Water-soluble vitamins: B complex and vitamin C
The ones you urinate out, which is why they are sold at absurd doses: what each B vitamin does, which one is non-negotiable on a vegan diet, and which one is genuinely toxic.
Water-soluble vitamins dissolve in water, are barely stored and the excess is urinated out. That makes them far safer than the fat-soluble ones and explains why multivitamins and energy drinks carry ridiculous amounts of the B group: it is cheap, it almost never causes problems, and the label gets to say 1,250% NRV, which looks impressive.
Why your urine turns fluorescent yellow
That is riboflavin (B2), which is fluorescent and is excreted intact. It does not prove the product "is working": it proves the opposite, that it carried far more than you can use. It is the most visible sign that a huge percentage on a label does not mean a better product.
The underlying rule is still the one from the general chapter: correcting a deficiency produces improvements, adding above requirement produces none. What changes here is that this group contains one guaranteed deficiency —B12 on an unsupplemented vegan diet— and one real toxicity —B6— and neither is obvious.
Do people who train need more B vitamins?
Requirements rise somewhat, but the extra food you already eat covers it
Woolf and Manore’s review describes why exercise may modestly increase the need for thiamine, riboflavin and B6: they are cofactors of energy metabolism and there are greater losses in sweat and urine. The practical conclusion is there too: people who train eat more, and that covers it. Where deficiencies do appear is in the other scenario, eating too little, and in that case a B complex is not the fix (what a sustained calorie deficit is).
B12: the only guaranteed deficiency on a vegan diet
B12 is made by bacteria, not by plants or animals: animals accumulate it because they ingest it. On a diet with no animal products there is no reliable source —not spirulina, not unfortified yeast, not tempeh— so this is not about optimising anything: it is a medical necessity.
Deficiency prevalence in unsupplemented vegans is very high
Pawlak’s review of studies across several countries finds deficiency in a very large fraction of vegetarians and in more than half of unsupplemented adult vegans, and in practically all children raised on unsupplemented vegan diets. Allen adds the other group almost nobody checks: adults over 60 with atrophic gastritis, in whom absorption fails even when intake is normal.
- Beef liver (100 g)~70 µg
- Clams (100 g)~49 µg
- Sardines (100 g)~9 µg
- Salmon (100 g)3.2 µg
- Beef (100 g)2.6 µg
- Fortified soy drink (250 ml)~1.2 µg
Only if it is fortified: check the label, not all of them are.
- Two eggs1.1 µg
- Spirulina, seaweed, tempeh0 usable µg
They contain inactive analogues that neither work nor count.
- Standard vegan dose
- 2,000 µg of cyanocobalamin once a week
- Form
- Cyanocobalamin
- How to measure it
- Serum B12 plus homocysteine or methylmalonic acid
- Who else should check
- Over 60, metformin, chronic PPI use, gastric surgery, Crohn’s
Or 25-250 µg daily. The doses look enormous because absorption collapses as the single dose rises.
The most studied and the cheapest. Methyl and adenosyl forms have not proven better in healthy people.
Serum B12 alone gives false normals. What to ask for.
In all of them absorption fails, not intake.
Neurological damage from B12 deficiency may not reverse
And high-dose folic acid corrects the anaemia without correcting the nerve problem, so it masks the picture and delays diagnosis. That is why a generic B complex with plenty of folate is no substitute for measuring B12 if you are at risk.
Folate (B9): the biggest public-health case
400 µg daily before and during the first trimester prevents neural tube defects
Crider’s review collects thirty years of data: periconceptional folic acid supplementation consistently reduces neural tube defects, which is why dozens of countries fortify flour. It is by a distance the best-supported supplementation recommendation in this entire guide — and it has nothing to do with performance. Outside pregnancy and that window, there is no reason to take folate on its own.
B6: the exception that is genuinely toxic
This is where the comfortable "you urinate it out, so nothing happens" rule breaks. Pyridoxine at high, sustained doses causes peripheral sensory neuropathy: tingling, numbness, clumsy gait. It usually reverses on stopping, but not always completely.
The cheap form at high doses can impair vitamin B6 function itself
Vrolijk describes the paradox with its mechanism: pyridoxine at high concentrations competes with the active form (pyridoxal-5-phosphate) at B6-dependent enzymes, so plenty of vitamin ends up behaving like an inhibitor. EFSA sets the upper level at 25 mg/day, and several European agencies have restricted sales above that threshold.
Check the B6 in what you already take
The reference intake is around 1.6 mg/day. A pre-workout, an energy drink and a multivitamin can each carry 10, 20 or 50 mg, and they add up. It is the commonest accidental vitamin overdose in people who train, and it does not come from eating badly: it comes from stacking products.
Thiamine, riboflavin, niacin, biotin and pantothenate
| Vitamin | What it does | When deficiency happens | Practical note |
|---|---|---|---|
| B1 · Thiamine | Carbohydrate metabolism | Chronic alcohol use, bariatric surgery, very poor diets | Requirement rises on very high-carbohydrate diets |
| B2 · Riboflavin | Cofactor in energy reactions | Diets with no dairy or eggs, very low intake | Responsible for the fluorescent colour of urine |
| B3 · Niacin | NAD/NADP, cellular repair | Rare on a varied diet | High doses cause flushing: warmth and redness. "No-flush" forms are not better |
| B7 · Biotin | Carboxylases; the hair-and-nails claim | Exceptional in healthy people | It interferes with blood tests: at high doses it skews troponin, TSH and thyroid hormones. Stop it several days before a test |
| B5 · Pantothenate | Coenzyme A | Practically never | It is in almost everything you eat. Its presence on a label is filler |
A B complex only improves anything cognitive when there was a deficiency behind it
Kennedy’s review of B vitamins and brain function reaches the conclusion that repeats throughout this guide, this time in the territory of focus and mood: effects appear when correcting low-intake states, and studies supplementing well-nourished people find little. It disposes in one go of almost every "energy and focus" claim on pre-workouts with a B complex.
Vitamin C: the one to dose downwards
It is the vitamin with the most sales and the fewest reasons. Antioxidant, cofactor for collagen synthesis and a facilitator of non-haem iron absorption: all true at the amounts food provides.
It does not prevent colds, and at high doses it can blunt training adaptations
Hemilä’s Cochrane review finds no prevention of the common cold in the general population; it does find a small reduction in duration, and a larger one in people under extreme physical stress —marathon runners, skiers, soldiers in the cold. On the other side, Paulsen’s trial and Merry and Ristow’s review describe how 1,000 mg a day switch off part of the oxidative signal that drives adaptation to training. Added up: little to gain and something to lose.
- Reference intake
- 95-110 mg/day in adults
- Where it does help
- With iron, 50-100 mg
- Upper level
- 1,000 mg/day
- When to avoid high doses
- During building blocks
A kiwi carries ~70 mg and a raw red pepper more than 100. Getting there is easy.
It improves non-haem iron absorption. There, the small dose is the useful one.
Above that, digestive upset and more urinary oxalate in people prone to stones.
In a competition week, where recovering beats adapting, the logic inverts.
The water-soluble ones, at a glance
| Vitamin | Reference intake | Supplement? |
|---|---|---|
| B12 | ~4 µg/day | Yes on a vegan or vegetarian diet, and in older adults with poor absorption |
| B9 · Folate | 330 µg/day | Yes in pregnancy and preconception (400 µg). Outside that, no |
| B6 | 1.6 mg/day | No. And watch what your products add up to: ceiling at 25 mg/day |
| B1, B2, B3, B5, B7 | Covered by a varied diet | No. Biotin also ruins blood tests |
| C | 95-110 mg/day | Not routinely. Yes, 50-100 mg alongside iron |
Takeaways
- Being urinated out does not make them harmless: B6 has real toxicity and a 25 mg/day ceiling.
- B12 is mandatory on a vegan diet: 2,000 µg of cyanocobalamin weekly.
- Measuring B12 by serum B12 alone gives false normals; ask for homocysteine or methylmalonic acid.
- Folic acid in pregnancy is the best-supported supplementation recommendation in the guide.
- High-dose biotin skews thyroid and troponin tests: stop it beforehand.
- Vitamin C from food; gram doses can subtract during building blocks.
Carry on here
The other vitamin family, the minerals, and the page that turns it into a decision.
Sources for this chapter
- [1]Woolf K, Manore MM (2006). B-vitamins and exercise: does exercise alter requirements?. International Journal of Sport Nutrition and Exercise Metabolism. review
- [2]Larson-Meyer DE, Woolf K, Burke L (2018). Assessment of Nutrient Status in Athletes and the Need for Supplementation. International Journal of Sport Nutrition and Exercise Metabolism. review
- [3]Pawlak R, Parrott SJ, Raj S, Cullum-Dugan D, Lucus D (2013). How prevalent is vitamin B12 deficiency among vegetarians?. Nutrition Reviews. review
- [4]Allen LH (2009). How common is vitamin B-12 deficiency?. American Journal of Clinical Nutrition. review
- [5]Crider KS, Qi YP, Yeung LF, et al. (2022). Folic acid and the prevention of birth defects: 30 years of opportunity and controversies. Annual Review of Nutrition. review
- [6]Vrolijk MF, Opperhuizen A, Jansen EHJM, Hageman GJ, Bast A, Haenen GRMM (2017). The vitamin B6 paradox: supplementation with high concentrations of pyridoxine leads to decreased vitamin B6 function. Toxicology in Vitro. trial
- [7]EFSA Scientific Committee on Food / NDA Panel (2006). Tolerable upper intake levels for vitamins and minerals. European Food Safety Authority. official guideline
- [8]Kennedy DO (2016). B vitamins and the brain: mechanisms, dose and efficacy — a review. Nutrients. review
- [9]Hemilä H, Chalker E (2013). Vitamin C for preventing and treating the common cold. Cochrane Database of Systematic Reviews. meta-analysis
- [10]Paulsen G, Cumming KT, Holden G, et al. (2014). Vitamin C and E supplementation hampers cellular adaptation to endurance training in humans: a double-blind, randomised, controlled trial. Journal of Physiology. trial
- [11]Merry TL, Ristow M (2016). Do antioxidant supplements interfere with skeletal muscle adaptation to exercise training?. Journal of Physiology. review
- [12]EFSA Panel on Dietetic Products, Nutrition and Allergies (2017). Dietary Reference Values for nutrients: summary report. EFSA Supporting Publications. official guideline