Chapter 13 — Key Takeaways
One page. The first chapter in this book to affirm anything — which is the point.
The Thirteen
| Fat-soluble | Water-soluble | |
|---|---|---|
| Which | A, D, E, K | C + eight Bs: B1 thiamin, B2 riboflavin, B3 niacin, B5 pantothenic acid, B6, B7 biotin, B9 folate, B12 |
| Absorbed | With dietary fat, via micelles | Directly, portal vein |
| Stored | Yes — liver, adipose | Largely not — except B12, stored for years |
| Toxicity risk | Higher | Lower, but not zero (B6, niacin) |
B12 is the exception in both directions, which is why it's the one that catches people out.
The DRI Framework
| Means | Use for | |
|---|---|---|
| EAR | Meets needs of 50% of a group | Populations |
| RDA | Meets needs of 97–98% of individuals | An individual target — deliberately set high |
| AI | Used when evidence is insufficient for an RDA | A weaker number (vitamin K, biotin, pantothenic acid, choline) |
| UL | Highest chronic intake unlikely to cause harm | A ceiling, not a target |
Labels show % Daily Value, not % RDA. The DV is one simplified figure that doesn't vary by age, sex, or pregnancy.
"Most people don't get enough" is usually derived by comparing intakes to the RDA — a figure designed to exceed what 97–98% of people need. The correct comparator for population adequacy is the EAR, and it produces much smaller numbers. It's an artefact of the wrong reference value, and it isn't even a lie.
🚪 The Threshold: Status Is Not Intake
Four routes by which they diverge:
| Route | Examples |
|---|---|
| Impaired absorption | Metformin and B12 · PPIs · celiac, IBD · bariatric surgery · age-related low acid · lost ileum |
| Increased requirement | Pregnancy, growth, illness, smoking |
| Increased losses | Some diuretics, dialysis |
| Endogenous synthesis | Vitamin D from skin — intake alone tells you almost nothing |
Plus the reverse: high intake, adequate status → expensive urine.
The curve: steep rise (deficiency) → long plateau (where most readers are) → eventual fall (toxicity). "More is better" assumes a straight line rising forever, which doesn't exist for any nutrient.
✅ Where Supplementation Genuinely Works
| Claim | Verdict | Why |
|---|---|---|
| Folic acid before conception prevents neural tube defects | ✅ Well supported | MRC Vitamin Study (1991) RCT + fortification data + mechanism. The clearest recommendation in this book. 400 µg/day, starting before conception — the neural tube closes by week four. |
| Vegans need B12 | ✅ Well supported, non-negotiable | Absent from unfortified plants; claimed sources contain inactive analogues. Stores last years, so "I feel fine" carries no information, and neurological damage can be irreversible. |
| Vitamin D in the genuinely deficient | ✅ Well supported | Corrects deficiency and treats/prevents bone disease. A different claim from supplementing the replete. |
What the three share: each corrects a genuine deficiency, or prevents one in a group with a genuinely elevated requirement. None claims to improve someone who already had enough.
Who Is Actually Deficient
Vegans (B12) · older adults (B12, D) · long-term metformin (B12) · long-term acid suppression (B12) · pregnancy/preconception (folate, D, iodine) · limited sun exposure, darker skin at high latitude (D) · malabsorption — celiac, IBD, bariatric, pancreatic (fat-soluble, B12, folate) · ⚠️ alcohol use disorder (thiamin) · food insecurity (multiple) · exclusively breastfed infants (D; K at birth).
Generally not deficient: a healthy adult in a wealthy country, varied diet including some animal foods, normal absorption, adequate sun, no relevant medication. Which is most readers.
The Isolated-Nutrient Graveyard
Observed in food → inferred to a compound → tested as a pill → nothing, or harm.
| Observed | Result | |
|---|---|---|
| Beta-carotene | Less lung cancer | ATBC/CARET: more. CARET stopped early |
| Vitamin E | Less CVD | Null; SELECT prostate signal |
| Folic acid + B12 (CVD) | Lower homocysteine → less CVD | Marker moved; outcome didn't — the cleanest surrogate failure in the book |
| Antioxidant combinations | Less cancer | Consistently null |
Four for four. Seven times across the whole book, counting L-carnitine, exogenous ketones, and fiber supplements.
Vitamin D — the special case
Deficiency correction: ✅. Supplementation in the replete: largely null in large RCTs (VITAL).
The threshold is genuinely disputed — roughly 20 ng/mL (National Academies, bone health) vs 30 ng/mL (Endocrine Society). Where you draw the line determines whether "most people are deficient."
And reverse causation is a serious candidate for much of the observational signal: obesity sequesters vitamin D in adipose tissue, and illness reduces time outdoors. Low vitamin D may often be a marker of poor health rather than a cause.
Practical: 1,000–2,000 IU in winter at high latitude is cheap, low-risk, reasonable. 5,000 IU indefinitely without testing is not.
⚠️ When More Is Harmful
| Harm | |
|---|---|
| A (preformed retinol) | ⚠️ Teratogenic; liver toxicity. Beta-carotene is not — the body regulates conversion |
| D | Hypercalcaemia, kidney damage |
| E | Bleeding; SELECT prostate signal |
| B6 | ⚠️ Peripheral neuropathy, sometimes irreversible — and it's in "nerve support" formulations |
| Niacin | Flushing; liver toxicity at high dose |
| Biotin | ⚠️ Interferes with immunoassays — troponin, thyroid. Not toxic; it makes results wrong |
| Folic acid | May mask B12 anemia while neurological damage progresses |
Three situations needing real caution: pregnancy (avoid high-dose preformed vitamin A and liver) · warfarin (vitamin K — consistency, not avoidance) · before any blood test (⚠️ tell the clinician about biotin, including "hair, skin and nails" formulations).
Verdict Summary
| Claim | Verdict |
|---|---|
| Folic acid before conception prevents NTDs | ✅ Well supported |
| Vegans need B12 supplementation | ✅ Well supported, non-negotiable |
| Vitamin D supplementation in the deficient | ✅ Well supported |
| High-dose antioxidant vitamins prevent cancer/CVD | ❌ Not supported (harmful for smokers on beta-carotene) |
| Vitamin C prevents colds | ❌ Not supported (🟡 in extreme physical stress) |
| Almost everyone is vitamin D deficient and should supplement | 🟡 Unclear — depends on your actual status, which is measurable |
| A daily multivitamin is good insurance | 🟠 Probably false for the well-nourished · 🟢 reasonable with genuinely poor intake |
What to Actually Do
- Eat varied food, including some animal products or deliberate fortified alternatives.
- If you're in a §13.5 group, act specifically. Vegan → B12. Could become pregnant → folic acid, now. Over 65, metformin, or acid suppression → get B12 checked. Limited sun → vitamin D.
- Match the intervention to the actual problem. Walt's multivitamin addressed intake when the bottleneck was absorption.
- Don't take high-dose isolated antioxidants.
- Respect the UL for fat-soluble vitamins and B6. Tell your clinician about biotin.
- Write down everything you take and put it in your phone.
🧾 Folic acid ~$0.03/day · B12 ~$0.05/day · vitamin D 1,000–2,000 IU ~$0.04/day. **All three: ~$44/year. Walt's cabinet was $2,244.**
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.