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

vitamins

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

There are 13 essential vitamins. A deficiency in even one of them — even a mild, subclinical deficiency — can impair your performance, recovery, immune response, and long-term health in ways that are often subtle but significant. Understanding what each of the vitamins does, where to find it, and how much you need is one of the most important foundations of complete nutrition.

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 are not stored in significant quantities by the body. Excess amounts are excreted in urine, which means they need to be replenished regularly through diet. There are 9 water-soluble vitamins: the 8 B vitamins and Vitamin C. While the lack of long-term storage reduces toxicity risk, it also means deficiencies can develop relatively quickly with inadequate intake.

Fat-soluble vitamins dissolve in fat and are stored in the liver and fatty tissues. Because they can be stored, deficiencies develop more slowly — but excessive supplementation over time can lead to toxicity since the body doesn’t excrete them as readily. There are 4 fat-soluble vitamins: A, D, E, and K.

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 — it’s a critical cofactor in the process of turning glucose into ATP, the energy currency of every cell in your body. Without adequate thiamine, energy production is impaired at the cellular level regardless of how much you eat. It also supports nervous system function, assists in blood formation, promotes hydrochloric acid production for digestion, and has a positive effect on cognitive function and learning capacity.

Key functions: Glucose to energy conversion, nervous system function, hydrochloric acid production, cognitive performance, blood formation.

Deficiency symptoms: Fatigue, irritability, poor concentration, nerve damage, in severe cases beriberi (neurological and cardiovascular damage) or Wernicke-Korsakoff syndrome (associated with alcohol dependence).

Best food sources: Whole grains, brown rice, pork, legumes, nuts and seeds, fortified cereals, nutritional yeast.

Recommended daily intake: 1.1–1.2mg for adults. Higher needs with intense physical activity or high carbohydrate intake.

2. Vitamin B2 — Riboflavin

Vitamin B2 (riboflavin) is central to energy production — it’s a key component of FAD and FMN, coenzymes that participate in the electron transport chain (the final stage of cellular energy production). Beyond energy metabolism, riboflavin is essential for red blood cell formation, antibody production, and cell respiration. It also plays an important role in maintaining healthy skin, eyes, and mucous membranes and is involved in converting other B vitamins (B6 and folate) into their active forms.

Key functions: Energy production (electron transport chain), red blood cell formation, antibody production, skin and vision health, activation of B6 and folate.

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

Best food sources: Beef liver, lean meats, fish, dairy products, eggs, dark leafy greens, almonds, 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 plays a fundamental role in energy metabolism, being a component of NAD and NADP, coenzymes involved in over 400 enzymatic reactions throughout the body. It supports a healthy nervous and digestive system, maintains skin health, assists in the production of hydrochloric acid, contributes to bile secretion, and is involved in the production of sex hormones and other important steroid hormones. It also has a well-documented memory-enhancing effect.

Key functions: NAD/NADP coenzyme function (energy metabolism), nervous system and digestive health, skin health, hydrochloric acid and bile production, sex hormone synthesis, cognitive function.

Deficiency symptoms: Pellagra — characterised by the “three Ds”: dermatitis (skin rash), diarrhoea, and dementia. Fatigue, depression, and headaches in milder deficiency.

Best food sources: Chicken breast, tuna, beef, salmon, peanuts, brown rice, avocado, mushrooms, fortified cereals.

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

Note: High-dose niacin supplementation (1000mg+) can cause flushing, liver stress, and other side effects. Stay within recommended ranges unless under medical supervision.

4. Vitamin B5 — Pantothenic Acid

Vitamin B5 (pantothenic acid) is named from the Greek “pantothen” meaning “from everywhere” — reflecting how widely it’s distributed in foods. It’s essential for the synthesis of coenzyme A (CoA), which is central to the metabolism of carbohydrates, fats, and proteins for energy. It’s also required for the production of red blood cells, adrenal hormones (including cortisol), and neurotransmitters. Pantothenic acid helps combat fatigue, depression, and anxiety, and plays a role in preventing certain forms of anaemia.

Key functions: Coenzyme A synthesis, carbohydrate, fat, and protein metabolism, red blood cell production, adrenal and steroid hormone synthesis, neurotransmitter production.

Deficiency symptoms: Fatigue, headaches, irritability, sleep disturbances, numbness and tingling in extremities (relatively rare due to wide food distribution).

Best food sources: Chicken liver, beef liver, sunflower seeds, avocado, sweet potato, mushrooms, salmon, eggs, dairy, legumes.

Recommended daily intake: 5mg for adults.

5. Vitamin B6 — Pyridoxine

Vitamin B6 (pyridoxine) is involved in over 100 enzymatic reactions in the body — primarily related to protein and amino acid metabolism. It’s essential for the metabolism of carbohydrates and proteins, the formation of haemoglobin, and the synthesis of several important neurotransmitters including serotonin, dopamine, GABA, and norepinephrine. It plays a key role in brain development and function, immune system regulation, and steroid hormone activity. For athletes, B6 is particularly important because protein metabolism demands — and therefore B6 requirements — increase with higher protein intake.

Key functions: Protein and amino acid metabolism, haemoglobin formation, neurotransmitter synthesis (serotonin, dopamine, GABA), brain development and function, immune regulation, steroid hormone activity.

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

Best food sources: Chicken, turkey, beef, salmon, tuna, potatoes, bananas, avocado, spinach, chickpeas, fortified cereals.

Recommended daily intake: 1.3–1.7mg for adults. Higher needs with high protein intake.

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 acts as a coenzyme for carboxylase enzymes that are central to fatty acid synthesis and gluconeogenesis (glucose production from non-carbohydrate sources). Biotin is also necessary for cell growth, bone marrow function, and nerve tissue health. It’s perhaps best known for its role in healthy hair, skin, and nails — and is one of the most popular beauty supplements as a result.

Key functions: Fatty acid synthesis, gluconeogenesis, protein metabolism, cell growth, bone marrow function, healthy hair and skin and nails.

Deficiency symptoms: Hair thinning, brittle nails, skin rashes, fatigue, depression, neurological symptoms (relatively rare).

Best food sources: Beef liver, eggs (particularly cooked egg yolk), salmon, dairy, nuts and seeds, sweet potato, avocado.

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.

7. Vitamin B9 — Folate

Vitamin B9 (folate) — also known as folic acid in its synthetic supplemental form — is essential for DNA synthesis, cell division, and the formation of red and white blood cells. It plays a critical role in protein metabolism and is essential for healthy cell division throughout the body. It’s particularly crucial during pregnancy — folate deficiency in early pregnancy significantly increases the risk of neural tube defects in the developing baby, which is why supplementation is universally recommended for women planning to conceive or in the first trimester. It also plays a role in mental health, with deficiency linked to depression and anxiety.

Key functions: DNA synthesis and repair, cell division, red and white blood cell formation, protein metabolism, fetal neural tube development, mental health support.

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

Best food sources: Dark leafy greens (spinach, kale, romaine), asparagus, Brussels sprouts, avocado, legumes, eggs, beef liver, fortified grains.

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

8. Vitamin B12 — Cobalamin

Vitamin B12 (cobalamin) is perhaps the most complex of all vitamins structurally, and one of the most important for long-term neurological health. It’s essential for the synthesis of myelin — the protective sheath surrounding nerve fibres — without which nerve signal transmission deteriorates progressively. It’s also required for protein synthesis, carbohydrate and fat metabolism, red blood cell production, iron utilisation (making it critical for preventing anaemia), and the production of acetylcholine, a neurotransmitter important for memory and learning. B12 is found almost exclusively in animal products — making deficiency extremely common in vegans and vegetarians who don’t supplement.

Key functions: Myelin sheath synthesis (neurological health), protein synthesis, carbohydrate and fat metabolism, red blood cell production, iron utilisation, acetylcholine production (memory and learning).

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: Beef liver, clams, fish, meat, poultry, eggs, dairy. Almost no plant sources contain meaningful B12.

Recommended daily intake: 2.4mcg for adults.

Particularly important for: Vegans, vegetarians, older adults (absorption decreases with age), anyone with digestive conditions affecting B12 absorption. Supplementation is strongly recommended for anyone avoiding animal products.

9. Vitamin C — Ascorbic Acid

Vitamin C is the most famous vitamin and one of the most biologically active — involved in over 300 biochemical processes in the body. It’s the primary water-soluble antioxidant, neutralising free radicals throughout the body’s aqueous environments. It’s essential for collagen synthesis — the structural protein that gives skin its firmness, maintains cartilage and joint health, supports blood vessel walls, and is critical for wound healing. It dramatically enhances iron absorption from plant sources (non-haem iron) — an important consideration for vegans and vegetarians. It supports immune function, contributes to adrenal hormone production, and is important for brain health through its role in neurotransmitter synthesis.

Key functions: Collagen synthesis, antioxidant protection, immune support, iron absorption enhancement, adrenal hormone production, neurotransmitter synthesis, wound healing.

Deficiency symptoms: Scurvy — characterised by weakened connective tissue, bleeding gums, poor wound healing, fatigue, and joint pain. Subclinical deficiency causes impaired immunity and poor recovery.

Best food sources: Red bell peppers (highest concentration), kiwi, citrus fruits (oranges, lemons, grapefruit), strawberries, broccoli, Brussels sprouts, papaya, guava.

Recommended daily intake: 75–90mg for adults. Smokers need an additional 35mg per day as smoking depletes Vitamin C. Many researchers suggest higher intakes (200–500mg) are optimal for health.

Fat-Soluble Vitamins

10. Vitamin A

Vitamin A is one of the most important fat-soluble vitamins, acting primarily as an antioxidant and playing 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 (beta-carotene) found in plant foods which the body converts to Vitamin A as needed. It’s critical for maintaining the integrity of mucous membranes — the first line of defence against pathogens. It’s involved in bone and tooth formation, fat storage regulation, and is essential for protein utilisation — the body cannot effectively use protein without adequate Vitamin A. It’s also required for new cell growth and healthy reproductive function.

Key functions: Vision (particularly night vision and colour vision), immune system maintenance, antioxidant protection, bone and tooth formation, fat storage regulation, protein utilisation, cell growth and reproduction.

Deficiency symptoms: Night blindness (often the first symptom), dry eyes, impaired immune function, dry skin, stunted growth in children.

Best food sources:

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

Toxicity note: Unlike most vitamins, preformed Vitamin A (retinol) can be toxic at high doses — excessive supplementation over time can cause liver damage, bone issues, and other serious problems. Stay within recommended ranges and be cautious with high-dose retinol supplements.

11. Vitamin D

Vitamin D is unique among vitamins — technically it functions more like a hormone than a vitamin, because your body can synthesise it when skin is exposed to UVB radiation from sunlight. Despite this, the vast majority of people in northern latitudes, office workers, and those who spend most of their time indoors are chronically deficient. Vitamin D deficiency is one of the most widespread nutritional deficiencies globally. It’s essential for calcium absorption and bone mineralisation — without adequate Vitamin D, calcium cannot be properly absorbed regardless of intake. It also regulates immune function, influences muscle function and strength, plays a role in mood regulation and mental health, and has receptors in virtually every tissue in the body suggesting widespread systemic importance.

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

Deficiency symptoms: Bone pain and weakness, muscle weakness and cramps, fatigue, depression, impaired immune function, increased risk of fractures. In children: rickets.

Best food sources: Fatty fish (salmon, mackerel, sardines, herring), beef liver, egg yolks, fortified dairy and plant milks, fortified cereals. Dietary sources alone are rarely sufficient.

Sunlight: 10–30 minutes of midday sun exposure on uncovered skin several times per week can produce adequate Vitamin D — but this varies significantly with latitude, season, skin tone, and age.

Recommended daily intake: 600–800 IU (15–20mcg) for adults. Many researchers and health professionals recommend higher intakes — 1000–2000 IU daily — particularly for those with limited sun exposure.

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 primary fat-soluble antioxidant — it protects cell membranes from oxidative damage by neutralising free radicals in fatty environments throughout the body. This is particularly important for red blood cell integrity, immune cell function, and the prevention of LDL cholesterol oxidation (oxidised LDL is a key driver of cardiovascular disease). It also supports skin health, reduces inflammation, and plays a role in immune function. Vitamin E exists in eight chemical forms, of which alpha-tocopherol is the most biologically active and the form used in the body.

Key functions: Cell membrane antioxidant protection, immune function, LDL cholesterol oxidation prevention, skin health, red blood cell integrity, anti-inflammatory effects.

Deficiency symptoms: Nerve and muscle damage, weakened immune function, impaired vision, haemolytic anaemia. True deficiency is relatively rare in healthy adults eating a balanced diet but common in people with fat absorption disorders.

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

Recommended daily intake: 15mg (22.4 IU) of alpha-tocopherol for adults.

Note: High-dose Vitamin E supplementation (above 1000mg) may increase bleeding risk by interfering with Vitamin K function. Whole food sources are preferable to high-dose supplements.

13. Vitamin K

Vitamin K is essential for blood coagulation — it’s required for the synthesis of several clotting factors in the liver, and without it the blood’s ability to clot is severely compromised. Beyond clotting, it plays a critical role in bone metabolism by activating osteocalcin, a protein that helps bind calcium into bone tissue. Research increasingly supports Vitamin K’s role in cardiovascular health — it activates matrix Gla protein (MGP), which prevents calcium from being deposited in artery walls. Vitamin K exists in two main forms: K1 (phylloquinone) found primarily in leafy greens and K2 (menaquinone) found in fermented foods and animal products. K2 appears to be particularly important for directing calcium into bones and away from arteries.

Key functions: Blood clotting (synthesis of clotting factors), bone mineralisation and strength, cardiovascular health (prevention of arterial calcification), wound healing.

Deficiency symptoms: Excessive bleeding and bruising, slow wound healing, weakened bones and increased fracture risk. True deficiency is rare in healthy adults but more common in people taking blood thinners, with fat absorption disorders, or with dysbiotic gut microbiomes (intestinal bacteria produce K2).

Best food sources:

Recommended daily intake: 90–120mcg for adults.

Important note: Vitamin K interferes with anticoagulant medications like warfarin. If you’re taking blood thinners, do not significantly change your Vitamin K intake without consulting your doctor.

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 (A, C, K, folate), citrus and berries (C), and orange and yellow vegetables (beta-carotene/A).

Quality protein sources — meat, fish, poultry, and eggs provide B12, B2, B3, B6, B7, A, and D that are difficult or impossible to obtain in adequate quantities from plant foods alone.

Healthy fats — nuts, seeds, and oils provide E, and fat itself is required for absorption of fat-soluble vitamins A, D, E, and K.

Fermented and organ foods — liver is one of the most nutrient-dense foods available, providing exceptional concentrations of A, B2, B12, folate, and many other vitamins.

Vitamins for Athletes and Active People

If you train regularly, several vitamins deserve particular attention:

Vitamin D — low levels are associated with reduced muscle function, impaired recovery, increased injury risk, and suppressed immune function. Regular testing and supplementation during winter months is a smart investment.

B vitamins (especially B1, B2, B3, B6) — energy metabolism depends heavily on B vitamins. High training volumes increase demands for all B vitamins, particularly those involved in energy production.

Vitamin C — supports collagen synthesis for connective tissue health and joint integrity, and aids recovery through its antioxidant properties. Exercise increases free radical production, increasing the demand for antioxidants.

Vitamin K2 — supports bone density and cardiovascular health — both important for long-term athletic performance and longevity.

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

Omega-3 — essential fatty acids and their benefits supports cardiovascular health. It helps with blood clotting (essential for synthesis of clotting factors in the liver)…

Basic Nutrition — macronutrients, calories, and nutrition fundamentals

Minerals — the essential minerals your body needs

Amino Acids — the building blocks of protein