Biochemistry
B vitamins as coenzymes in metabolism
B vitamins don't provide energy themselves — they're the coenzymes without which the pathways that release energy cannot run.
The coenzyme map
| Vitamin | Coenzyme form | Role | Deficiency disease |
|---|---|---|---|
| B1 (thiamine) | TPP | Pyruvate → acetyl-CoA; α-ketoglutarate step | Beriberi; Wernicke-Korsakoff |
| B2 (riboflavin) | FAD, FMN | Electron carrier in Krebs and beta-oxidation | Ariboflavinosis |
| B3 (niacin) | NAD+, NADP+ | Principal electron carrier across metabolism | Pellagra |
| B5 (pantothenic acid) | Coenzyme A | Carries acyl groups; forms acetyl-CoA | Rare |
| B6 (pyridoxine) | PLP | Amino acid transamination | Anaemia, neuropathy |
| B7 (biotin) | Biotin | Carboxylation reactions | Rare |
| B9 (folate) | THF | One-carbon transfer; DNA synthesis | Megaloblastic anaemia; neural tube defects |
| B12 (cobalamin) | Methylcobalamin | Works with folate; myelin maintenance | Megaloblastic anaemia; nerve damage |
Why deficiency symptoms are so broad
Because these coenzymes serve many enzymes across many pathways at once, deficiency rarely produces one isolated symptom. Fatigue is near-universal, since energy metabolism itself is impaired. Neurological symptoms are also common — nervous tissue has high metabolic demand and little capacity to compensate.
The folate and B12 trap
High folate intake can correct the anaemia of B12 deficiency while the neurological damage continues unchecked — the blood picture normalizes, but nerve damage progresses and can become irreversible. This is why B12 status matters independently, particularly on a vegan diet.
Water-soluble, with a caveat
Most B vitamins are water-soluble and excess is excreted, so toxicity is uncommon — but not impossible. High-dose B6 supplementation over long periods can cause peripheral neuropathy, and B12 is stored in the liver for years, which is why deficiency there develops slowly. See fat-soluble vitamins for the contrast.
Frequently asked questions
Do B vitamins give you energy?
Not directly. They contain no calories. They act as coenzymes that allow the pathways releasing energy from carbohydrate, fat, and protein to function, so deficiency causes fatigue but supplementation does not add energy when status is already adequate.
Which B vitamin becomes NAD+?
Niacin (vitamin B3) forms NAD+ and NADP+, the principal electron carriers used throughout glycolysis, the Krebs cycle, and beta-oxidation.
Why is taking folate without B12 a problem?
Folate can correct the megaloblastic anaemia caused by B12 deficiency while the neurological damage continues undetected. Masking the blood abnormality allows nerve damage to progress and potentially become irreversible.
Keep reading: Enzymes explained · B12 on a vegan diet · Micronutrients vs. macronutrients