt's supplement audit kept vitamin D and magnesium and stopped three others. Using §13.8, explain why the vitamin D decision was right and why the dose changed.

Right to keep: his 25(OH)D was 22 ng/mL — genuinely low, and correcting deficiency is ✅. Dose reduced from 5,000 to 2,000 IU because the evidence supports correcting deficiency, not maintaining high intakes indefinitely, and 5,000 IU without monitoring is above what the situation warranted.

Project Checkpoint: Your Vitamin Gap Analysis

Component thirteen, and Phase 3 — Debiasing begins. Thirty minutes.

Step 1 — Check the §13.5 list first. Before any arithmetic, go through the risk groups. Are you in any of them? Is anyone you feed?

Groups I'm in: ______

For most readers the honest answer is "none," and that is the most useful output of this checkpoint. For a meaningful minority it's one or two, and those are worth acting on today.

Step 2 — From food, not from labels. Using your three-day diary and Appendix A or USDA FoodData Central, estimate intake for the four vitamins most commonly short in real diets:

Precision is not the point. You are looking for order of magnitude — is this roughly adequate, or obviously not?

Step 3 — Read your own labels. If you take anything, write down what's actually in it and compare each to the UL (Appendix A), not just the RDA. Pay particular attention to B6, preformed vitamin A, and biotin.

Anything above the UL? ______

Step 4 — Decide about testing. Using §13.11: is there a specific reason to suspect a specific deficiency? If yes, that's a conversation with a physician. If no, don't order a panel — it will generate findings rather than answers.

Step 5 — The list. Write down every supplement, vitamin, herbal product and fortified "functional" food you take. Put it in your phone. Take it to your next medical appointment.

This takes four minutes and it is the single most useful clinical act in this chapter. Walt's list contained a biotin dose that nearly cost him a correct emergency-room diagnosis, and nobody treating him knew.

Non-tracking alternative. Skip Step 2 entirely. Do Steps 1, 3, 4 and 5 — the risk-group check, the label check, the testing decision, and the list. Those four contain nearly all of the actionable value, and none of them requires tracking food.

Next checkpoint (Chapter 14): your mineral gap analysis, with particular attention to iron, calcium and sodium.

Chapter Summary

Thirteen vitamins. Four fat-soluble (A, D, E, K — stored, higher toxicity risk), nine water-soluble (C and eight Bs — largely not stored, mostly excreted). B12 is the exception in both directions, and therefore the one that catches people out.

The DRI framework: EAR (50% of a group — for populations), RDA (97–98% — an individual target, deliberately set high), AI (used when evidence is insufficient for an RDA), UL (a ceiling with safety margin, not a harm threshold). Labels show % Daily Value, which is not % RDA.

"Most people don't get enough" is usually derived by comparing intakes to the RDA — a number designed to exceed what 97–98% of people need. It is an artefact of using the wrong reference value, and it isn't even a lie.

🚪 The threshold: status is not intake. They diverge through impaired absorption, increased requirement, increased losses, and endogenous synthesis — and in the other direction through high intake with adequate status, which is the expensive urine.

Where supplementation genuinely works:

The isolated-nutrient graveyard: beta-carotene (harm in smokers) · vitamin E (SELECT prostate signal) · folic acid/homocysteine (marker moved, outcome didn't) · antioxidant combinations (null). Four for four.

This chapter's other verdicts:

The one thing to remember: Walt was taking a multivitamin the entire time. The bottle said B12; his blood said otherwise. Intake is not status, and the appearance of coverage is worse than no coverage, because it stops anyone looking.

What's Next

Chapter 14 does the same job for minerals, and it contains the one that has been waiting since Chapter 4.

Devi's ferritin is 11 ng/mL and her hemoglobin is 12.6 — which is technically normal, which is why nobody caught it. Iron-deficient without being anemic is a state that standard screening misses, that affects a large number of menstruating women and endurance athletes, and that explains a great deal of unexplained fatigue.

Plus calcium, magnesium, zinc, selenium, iodine — and sodium, which is the most genuinely contested number in the whole of nutrition.