Fat-soluble vitamins Fat-soluble vitamins include the vitamins A, D, E and K. They're essential for general good health, the daily repair of body cells and efficient functioning of body organs. As the name suggests, they're carried into the body by fat.

In this article

Vitamin A Vitamin D

Vitamin E Vitamin K

If your diet contains more of these vitamins than required at any one time, they can be stored in your fat and liver cells. However, because these vitamins can be stored, excessive intakes can accumulate and may be harmful. A recent report which analysed 67 research studies, found that synthetic vitamin A and E supplements can actually increase risk of premature death. A balanced diet will provide most people with enough of these vitamins and it's not necessary to take specific supplements unless advised by your GP.

A deficiency of fat-soluble vitamins may occur in people with poor diets or those suffering from long-term conditions that affect their ability to absorb fats from the intestine, such as cystic fibrosis or Crohn's disease. These people may be recommended to take a vitamin supplement by their doctor or dietitian.

Role

Vitamin A has an essential role in vision (especially night vision), normal bone growth, reproduction and the health of skin and mucous membranes. It also acts as an antioxidant, protecting the body from harmful free radicals - this may help to reduce the risk of certain forms of cancer.

Sources

There are two sources of dietary vitamin A. The first, or active form (retinol), is immediately available to the body and can be obtained from animal products such as milk, egg yolk and liver. The second, or precursor form (beta-carotene), can be obtained from fruit and vegetables, and is converted to the active form in the body.

Deficiency

Vitamin A deficiency is rare in the UK, but in developing countries it can be a problem. Pregnant women and infants are most at risk. The deficiency generally affects the skin and functioning of the mucous membranes throughout the body. This can lead to xerophthalmia, a disorder of the eye in which there is hardening of the cornea. Other results of deficiency may include growth problems in children, poor wound healing and follicular hyperkeratosis (when the skin becomes dry and bumpy).

Toxicity

Vitamin A is potentially harmful in large amounts during pregnancy. There's a relationship between the incidence of birth defects in infants and high vitamin A during pregnancy. If you're pregnant or thinking about becoming pregnant, it's advisable not to eat liver or liver products rich in vitamin A, and not to take vitamin A supplements unless specifically advised to do so by your GP.

For adult men and women the dietary recommended values (DRV) are respectively 700 and 600 micrograms of retinol per day. Regular high daily intakes (more than 1500 micrograms per day) are toxic, and in some cases can result in permanent damage to liver, bone, and vision.

Role

Vitamin D is important for the absorption and use of calcium and phosphorus by the body. It's essential for the formation and health of bones, teeth and cartilage.

Sources

There are two forms - D2 is found in some foods and D3 is produced within the body when the skin is exposed to sunlight.

Dietary vitamin D2 is found naturally in egg yolk, mackerel, cod and halibut liver oils, salmon and sardines. Vitamin D is also added to some foods. In the UK, margarine has to be fortified with vitamin D by law. In the US milk is fortified with vitamin D.

Deficiency

Sunlight is a stimulus to the production of vitamin D in the body, and it's assumed that most people will get all of the vitamin D they need from some exposure of their skin to the sun. The food sources of vitamin D are especially important for people who may not produce enough vitamin D in their bodies from sunlight alone. These include:

• older adults, especially those who are house-bound or bed-ridden
• those who cover themselves for religious or cultural reasons
• pregnant/breastfeeding women

For these vulnerable groups, an intake of 10 micrograms per day from a supplement is recommended.

Prolonged deficiency can result in weakness and softening of the bones. In children this is known as rickets and symptoms include bone pain, skeletal and dental deformities, muscle cramps and impaired growth. In adults softening of the bones due to inadequate vitamin D is called osteomalacia, symptoms of which include skeletal pain, muscle weakness and fractures.

Toxicity

Very high levels of vitamin D from supplements may lead to hypercalcaemia which can result in the deposition of calcium in soft tissues, demineralisation of bones and renal and cardiovascular damage.