Training
B vitamins and energy
They are called that because they release energy from food, which is true and is not the same as providing it. If you are already replete, the extra goes into your urine, brightly.
What most people believe
A B-complex before training gives you energy, and since they are water-soluble there is no downside to taking plenty.
What the evidence actually says
What "energy vitamin" actually means
B vitamins are cofactors, not fuel. They carry no calories. What they do is enable the enzymes that convert carbohydrate, fat and protein into usable energy — thiamin at the entry to the citric acid cycle, riboflavin and niacin as the electron carriers, B6 across the amino acid reactions.
The consequence is that a shortage produces fatigue and correcting it removes the fatigue, while adding surplus to an already-working system does nothing. The enzyme is either enabled or it is not; it does not run faster with more cofactor sitting around.
This is the whole story of the category, and it explains both halves of the observation people find contradictory: that deficiency really does cause exhaustion, and that supplements really do nothing for most people.
Why training does raise the requirement
More energy through the system means more cofactor turnover, and some is lost in sweat and urine. Requirements for several of the B vitamins scale with energy expenditure rather than with body size — which is why they are sometimes expressed per thousand calories.
The increase is real and modest. It is also usually self-correcting, because an athlete eating more food is eating more of everything in it. The exception is the athlete eating more energy from foods that carry little else, which brings this back to the same place as most of these guides.
The two that are worth actual attention
B12, because it is only in animal foods and fortified products, and because the consequences of a long shortfall are neurological and can be permanent. Any athlete eating no animal foods needs a supplement or fortified foods — this is not a preference question.
And B6, because it is the one water-soluble vitamin with a real upper limit. Sustained high doses cause peripheral neuropathy — numbness and unsteadiness — that is sometimes only partly reversible. Doses in that range are sold routinely, which is the part worth knowing.
The bright yellow urine
That is riboflavin being excreted, and it is harmless. It is also the most honest feedback any supplement gives: it is the visible portion of a dose the body had no use for.
The numbers, and how firmly they are held
| What it describes | Figure | Evidence |
|---|---|---|
| Requirement in heavy training, relative to sedentary Scales with energy throughput | 1.5–2× | Probable |
| Performance benefit in replete athletes Consistent across the position stands | none if replete | Established |
| Upper limit for vitamin B6 The one water-soluble vitamin where excess causes nerve damage | 100 mg | Established |
Established means the position stands agree and you can act on it. Probable means the weight of evidence points one way with real dissent. Contested means it is genuinely unsettled, and anyone selling you certainty about it is selling something.
What follows is easier when you know what you are already eating.
What to actually do
Check the B6 in what you already take
Doses at or above the upper limit are sold routinely in B-complex and "energy" products. Sustained, they cause a neuropathy that is not always fully reversible.
If you eat no animal foods, B12 is not optional
Supplement or fortified foods. Spirulina and fermented foods contain analogues that occupy the receptor without doing the job.
Treat fatigue as a question, not a diagnosis
It matches a dozen shortfalls, poor sleep, low iron and an underactive thyroid. A B-complex is a poor way to find out which.
The nutrients behind this
Sources
- ACSM, AND and DC — joint position stand on nutrition and athletic performance
- NIH Office of Dietary Supplements — Vitamin B6
- NIH Office of Dietary Supplements — Thiamin
This page is education, not medical advice. It does not diagnose anything and it is not a substitute for a clinician who knows your history. Sports nutrition in particular is a field where the evidence moves, and where a figure that suits a trained twenty-five-year-old may be wrong for you.
Every nutrient named above, measured — in the app.