Vitamin overview

If you know vitamins are micronutrients that are good for you but struggle to name more than 2 or 3, this vitamin overview is for you.

In this guide you will find the following information (when applicable or available) for all vitamins:

• Forms
• Solubility
• Sources
• Bioavailability (i.e. the extent at which the vitamin can be absorbed, utilised and stored) in percentages, along with enhancing and inhibiting factors

Vitamin A

Forms: preformed and provitamin A carotenoids (e.g. β-carotene, α-carotene and β-cryptoxanthin), which can be converted to vitamin A.

Solubility: fat-soluble.

Sources: preformed vitamin A is found in animal foods and carotenoids are found in oils, green leafy vegetables (e.g. spinach and kale), carrots, fruits (e.g. mango, papaya, orange).

The conversion factor of β-carotene to vitamin A depends on the source. Making 1 ug of vitamin A requires: 12 ug from fruits, 2.8-13.4 ug from tubers, 10-28 ug from cooked vegetables, 13-77 ug from raw vegetables.

Bioavailability: about 15% from dairy foods, 0-36% from fruits and vegetables. Absorption is decreased by the food matrix, and therefore increased by mechanical processing (cutting up food, cooking), also by dietary fat.

Thiamin (vitamin B1)

Solubility: water-soluble.

Sources: mostly in cereal foods.

Riboflavin (vitamin B2)

Solubility: water-soluble.

Sources: milk and milk products, fortified breads and cereals.

Bioavailability: high (about 95%) on average, about 67% from milk. Intestinal absorption is moderate and limited by intake; if intake is high, a limited amount is absorbed and the rest excreted.

Niacin (vitamin B3)

Sources: preformed niacin is present in beef, pork, wholegrain cereals, eggs and cow’s milk.

Bioavailability: low in mature grains (about 30%) but can be increased by treatment with alkali, more bioavailable in meats and most bioavailable in beans and liver. Tryptophan can be converted by niacin but this conversion is limited if iron, riboflavin or vitamin B6 levels are inadequate.

Pantothenic acid (vitamin B5)

Sources: chicken, beef, potatoes, oat-based cereals, tomato products, liver, kidney, egg yolks and whole grains.

Bioavailability: around 40 to 61%. Absorption is less efficient at higher intakes.

Vitamin B6

Sources: organ meats, muscle meats, breakfast cereals, vegetables and fruits.

Bioavailability: around 75% in a mixed diet.

Biotin (vitamin B7)

Forms: free and protein-bound.

Sources: liver is the best source, followed by far by meats and cereals.

Bioavailability: lower in cereals than other sources. Absorption is prevented by a protein found in raw egg whites.

Folate (vitamin B9)

Sources: cereals, cereal-based foods, green leafy vegetables, legumes, citrus fruit, fortified orange juice.

Bioavailability: about 50–60% from naturally occurring folate in food (although other sources estimate 60-98% from a diet high in vegetables and fruits), about 85% from foods fortified with folic acid, almost 100% from folic acid as a supplement when taken on an empty stomach.

Bioavailability is enhanced by cow’s milk intake and inhibited by the food matrix, dietary fibre and low pH.

Vitamin B12

Sources: mostly animal foods (e.g. red meats, milk and dairy products). The only sources are certain algae and plants exposed to bacterial action.

Bioavailability: about 65% from dairy foods, ≤50% from other animal foods, <5% from synthetic supplements.

Absorption increases with increasing intake and, in addition, is inversely proportional to the amount of B12 in the food, e.g. 11% from liver, 24–40% from eggs and trout, more than 60% from mutton and chicken. Better absorbed from fortified foods when added in low amounts.

Vitamin C

Sources: fruits (e.g. blackcurrants, guava, citrus and kiwifruit), vegetables (e.g. broccoli and sprouts).

Content varies depending on the food and, moreover, is affected by season, transport, shelf life, storage time, cooking practices and chlorination of water. For example, cutting, bruising and heating decrease the vitamin C content of the food. Vitamin C also leaches into water during cooking. Vitamin E might preserve vitamin C.

Bioavailability: 80-90% from fruits and synthetic vitamin C. It is decreased by the food matrix.

Absorption is 70–90% when intake is < 1 g/day, decreasing to <e; 50% with higher intake. Smoking and flavonoids might reduce absorption further.

Vitamin D

Forms: D3 (cholecalciferol) produced by sunlight on the skin, D3 (ergocalciferol) found in some foods. D2 is less biologically active.

Sources: fatty fish (e.g. salmon, herring, mackerel), eggs, mushrooms exposed to UV light, fortified foods (e.g. margarine and some dairy products).

Solubility: fat-soluble.

Absorption: enhanced by dietary fat.

Vitamin E

Forms: many; the naturally-occurring α-tocopherol is the most biologically active form.

Solubility: water-insoluble.

Sources: fats and oils, some vegetables, in the fats of meat, poultry and fish and, to lesser degrees, in cereals and dairy foods.

Absorption: low, enhanced by dietary fat.

Vitamin K

Forms: phylloquinone (vitamin K1) found in plants, menaquinones (vitamin K2) produced by bacterial fermentation.

Solubility: fat-soluble.

Sources: K1 is found in green leafy vegetables and herbs (e.g. kale, parsley, spinach, salad greens, green cabbage, broccoli and Brussel sprouts), some fruits (kiwifruit and avocado), certain plant oils (e.g. soybean and canola), margarines and salad dressings made from them.

K2 is found in fermented dairy product (e.g. yoghurt and cheese) and fermented vegetables (e.g. sauerkraut).

Bioavailability: <5% from dark leafy vegetables. Enhanced by dietary fat.

Absorption from plant sources including plant oils is < 20% than from supplements.

References

• National Health and Medical Research Council. Nutrient Reference Values for Australia and New Zealand [Internet]. Canberra: National Health and Medical Research Council; 2006. Available from: http://www.nhmrc.gov.au/_files_nhmrc/publications/attachments/n35.pdf
• Melse-Boonstra A. Bioavailability of Micronutrients From Nutrient-Dense Whole Foods: Zooming in on Dairy, Vegetables, and Fruits. Front Nutr. 2020;7:101.

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