The 13 Essential Vitamins: Complete Guide to Functions, Sources, and Benefits

Vitamins are micronutrients — compounds your body needs in relatively small amounts but cannot function without. Unlike macronutrients (protein, carbohydrates, and fat), vitamins don’t provide calories. Instead, they act as essential cofactors and signalling molecules in thousands of biochemical reactions — enabling energy production, immune function, tissue repair, bone formation, and much more.

There are 13 essential vitamins. A deficiency in any one of them can affect your health, energy, recovery, and immune response — sometimes in subtle ways before obvious symptoms appear. Understanding what each vitamin does, where to find it, and how much you need is one of the most important foundations of complete nutrition — and so is knowing where more isn’t better.

For a broader overview of nutrition fundamentals, visit our Basic Nutrition page.

Water-Soluble vs. Fat-Soluble Vitamins

The 13 essential vitamins are divided into two categories based on how they’re absorbed, transported, and stored in the body:

Water-soluble vitamins dissolve in water, and most aren’t stored in large amounts — excess is generally excreted in urine, so they need to be replenished regularly through diet. There are 9 water-soluble vitamins: the 8 B vitamins and Vitamin C. Deficiencies can develop relatively quickly with inadequate intake. There are two important exceptions to the “no storage, no risk” rule: Vitamin B12 is stored in the liver in amounts that can last several years, and high-dose supplements of Vitamin B6 and niacin (B3) can cause real side effects.

Fat-soluble vitamins dissolve in fat, need dietary fat to be absorbed well, and are stored in the liver and fatty tissues. There are 4 fat-soluble vitamins: A, D, E, and K. Vitamins A and D are stored in large amounts, so deficiencies develop more slowly — but excessive supplementation over time can lead to toxicity. Vitamin K, by contrast, is stored only in small amounts and turns over quickly.

Water-Soluble Vitamins

The B Vitamin Complex

The B vitamins are a group of 8 chemically distinct vitamins that work together and individually to support energy metabolism, nervous system function, red blood cell production, and cellular health. Despite being grouped together, each B vitamin has its own unique functions and deficiency symptoms.

1. Vitamin B1 — Thiamine

Vitamin B1 (thiamine) is essential for converting carbohydrates into usable energy — its active form is the cofactor for the “gateway” step that lets glucose enter aerobic metabolism and produce ATP, the energy currency of every cell. Without adequate thiamine, energy production is impaired at the cellular level regardless of how much you eat. It’s also essential for normal nerve and heart function, and severe deficiency affects the brain, causing confusion and memory problems.

Key functions: Glucose to energy conversion, nerve and muscle function (including the heart), branched-chain amino acid breakdown, making the sugars needed for DNA and RNA.

Deficiency symptoms: Fatigue, irritability, poor concentration, tingling and nerve damage; in severe cases, beriberi (nerve and heart damage) or Wernicke-Korsakoff syndrome (most often associated with alcohol dependence).

Best food sources: Pork, sunflower seeds, nuts, legumes, whole grains, fortified cereals and bread, nutritional yeast.

Recommended daily intake: 1.1–1.2mg for adults. Needs rise with energy intake, so very active people eating more food need somewhat more.

2. Vitamin B2 — Riboflavin

Vitamin B2 (riboflavin) is central to energy production — it’s the building block of FAD and FMN, coenzymes that work in the electron transport chain (the final stage of cellular energy production) and in burning fat for fuel. Riboflavin also powers the recycling of glutathione, one of the body’s key antioxidants, and is needed to convert other B vitamins (B6 and folate) into their active forms. It supports red blood cell production and helps maintain healthy skin, eyes, and mucous membranes.

Key functions: Energy production (electron transport chain and fat burning), antioxidant recycling, activation of B6 and folate, red blood cell production, skin and eye health.

Deficiency symptoms: Cracked lips and mouth corners (angular cheilitis), inflamed tongue, sore throat, skin rashes, sensitivity to light, anaemia.

Best food sources: Liver and kidney, dairy products, eggs, lean meats, almonds, mushrooms, fortified cereals.

Recommended daily intake: 1.1–1.3mg for adults.

3. Vitamin B3 — Niacin

Vitamin B3 (niacin) exists in two main forms — nicotinic acid and nicotinamide — and is the building block of NAD and NADP, coenzymes involved in over 400 enzyme reactions throughout the body, more than any other vitamin-derived molecule. These cover energy production, DNA repair, and gene regulation, and NADPH supplies the power for making fatty acids, cholesterol, and steroid hormones. Niacin supports healthy skin and normal nervous and digestive system function, and the body can also make it from the amino acid tryptophan.

Key functions: NAD/NADP coenzyme function (energy metabolism), DNA repair, cholesterol and steroid hormone production (via NADPH), skin health, nervous and digestive system function.

Deficiency symptoms: Pellagra — known for the “4 Ds”: dermatitis (a rash on sun-exposed skin), diarrhoea, dementia, and, if untreated, death. Fatigue, depression, and headaches in milder deficiency.

Best food sources: Fish (especially anchovies and tuna), chicken and turkey breast, beef, salmon, peanuts, mushrooms, whole grains, fortified cereals.

Recommended daily intake: 14–16mg NE (niacin equivalents) for adults.

Note: Supplemental nicotinic acid above the 35mg upper limit can cause flushing — redness, warmth, and itching. The much higher doses once used for cholesterol (1,000mg or more) can cause liver damage and raise blood sugar, and should only be used under medical supervision. Niacinamide doesn’t cause flushing.

4. Vitamin B5 — Pantothenic Acid

Vitamin B5 (pantothenic acid) is named from the Greek “pantothen,” meaning “from everywhere” — reflecting how widely it’s found in foods. It’s essential for making coenzyme A (CoA), which is central to getting energy from carbohydrates, fats, and proteins. Through CoA, it’s also needed to make fatty acids, cholesterol and the steroid hormones made from it (including cortisol), the neurotransmitter acetylcholine, and heme, the oxygen-carrying part of haemoglobin.

Key functions: Coenzyme A synthesis, carbohydrate, fat, and protein metabolism, cholesterol and steroid hormone production, acetylcholine production, heme production.

Deficiency symptoms: Fatigue, headaches, irritability, sleep disturbances, numbness and burning or tingling in the feet and hands (very rare, because it’s found in so many foods).

Best food sources: Liver, mushrooms (especially shiitake), chicken, salmon, eggs, avocado, sweet potato, dairy, legumes.

Recommended daily intake: 5mg for adults.

5. Vitamin B6 — Pyridoxine

Vitamin B6 (pyridoxine) is involved in over 100 enzyme reactions in the body — mainly related to protein and amino acid metabolism. It’s essential for making haemoglobin, releasing stored glycogen for energy, and producing several important neurotransmitters, including serotonin, dopamine, and GABA (and, indirectly, noradrenaline). It plays key roles in brain development and function and in immune function. For athletes, B6 needs rise somewhat with protein intake — though high-protein foods such as meat, fish, and poultry are also rich in B6.

Key functions: Protein and amino acid metabolism, haemoglobin formation, glycogen release, neurotransmitter synthesis (serotonin, dopamine, GABA), brain development and function, immune function.

Deficiency symptoms: Anaemia, depression, confusion, weakened immune function, skin rashes, inflamed tongue.

Best food sources: Pistachios, sunflower seeds, chicken, turkey, beef, salmon, tuna, potatoes, bananas, chickpeas, fortified cereals.

Recommended daily intake: 1.3–1.7mg for adults.

Safety note: B6 is one of the few water-soluble vitamins that can cause harm in excess. Long-term high-dose supplements can cause nerve damage (tingling and numbness in the hands and feet) — cases have been reported at 50mg a day or less, and Europe’s upper limit is just 12mg a day. Check all your supplements for added B6, including magnesium products and energy drinks.

6. Vitamin B7 — Biotin

Vitamin B7 (biotin) — sometimes called Vitamin H — is essential for the metabolism of all three macronutrients: protein, carbohydrates, and fat. It’s the cofactor for five carboxylase enzymes that are central to making fatty acids, producing glucose from non-carbohydrate sources (gluconeogenesis), and breaking down amino acids. It’s best known for hair, skin, and nails, because deficiency causes hair loss, rashes, and brittle nails — but biotin supplements haven’t been shown to help hair or nails in people who aren’t deficient.

Key functions: Fatty acid synthesis, gluconeogenesis, amino acid breakdown, gene regulation, healthy hair, skin, and nails.

Deficiency symptoms: Hair thinning, brittle nails, scaly red rash around the eyes, nose, and mouth, fatigue, depression, neurological symptoms (very rare).

Best food sources: Liver, eggs (cooked), salmon, pork, beef, sunflower seeds, sweet potato, nuts.

Note: Raw egg whites contain avidin, a protein that binds biotin and prevents its absorption. Cooking eggs neutralises avidin — another reason to eat cooked rather than raw eggs.

Recommended daily intake: 30mcg for adults.

Safety note: High-dose biotin supplements (common in hair, skin, and nail products) can seriously distort many blood tests, including thyroid and heart attack tests. Stop high-dose biotin at least 48–72 hours before blood tests, and tell your doctor you take it.

7. Vitamin B9 — Folate

Vitamin B9 (folate) — called folic acid in its synthetic form, used in supplements and fortified foods — is essential for DNA synthesis, cell division, and the formation of red and white blood cells. It works closely with Vitamin B12 in the methylation cycle, which regulates genes and clears homocysteine. It’s particularly crucial in early pregnancy — low folate around conception significantly increases the risk of neural tube defects in the developing baby, which is why 400mcg of folic acid a day is recommended for anyone who could become pregnant, starting at least a month before conception. Low folate is also linked with depression.

Key functions: DNA synthesis and repair, cell division, red and white blood cell formation, methylation and homocysteine clearance, fetal neural tube development.

Deficiency symptoms: Megaloblastic anaemia (large, abnormal red blood cells), fatigue, weakness, sore tongue, neural tube defects in pregnancy, low mood.

Best food sources: Legumes (edamame, lentils, chickpeas), dark leafy greens (spinach, romaine), asparagus, broccoli, Brussels sprouts, avocado, liver, fortified grains.

Recommended daily intake: 400mcg DFE (dietary folate equivalents) for adults. 600mcg DFE during pregnancy.

Note: High-dose folic acid can hide a Vitamin B12 deficiency while its nerve damage progresses, so keep folic acid from supplements and fortified foods below 1,000mcg a day unless your doctor advises otherwise, and make sure your B12 intake is adequate.

8. Vitamin B12 — Cobalamin

Vitamin B12 (cobalamin) is the largest and most structurally complex vitamin — the only one containing a metal, cobalt — and one of the most important for long-term neurological health. It’s needed to maintain the myelin sheath that protects nerve fibres; without enough B12, nerve damage can develop gradually and may become permanent. It works with folate in making DNA, so deficiency causes megaloblastic anaemia, and it’s needed to recycle homocysteine and to break down certain fatty acids and amino acids for energy. B12 is found almost exclusively in animal products, so deficiency is common in vegans and vegetarians who don’t take a supplement or eat fortified foods.

Key functions: Nerve and myelin health, red blood cell production, DNA synthesis (with folate), homocysteine clearance, energy metabolism of certain fatty acids and amino acids.

Deficiency symptoms: Megaloblastic anaemia, fatigue, weakness, neurological damage (numbness, tingling, balance problems), memory issues, depression. Neurological damage from prolonged deficiency can be irreversible.

Best food sources: Clams and other shellfish, liver, oily fish (mackerel, sardines, salmon), meat, poultry, eggs, dairy. Plant foods don’t contain reliable B12 — fortified foods and supplements are the reliable plant-based sources.

Recommended daily intake: 2.4mcg for adults.

Particularly important for: Vegans and vegetarians, older adults (absorption from food falls with age), people taking metformin or acid-reducing medicines, and anyone with digestive conditions affecting absorption. A reliable B12 supplement or fortified foods are essential for anyone avoiding animal products.

9. Vitamin C — Ascorbic Acid

Vitamin C is the most famous vitamin and the body’s main water-soluble antioxidant, neutralising free radicals in the body’s watery environments and regenerating Vitamin E. It’s essential for collagen synthesis — the structural protein that gives skin its firmness, maintains cartilage and joints, supports blood vessel walls, and is critical for wound healing. It substantially boosts iron absorption from plant foods (non-haem iron), roughly doubling it when eaten with a meal — an important consideration for vegans and vegetarians. It also supports immune function, is needed to make L-carnitine for fat burning, and helps the body make the stress hormone noradrenaline.

Key functions: Collagen synthesis, antioxidant protection, immune support, iron absorption, carnitine and noradrenaline production, wound healing.

Deficiency symptoms: Scurvy — characterised by weakened connective tissue, bleeding gums, poor wound healing, fatigue, and joint pain. Milder deficiency can cause fatigue, easy bruising, and slow healing.

Best food sources: Guava, red bell peppers, kiwi, broccoli, Brussels sprouts, strawberries, citrus fruits (oranges, lemons, grapefruit), papaya.

Recommended daily intake: 75–90mg for adults. Smokers need an additional 35mg per day, as smoking depletes Vitamin C. Around 200mg a day from food — easily reached with five or more servings of fruit and vegetables — brings blood levels close to their natural ceiling; above about 400mg, most extra is simply excreted.

Fat-Soluble Vitamins

10. Vitamin A

Vitamin A plays essential roles in vision, immune function, growth, and reproduction. It exists in two main forms: preformed Vitamin A (retinol), found in animal foods, and provitamin A carotenoids (such as beta-carotene), found in plant foods, which the body converts to Vitamin A as needed. One form, retinal, is the light-sensing molecule in the eye that makes night vision possible; another, retinoic acid, acts like a hormone, switching hundreds of genes on and off to control cell growth and development. It’s critical for maintaining the mucous membranes — the body’s first line of defence against pathogens — and for normal bone development, new cell growth, and reproduction. Beta-carotene from plants also works as an antioxidant.

Key functions: Vision (particularly night vision), immune system maintenance, gene regulation, cell growth and development, bone development, reproduction.

Deficiency symptoms: Night blindness (often the first symptom), dry eyes, impaired immune function, dry skin, stunted growth in children. Worldwide, Vitamin A deficiency is the leading preventable cause of childhood blindness.

Best food sources:

• Preformed Vitamin A (retinol): Liver (exceptionally high), fish liver oils, butter, cheese, eggs
• Beta-carotene (converted to Vitamin A): Sweet potato, carrots, pumpkin, butternut squash, dark leafy greens, red bell pepper, mango

Recommended daily intake: 700–900mcg RAE (retinol activity equivalents) for adults.

Toxicity note: Preformed Vitamin A (retinol) can be toxic — the adult upper limit is 3,000mcg a day, and long-term high intakes have been linked with liver damage and weaker bones. High doses in early pregnancy can cause birth defects, so pregnant women should avoid high-dose retinol supplements (and, in UK guidance, liver). Beta-carotene from food doesn’t carry these risks.

11. Vitamin D

Vitamin D is unique among vitamins — it works more like a hormone, and your body can make it when skin is exposed to UVB rays from sunlight. Even so, low Vitamin D is common, especially in winter at northern latitudes and among people who spend most of their time indoors or have darker skin. It’s essential for calcium absorption and bone mineralisation — with too little Vitamin D, the body absorbs far less calcium, however much you eat. It also influences immune function and muscle function, and its receptors are found in almost every tissue in the body.

Key functions: Calcium absorption and bone health (prevents rickets and osteomalacia), immune system regulation, muscle function and strength, cell growth regulation.

Deficiency symptoms: Bone pain and weakness, muscle weakness and cramps, fatigue, impaired immune function, increased risk of fractures. In children: rickets. Low Vitamin D is also linked with low mood.

Best food sources: Oily fish (salmon, mackerel, sardines, herring), cod liver oil, egg yolks, fortified dairy and plant milks, fortified cereals, UV-exposed mushrooms. Food alone rarely provides enough.

Sunlight: Short spells of midday summer sun on uncovered skin — often around 10–30 minutes, several times a week, well short of burning — can produce good amounts of Vitamin D, but this varies significantly with latitude, season, skin tone, and age. In winter, at latitudes above about 35°, the sun is too weak to make meaningful Vitamin D. Avoid sunburn, which raises skin cancer risk.

Recommended daily intake: 600–800 IU (15–20mcg) for adults. When sun exposure is low — such as in autumn and winter at northern latitudes — a supplement makes sense: the UK advises 400 IU (10mcg) a day from October to March, and many people take 1,000–2,000 IU, which is safe and well below the 4,000 IU upper limit. Large trials found little extra benefit from higher doses in people who weren’t deficient.

Supplementation: Vitamin D3 (cholecalciferol) is the preferred supplemental form, being more effective than D2 at raising blood levels.

12. Vitamin E

Vitamin E is the body’s main fat-soluble antioxidant — it protects cell membranes from oxidative damage by stopping the chain reactions that damage their fats. This matters for red blood cells, immune cells, nerves, and for protecting LDL cholesterol from oxidation, a key step in artery disease (though Vitamin E supplements haven’t prevented heart disease in trials). Vitamin E comes in eight chemical forms; alpha-tocopherol is the form the body keeps and the only one counted toward daily requirements.

Key functions: Cell membrane antioxidant protection, immune function, nerve and muscle health, red blood cell protection, protection of LDL cholesterol from oxidation.

Deficiency symptoms: Nerve and muscle damage, poor coordination, weakened immune function, impaired vision, haemolytic anaemia. True deficiency is rare in healthy adults but occurs in people with fat absorption disorders and in premature babies.

Best food sources: Wheat germ oil (highest concentration), sunflower oil and seeds, almonds, hazelnuts, peanuts, spinach, broccoli, avocado, olive oil.

Recommended daily intake: 15mg of alpha-tocopherol for adults (about 22 IU of natural or 33 IU of synthetic Vitamin E).

Note: Long-term high-dose supplements (400 IU a day or more) have been linked with more haemorrhagic strokes and, in one large trial, more prostate cancer, and high doses can interfere with Vitamin K’s role in blood clotting. Food sources are preferable to high-dose supplements.

13. Vitamin K

Vitamin K is essential for blood clotting — it activates several clotting factors made in the liver, and without it, the blood’s ability to clot is severely compromised. Beyond clotting, it activates osteocalcin, a protein involved in bone mineralisation, and matrix Gla-protein (MGP), which helps keep calcium out of artery walls. Vitamin K exists in two main forms: K1 (phylloquinone), found mainly in leafy greens, and K2 (menaquinones), found in fermented foods and animal products. Higher K2 intake has been linked with less arterial calcification in observational studies, though trials haven’t yet confirmed a benefit from supplements.

Key functions: Blood clotting (activating clotting factors), bone mineralisation, limiting calcium deposits in blood vessels, wound healing.

Deficiency symptoms: Excessive bleeding and bruising, slow wound healing; long-term low intake is linked with weaker bones. True deficiency is rare in healthy adults, but newborns are at risk — which is why babies receive a vitamin K injection at birth — as are people with fat absorption disorders and those taking long courses of antibiotics while eating little vitamin K.

Best food sources:

• Vitamin K1: Dark leafy greens (kale, spinach, collards, Swiss chard), broccoli, Brussels sprouts, parsley
• Vitamin K2: Natto (fermented soy — exceptionally high), hard cheeses, egg yolks, chicken, liver

Recommended daily intake: 90–120mcg for adults.

Important note: Vitamin K directly opposes warfarin and similar anticoagulants. If you take one of these, keep your vitamin K intake consistent and don’t start vitamin K supplements without consulting your doctor. Newer anticoagulants (DOACs, such as apixaban and rivaroxaban) aren’t affected by vitamin K.

Getting Enough Vitamins from Your Diet

The most reliable way to meet your vitamin needs is through a varied, whole-food diet that includes:

Abundant vegetables and fruits — particularly dark leafy greens (folate, K, and beta-carotene for A), peppers, kiwi, citrus, and berries (C), and orange and yellow vegetables (beta-carotene for A).

Quality protein sources — meat, fish, poultry, eggs, and dairy are rich in B12, B2, B3, B6, and preformed Vitamin A, and oily fish is one of the few good food sources of Vitamin D. B12 is the one vitamin that’s essentially impossible to get from unfortified plant foods, so plant-based eaters need fortified foods or a supplement.

Legumes, whole grains, nuts, and seeds — major sources of folate, B1, B6, and Vitamin E.

Healthy fats — nuts, seeds, and plant oils provide Vitamin E, and eating some fat with meals helps you absorb the fat-soluble vitamins A, D, E, and K.

Fermented and organ foods — liver is one of the most nutrient-dense foods available, providing exceptional amounts of A, B2, B12, and folate (though its Vitamin A content is so high that it’s best eaten occasionally rather than daily, and avoided in pregnancy). Fermented foods such as hard cheese and natto provide Vitamin K2.

Vitamins for Athletes and Active People

If you train regularly, several vitamins deserve particular attention:

Vitamin D — low levels are linked with reduced muscle function, more stress fractures, and more respiratory infections, and are common in athletes who train indoors or through the winter. A supplement during the darker months is a sensible precaution; a blood test is worthwhile if you have symptoms, a history of stress fractures, or another medical reason.

B vitamins (especially B1, B2, B3, B6) — energy metabolism depends heavily on B vitamins, and needs rise roughly in line with how much you eat and burn. A varied diet that covers your extra calories usually covers the extra B vitamins too. Supplements haven’t been shown to improve performance in athletes who aren’t deficient.

Vitamin B12 — essential for vegan and vegetarian athletes, who need a reliable supplement or fortified foods to avoid anaemia and nerve problems.

Vitamin C — supports collagen synthesis for connective tissue and joints, and regular intake (around 250mg–1g a day) has roughly halved the risk of colds in people under heavy physical stress, such as marathon runners. But high-dose antioxidant supplements taken daily around training may blunt some training adaptations, so food sources are best for everyday needs.

Vitamin K2 — plays a role in bone health, and is often paired with Vitamin D supplements — a plausible approach for long-term bone and cardiovascular health, though its benefits haven’t yet been proven in trials.

Related Nutrition Pages

For more on nutrition fundamentals and how vitamins work alongside other nutrients:

Omega-3 — essential fatty acids and their benefits

Basic Nutrition — macronutrients, calories, and nutrition fundamentals

Minerals — the essential minerals your body needs

Amino Acids — the building blocks of protein