Can Our Bodies Make Our Own Vitamins?

Vitamins are substances needed by everybody in small amounts to stay healthy. The human body can make its own vitamin D in the skin when it is exposed to sunlight, and friendly bacteria in the gut provide small amounts of vitamin K. Animals can make their own vitamin C, though unfortunately humans cannot. People need to get all the rest of the required vitamins from eating a healthy, balanced and varied diet.

Vitamin D

The only vitamin that the human body can make on its own is vitamin D. The action of the sun on the skin turns a compound called 7-dehydrocholesterol into a form of vitamin D called vitamin D3 (cholecalciferol). According to the Mayo Clinic in the USA, about 15 minutes of sun exposure on the skin a couple of times a week is enough to create the required levels of vitamin D. Vitamin D helps the absorption of calcium and maintains healthy bones and teeth (see 'Vitamin D').

People who do not get enough sun exposure, for example people who are bedridden, who have to keep entirely covered up outside for cultural or religious reasons, or who have dark skins but live in northern climates (especially during the winter) can find their levels of vitamin D dropping too low.

According to the World Health Organisation, adults need an average of 5-10 mcg of vitamin D daily, though the body can store this for a while, as vitamin D is a fat-soluble vitamin. Good food sources of vitamin D include oily fish, eggs and dairy.

Vitamin K

While the human body cannot make vitamin K on its own, the friendly bacteria in the gut can make vitamin K2 (menaquinone or menatetrenone). People on high doses of antibiotics can become deficient in vitamin K, because the drugs destroy the friendly gut bacteria. Doctors give newborn babies injections of vitamin K, as their guts have not yet been colonised with the right friendly bacteria.

Vitamin K is important to help blood clotting (see ‘Other Vitamins’), and may also be needed to keep bones strong and healthy. According to the World Health Organisation, adults need an average of 55-65 mcg of vitamin K daily, though the body can store this for a while, as vitamin K is a fat-soluble vitamin. Good food sources of vitamin K include vegetable oils, green leafy vegetables and wholegrains.

Vitamin B3 (niacin)

Many animals, including humans, can convert the amino acid tryptophan (found in foods rich in protein) to vitamin B3. This takes place in the liver, which makes 1 mg of vitamin B3 from 60 mg of tryptophan. Vitamin B3 helps release energy from food (see 'The Importance of B Vitamins'). According to the World Health Organisation, adults need 1.3-1.7 mg vitamin B3 a day, and it is found in meat, wholegrains, legumes and peanuts.

Vitamin C

Vitamin C acts as an antioxidant, helps keep cells healthy, and helps people absorb iron from food (see ‘Getting the Right Amount of Vitamin C’).

Most animals can make their own vitamin C, except apes (which includes humans), monkeys, tarsiers, bats, guinea pigs, some birds and some fish. According to the World Health Organisation, adults need 45 mg of vitamin C daily, and they must get this from food every day, as vitamin C is a water-soluble vitamin, so is not stored in the body. Eating five portions of a variety of fruit and vegetables a day should supply the required amount of vitamin C.