Can Our Bodies Make Our Own Vitamins?

Which vitamins can the body make?

Vitamins are substances your body needs in small amounts to work properly. We get most of them from food, but the body can make some vitamins or produce them from substances in food.

The skin makes vitamin D when exposed to sunlight. The body can also convert some nutrients into vitamins. It turns beta carotene, found in carrots and leafy greens, into vitamin A, and it can make some niacin (vitamin B3) from tryptophan, an amino acid in protein. Both of these still depend on what we eat. Bacteria living in the gut also produce some vitamin K, but that is the work of the bacteria rather than our own cells.

None of these processes removes the need to get most vitamins from food.

Vitamin D and sunlight

Unlike most vitamins, vitamin D can be made in the skin. UVB radiation from sunlight starts a process that turns a compound in the skin related to cholesterol into vitamin D3, also called cholecalciferol. The liver and kidneys then convert this into the active form the body uses.

Vitamin D helps the body absorb calcium and phosphate, which keep bones, teeth and muscles healthy. A lack of it can lead to rickets in children and bone pain and weakness in adults.

In the UK, most people can make some vitamin D from sunlight from about late March or early April to the end of September. How much they make depends on the weather, time of day, skin colour and how much skin is exposed. During autumn and winter, sunlight is generally not strong enough for adequate production. Oily fish, red meat, egg yolks and some fortified breakfast cereals and spreads contain vitamin D, but diet alone may not provide enough.

The NHS vitamin D guidance recommends:

• adults and children aged 4 and over, including during pregnancy and breastfeeding, should consider taking 10 micrograms a day in autumn and winter
• children aged 1 to 4 should have 10 micrograms a day all year round
• breastfed babies under 1 should have 8.5 to 10 micrograms a day all year round
• babies having more than 500ml of infant formula a day do not need a supplement, because formula is already fortified

People who are rarely outdoors, such as those who are housebound or living in a care home, and people who usually cover most of their skin outside should take 10 micrograms a day all year round. People with dark skin, for example those of African, African Caribbean or South Asian origin, should consider doing the same, as their skin may make less vitamin D from the same amount of sunlight.

You do not need to burn to make vitamin D, and too much sun raises the risk of skin cancer, so protect your skin from sunburn.

Your situation may be slightly different. ask a question below ↓ and our editorial team will reply with our advice.

Vitamin K and gut bacteria

Vitamin K is needed for blood clotting and plays a part in bone health. Some gut bacteria produce menaquinones, a group of compounds collectively known as vitamin K2. How much these contribute to our vitamin K needs remains uncertain. Most adults get enough from green leafy vegetables, vegetable oils and cereal grains.

Newborn babies have very little vitamin K. All newborns are offered it to help prevent vitamin K deficiency bleeding, usually as a single injection after birth. An oral course is an alternative, but requires further doses.

If you take warfarin, ask your GP or pharmacist before taking vitamin K supplements. You do not need to avoid green vegetables, but try to keep the amount you eat fairly consistent.

Gut bacteria may also make small amounts of some B vitamins. Their contribution to our intake is uncertain and varies between vitamins.

Why humans cannot make vitamin C

Most animals make their own vitamin C in the liver or kidneys. Humans cannot, because we lack a working version of one of the enzymes needed. Humans, other apes, monkeys and guinea pigs are among the animals that cannot make it.

Good sources include citrus fruits, berries, peppers, broccoli, Brussels sprouts and potatoes. The body has only limited stores of vitamin C, so you need a regular dietary supply. Eating a range of fruit and vegetables gives most people enough.

Most people do not need a multivitamin. A balanced diet generally supplies what the body needs, and vitamin D is the main exception in the UK. If you are worried about your vitamin levels, speak to your GP or a pharmacist, or see the British Dietetic Association food fact sheets.