Nutrition basicsVitaminsDoses & safety
Vitamins are organic compounds needed in small amounts that the body mostly can't make itself. There are exactly 13. This page gives each one in clinical depth — including deficiency symptoms, therapeutic use and the toxic upper limit. Minerals have their own dedicated guide.
Read first — about the doses on this page. The therapeutic doses listed describe what is used in research and clinical practice. They are information, not a prescription. High-dose, "therapeutic" supplement use can be harmful and can interact with medication. Always involve a doctor or pharmacist before using any vitamin therapeutically, in pregnancy or breastfeeding, for a child, or alongside other medicines. The "upper limit" is the official Tolerable Upper Intake Level (UL) — the most you can take long-term without expected harm.
Two families. Fat-soluble (A, D, E, K) are stored in the body, so high doses build up — this is where most vitamin toxicity happens. Water-soluble (C and the B's) are mostly passed in urine, so they rarely reach toxic levels — vitamin B6 is the key exception.
Fat-soluble vitamins
Vitamin A — retinol & beta-carotene
Adult need ≈ 700–900 µg RE/day
What it does: vision in dim light, immune defence, and healthy growth of skin and the linings of the body.
Where you find it: retinol in liver, egg yolk, dairy and oily fish; beta-carotene (converted to vitamin A) in carrots, sweet potato, pumpkin and dark greens.
DeficiencyNight blindness, dry eyes (xerophthalmia), more frequent infections, dry skin.
Therapeutic useCorrecting deficiency; WHO high-dose vitamin A in children with measles/deficiency; prescription retinoids (topical/oral) for acne and skin — specialist-only.
Upper limit3,000 µg/day (preformed retinol). Excess: headache, liver damage, bone thinning, hair loss — and birth defects, so never high-dose retinol in pregnancy. Beta-carotene from food does not cause this — but beta-carotene supplements are a different matter: in two large trials they increased lung cancer in smokers and asbestos-exposed workers (ATBC +18%; CARET +28%, and +17% all-cause mortality, stopped early). If you smoke or have smoked, avoid beta-carotene supplements entirely.
Vitamin D — the "sunshine" vitamin
Adult need ≈ 10–20 µg (400–800 IU)/dayLow in winter
What it does: lets the gut absorb calcium and phosphorus — the basis of strong bone — and supports muscle and immune function.
Where you find it: summer sunlight on skin, oily fish, egg yolk and fortified foods (milk and plant drinks).
DeficiencyRickets in children; in adults soft bones (osteomalacia), bone pain and muscle weakness. Common in winter at our latitude.
Therapeutic useTreating deficiency (maintenance ≈ 20–50 µg/800–2000 IU; short higher "loading" doses only under medical care); with calcium for osteoporosis.
Upper limit100 µg/day (4,000 IU). Excess from supplements raises blood calcium → nausea, kidney stones, kidney damage. You cannot overdose from sunlight.
Vitamin E — tocopherols
Adult need ≈ 11–13 mg/day
What it does: the body's main fat-phase antioxidant, protecting cell membranes and blood fats from oxidative damage; supports immunity.
Where you find it: vegetable oils, nuts (almonds, hazelnuts), seeds, wheatgerm and avocado.
DeficiencyRare — mainly in fat-absorption disorders; can cause nerve and muscle problems and anaemia.
Therapeutic useLimited proven role beyond correcting genuine deficiency; routine high-dose supplements are not recommended.
Upper limit300 mg/day (EFSA) · 1,000 mg (US). Excess increases bleeding risk, especially with blood-thinning medication — and the harm signal goes further: in the SELECT trial (35,533 men) 400 IU/day raised prostate cancer risk by 17%, and meta-analyses link supplementation to a small but significant rise in all-cause mortality at doses below the upper limit. This is a vitamin to get from food.
Vitamin K — K1 & K2
Adult need ≈ 70–120 µg/day
What it does: essential for blood clotting and for binding calcium into bone.
Where you find it: K1 in dark leafy greens (kale, spinach, broccoli); K2 in fermented foods (natto), some cheeses and animal foods.
DeficiencyEasy bruising and bleeding. Newborns are born low and are given vitamin K at birth to prevent bleeding.
Therapeutic useReverses dangerous over-thinning from warfarin (medical use); routine newborn injection.
Upper limitNo formal UL — toxicity is very low. But vitamin K opposes warfarin: if you take it, keep intake steady and consult your doctor.
Vitamin C
Vitamin C — ascorbic acid
Adult need ≈ 75–110 mg/day
What it does: antioxidant; builds collagen for skin, blood vessels and gums; supports immunity; and boosts absorption of plant iron in the same meal.
Where you find it: citrus, berries, kiwi, peppers, broccoli, tomatoes, potatoes. Fragile — lost to heat and long storage.
DeficiencyScurvy — bleeding gums, slow wound healing, bruising, fatigue, loose teeth.
Therapeutic useTreats and prevents scurvy; modestly shortens (does not prevent) the common cold; taken with meals to raise iron absorption in iron-deficiency.
Upper limit2,000 mg/day. Excess: diarrhoea and stomach upset; higher kidney-stone risk in susceptible people.
The B-complex — all eight
Eight water-soluble vitamins that act as coenzymes — releasing energy from food and building DNA, blood cells and brain chemistry. Every one is below.
B1 — Thiamin
Adult need ≈ 1.1–1.2 mg/day
What it does: converts carbohydrate into energy; essential for nerve and heart-muscle function. Where: whole grains, pork, legumes, seeds, nuts.
DeficiencyBeriberi (nerve and heart problems); with heavy alcohol use, the serious brain disorder Wernicke–Korsakoff.
Therapeutic useHigh-dose (often intravenous) thiamine in alcohol-related deficiency and Wernicke's — an emergency hospital treatment.
Upper limitNone set — surplus is harmlessly excreted in urine.
B2 — Riboflavin
Adult need ≈ 1.1–1.6 mg/day
What it does: energy metabolism; helps activate B6 and folate; supports skin, eyes and the mouth lining. Where: dairy, eggs, lean meat, almonds, green vegetables.
DeficiencyCracked lips and mouth corners, sore red tongue, skin and eye irritation.
Therapeutic useHigh-dose (≈ 400 mg/day) has reasonable evidence for migraine prevention.
Upper limitNone set; surplus is excreted (it turns urine bright yellow — harmless).
B3 — Niacin
Adult need ≈ 14–16 mg NE/day
What it does: a workhorse of energy metabolism in every cell; supports skin, nerves and digestion. Where: meat, poultry, fish, peanuts, whole grains.
DeficiencyPellagra — the "three D's": dermatitis, diarrhoea, dementia.
Therapeutic usePrescription high-dose nicotinic acid (1–2 g) was once used for cholesterol — specialist-only and now largely superseded by safer drugs.
Upper limit35 mg/day (supplemental nicotinic acid). Excess: skin flushing and itching; liver strain at gram-level doses.
B5 — Pantothenic acid
Adult need ≈ 5 mg/day
What it does: forms coenzyme A, central to releasing energy from fat, carbs and protein, and to making hormones. Where: almost all foods — meat, eggs, whole grains, avocado, mushrooms, legumes.
DeficiencyVery rare because it is so widespread; can cause fatigue and tingling feet when it does occur.
Therapeutic useNo well-established therapeutic role.
Upper limitNone set; very low toxicity (large doses may loosen the stomach).
B6 — Pyridoxine
Adult need ≈ 1.3–1.7 mg/day
What it does: protein and amino-acid metabolism; makes neurotransmitters (serotonin, dopamine, GABA) and haemoglobin. Where: poultry, fish, potatoes, bananas, chickpeas.
DeficiencyAnaemia, dermatitis, cracked lips, low mood and irritability, weakened immunity.
Therapeutic useNausea/vomiting in pregnancy (≈ 10–25 mg, often with doxylamine — the upper half of that range exceeds the EFSA limit below, so it is a prescribed and supervised dose, not a self-treatment); given with the TB drug isoniazid to prevent nerve damage; sometimes used for PMS.
Upper limit12 mg/day (EFSA, revised 2023) · 100 mg (US). The one water-soluble vitamin that is genuinely toxic in excess: chronic high doses cause peripheral nerve damage (numbness, unsteady gait).
B7 — Biotin
Adult need ≈ 30–40 µg/day
What it does: a coenzyme in fat, carb and protein metabolism; popularly linked to hair, skin and nails. Where: egg yolk, nuts, seeds, salmon, sweet potato; gut bacteria make some.
DeficiencyRare — hair thinning, a scaly red rash around the eyes/nose, brittle nails.
Therapeutic useWidely sold for hair/skin/nails, but the evidence is weak unless you are actually deficient.
Upper limitNone set. Important: high-dose biotin can falsify common blood tests (thyroid, troponin) — tell your doctor if you take it.
B9 — Folate (folic acid)
Adult need ≈ 330–400 µg DFE/dayCritical pre-pregnancy
What it does: builds DNA and new cells (including red blood cells), so needs rise whenever the body grows. Where: dark leafy greens, legumes, beetroot, citrus, fortified grains.
DeficiencyMegaloblastic anaemia (large immature red cells), fatigue; in early pregnancy, raised risk of neural-tube birth defects.
Therapeutic use400 µg/day folic acid before and in early pregnancy to prevent NTDs; 4–5 mg (prescription) for higher-risk pregnancies; treats folate-deficiency anaemia.
Upper limit1,000 µg/day of folic acid (the synthetic form). High intake can mask a B12 deficiency, delaying its diagnosis.
B12 — Cobalamin
Adult need ≈ 2.4–4 µg/dayVegans must supplement
What it does: nerve function, red-blood-cell formation and DNA synthesis; works with folate. Where: essentially only animal foods — meat, fish, eggs, dairy — plus fortified foods and supplements.
DeficiencyMegaloblastic anaemia, fatigue, tingling/numbness, memory and mood changes — and, if prolonged, permanent nerve damage. Risk: vegans and many older adults.
Therapeutic useHigh-dose oral (≈ 1,000 µg) or injections for pernicious anaemia and deficiency; routine supplement for vegans.
Upper limitNone set — toxicity is very low even at high doses.
Vitamin-like nutrients: choline & relatives
These are related compounds, often grouped with the B vitamins. Choline is the key player — an officially essential nutrient — and lecithin is the natural phospholipid mixture that's a practical source of it. Inositol is a related compound once called "vitamin B8," and phosphatidylserine is a different phospholipid again — built on the amino acid serine rather than choline, and of interest for the brain.
Choline
Adult AI ≈ 400 mg/day (EFSA) · 425 ♀ / 550 ♂ (US)Essential nutrient
What it does: makes acetylcholine (the neurotransmitter for memory and muscle control), builds cell membranes via phosphatidylcholine, transports fat out of the liver, and is vital for fetal brain development.
Where you find it: egg yolk and liver are the richest, then meat, fish, soybeans, cruciferous vegetables (broccoli, Brussels sprouts) and peanuts.
DeficiencyFatty liver and liver/muscle damage. Uncommon on a varied diet, but intake is often below target in pregnancy, when needs rise.
Therapeutic useSupporting fetal brain development in pregnancy (egg-rich diet or supplement); studied in non-alcoholic fatty liver disease.
Upper limit3,500 mg/day (US; EFSA sets none). Excess: a fishy body odour, sweating, low blood pressure and GI upset.
Inositol (myo-inositol)
No official requirementOnce called "vitamin B8"
What it does: acts as a "second messenger" that helps insulin signalling and nerve/cell communication. It is not a true vitamin — your body makes it, so there is no recognised dietary deficiency.
Where you find it: fruit (especially citrus and cantaloupe), beans, grains and nuts; the body also synthesises its own.
DeficiencyNot a recognised dietary deficiency in healthy people — the body produces enough.
Therapeutic useBest evidence is in PCOS: myo-inositol ≈ 4 g/day (often combined with D-chiro-inositol in a 40:1 ratio) improved menstrual regularity, ovulation and insulin/androgen markers in trials informing the 2023 international PCOS guideline.5 Also studied for metabolic and mood symptoms.
Upper limitNo formal UL; generally well tolerated — mild nausea or loose stools at high doses. Note: high-dose D-chiro-inositol alone may worsen egg quality, so myo-inositol is the better-evidenced form.
Lecithin
Natural phospholipid mixture (food additive E322)
What it is: not a single substance but a mixture of phospholipids — mainly phosphatidylcholine — extracted from soy, sunflower or egg. It is widely used as a natural emulsifier in food (additive E322) and sold as a supplement, and it is a practical dietary source of choline.
Where you find it: naturally in egg yolk and soybeans; as an added emulsifier in chocolate, spreads, baked goods and many processed foods.
DeficiencyNot applicable — it is a food ingredient and choline source rather than a required nutrient itself.
Therapeutic useMarketed for cholesterol and liver health, but the clinical evidence is limited; its clearest value is as a choline/phosphatidylcholine source.
Upper limitGenerally recognised as safe; high supplement doses may cause GI upset. Soy lecithin contains very little soy protein, so it rarely troubles soy allergy.
Phosphatidylserine
Studied: 100–300 mg/dayBrain cell-membrane phospholipid
What it does: a phospholipid concentrated in brain cell membranes, where it supports cell signalling and the health of nerve cells. Note it is not a choline carrier — it is built on the amino acid serine, and its interest is cognitive rather than as a choline source.
Where you find it: small amounts in organ meats and fish; supplements are now made from soy or sunflower lecithin (older products used bovine brain, now avoided over prion-disease concerns).
DeficiencyNot a dietary deficiency — the body makes its own.
Therapeutic useStudied for age-related memory and cognitive decline at 100–300 mg/day (effects take ~4–6 weeks); some study in ADHD and exercise/stress (cortisol). Honest evidence note: the US FDA allows only a deliberately qualified (weak) cognitive claim, and EFSA reviewed the memory claims and did not accept them — so treat it as promising but not proven.6
Upper limitNo formal UL; generally well tolerated (mild GI upset or insomnia at higher doses). Soy/sunflower-derived PS is recognised as safe. Caution if you take blood thinners.
The honest bottom line: a varied diet supplies almost all of these vitamins — and plenty of choline if you eat eggs, fish or soy. For most people at northern latitudes, the genuinely useful supplements are vitamin D in winter, plus folic acid before pregnancy and B12 for vegans; choline matters most in pregnancy, and inositol has a real niche in PCOS. Therapeutic high doses are a medical decision, not a shopping decision.
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Bodil Isaksson, Licensed Biomedical Scientist (Sweden)
At Food Health Lab we explain what the science says in plain language, and we cite our sources so you can check for yourself.
References
Reference intakes and upper limits are general adult values from official bodies, rounded for clarity; exact values vary by age, sex and life stage. "Upper limit" = Tolerable Upper Intake Level (UL).
- EFSA. Overview on Tolerable Upper Intake Levels — consolidated UL summary for vitamins and minerals (2024). efsa.europa.eu (UL summary PDF) · DRV topic
- US NIH, Office of Dietary Supplements. Individual vitamin fact sheets — functions, food sources, recommended intakes, upper limits, deficiency and therapeutic uses. ods.od.nih.gov/factsheets
- Nordic Council of Ministers. Nordic Nutrition Recommendations 2023 — reference values and Nordic-specific guidance. pub.norden.org
- EFSA & US NIH ODS. Choline — Dietary Reference Values (EFSA AI 400 mg/day) and Health Professional fact sheet (functions, sources, UL 3.5 g/day). EFSA 2016 · NIH ODS Choline
- Greff D, et al. Inositol for PCOS: a systematic review and meta-analysis to inform the 2023 update of the International Evidence-based PCOS Guidelines. J Clin Endocrinol Metab, 2024. JCEM 109(6):1630
- EFSA Panel (NDA). Scientific Opinion on the substantiation of health claims related to phosphatidylserine (memory/cognition) — claims not substantiated. EFSA Journal, 2010. EFSA J 2010;8(10):1749
This article summarizes nutrition science from official health authorities for education. It is not a substitute for individual medical advice, and the doses given are not prescriptions — always consult a professional before therapeutic supplement use, in pregnancy, or alongside medication.