There are eight B vitamins, each with distinct functions. They are often grouped together because they work synergistically in energy metabolism and cellular function, but they are not interchangeable — B12 deficiency produces neurological damage while B2 deficiency produces mouth sores; the symptoms and consequences are completely different. Here is what each B vitamin does, what deficiency looks like, and who is most at risk in the UK.
Why Are They Called B Vitamins?
Historically, vitamins were classified alphabetically as they were discovered. What was initially thought to be a single “vitamin B” was later found to be a family of chemically distinct compounds with different functions, all water-soluble. They retained the B designation with numbers. Some numbers are missing (B4, B8, B10, B11) because they were initially thought to be vitamins but were later reclassified.
What Each B Vitamin Does: The Complete Guide
Vitamin B1 (Thiamine)
Primary function: Cofactor for enzymes that convert carbohydrates into energy (pyruvate dehydrogenase complex). Essential for the nervous system — the brain and nerves are almost exclusively dependent on glucose, making B1 critical for neurological function. Severe deficiency causes beriberi (heart failure and neurological damage) and Wernicke’s encephalopathy. Moderate deficiency produces fatigue, irritability and poor concentration.
UK deficiency risk: Low in most adults, but elevated in heavy alcohol users (alcohol severely impairs B1 absorption and metabolism) and people with very high refined carbohydrate diets.
Food sources: Fortified cereals, pork, legumes, nuts, seeds. UK Reference Nutrient Intake: 0.8mg women, 1.0mg men.
Vitamin B2 (Riboflavin)
Primary function: Component of FAD and FMN coenzymes central to the electron transport chain (energy production). Also required for the activation of B6 and folate. Deficiency produces cheilosis (cracked corners of the mouth), glossitis (inflamed tongue) and photophobia.
UK deficiency risk: NDNS data shows UK women aged 20–59 have significantly lower riboflavin intake than men. Vegans avoiding dairy (milk is a major B2 source) are at elevated risk.
UK RNI: 1.1mg women, 1.3mg men.
Vitamin B3 (Niacin)
Primary function: Precursor to NAD+ and NADP+ — the most abundant coenzymes in the body, involved in over 400 enzymatic reactions including energy production, DNA repair and cell signalling. Severe deficiency causes pellagra (dermatitis, diarrhoea, dementia — the “3 Ds”). Niacin at therapeutic doses (1,000–3,000mg) significantly raises HDL cholesterol and lowers triglycerides — this is a pharmaceutical use, not a standard supplement dose.
EU authorised claim: Niacin contributes to normal psychological function and the reduction of tiredness and fatigue.
UK RNI: 13mg women, 17mg men. The body can synthesise small amounts from tryptophan, reducing the absolute dietary requirement.
Vitamin B5 (Pantothenic Acid)
Primary function: Component of coenzyme A (CoA), central to fatty acid metabolism, the Krebs cycle and synthesis of steroid hormones, haemoglobin and acetylcholine. Deficiency is rare because B5 is found in virtually all foods (“pantothenic” derives from the Greek for “from everywhere”).
UK deficiency risk: Very low. Isolated deficiency essentially does not occur in people eating any varied diet.
Vitamin B6 (Pyridoxine)
Primary function: Cofactor for over 140 enzymatic reactions, primarily involving amino acid metabolism. Required for the synthesis of serotonin, dopamine, GABA and norepinephrine (neurotransmitters). Also involved in haemoglobin synthesis and immune function.
EU authorised claims: B6 contributes to normal psychological function, normal nervous system function, the regulation of hormonal activity and the reduction of tiredness and fatigue.
UK safety alert: The MHRA issued guidance in 2023 warning that high-dose B6 supplements (above 10mg daily) taken long-term can cause peripheral neuropathy — painful nerve damage to the hands and feet. Many UK supplements historically contained 50–100mg B6. This is a genuine safety concern at higher doses.
UK RNI: 1.2mg women, 1.4mg men. Standard maintenance supplementation at 1–2mg is appropriate; avoid high-dose B6 supplements without medical reason.
Vitamin B7 (Biotin)
Primary function: Cofactor for carboxylase enzymes involved in fatty acid synthesis, gluconeogenesis and amino acid metabolism. Also involved in keratin production (hence the hair/nail connection). Deficiency is rare in adults eating varied diets. For detailed information including why biotin supplements are widely oversold for hair, see our dedicated biotin and hair guide.
UK RNI: 50mcg for adults.
Vitamin B9 (Folate/Folic Acid)
Primary function: DNA synthesis and methylation. Critical for rapidly dividing cells. Most important for neural tube development during early pregnancy. Works synergistically with B12 in homocysteine metabolism. For the full guide including the MTHFR variant affecting 40% of the population, see our folic acid vs methylfolate guide.
UK RNI: 200mcg adults, 400mcg for pregnancy.
Vitamin B12 (Cobalamin)
Primary function: Myelin sheath maintenance, DNA synthesis, red blood cell formation and neurotransmitter production. The most clinically consequential B vitamin deficiency — causes irreversible neurological damage if untreated. For the complete guide including methylcobalamin vs cyanocobalamin and who is at risk, see our B12 deficiency guide.
UK deficiency risk: High in vegans, vegetarians, adults over 50 and people on metformin or PPIs.
B Complex vs Individual B Vitamins: Which Should You Take?
For most people, a B complex supplement covering all eight B vitamins at maintenance doses (100% NRV) is more appropriate than individual B vitamins. The B vitamins work interdependently — B2 activates B6 and folate; B12 and folate work together in methylation; niacin and B2 are both required for the electron transport chain. Taking one in isolation without others can create relative deficiencies in the others.
Individual supplements at higher doses are appropriate for diagnosed deficiencies (e.g. B12 deficiency confirmed by blood test) or specific therapeutic applications under medical guidance.
Who Should Consider B Complex Supplementation?
- Vegans — B12, riboflavin and B6 are all lower in plant-based diets
- People under chronic stress — B vitamins are rapidly consumed in the stress response
- People with fatigue — B vitamins are involved in virtually every energy-producing pathway
- Anyone taking metformin or PPIs long-term — both impair B12 absorption
- Women of reproductive age — NDNS data shows lower B vitamin intake in UK women vs men
- Adults over 50 — absorption of B12 and B6 declines with age
ZenBlend by BioBodyBoost includes a full B-vitamin complex alongside 5 adaptogens — particularly relevant for stress-related B vitamin depletion. Daily Multi Complex provides all 13 essential vitamins including all 8 B vitamins at comprehensive doses. Both halal certified, vegan, UK GMP manufactured. Browse the full range.
