Fat-soluble vitamins

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.

Vitamin A

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.

Vitamin D

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.

Vitamin E

Role

Vitamin E is a powerful antioxidant that protects the body against free radicals. It helps to make red blood cells and prevents blood from clotting.

Sources

Foods containing large amounts of polyunsaturated fatty acids will generally contain large amounts of vitamin E. Good sources include wheat germ, prawns, almonds, hazelnuts, peanuts, pistachio nuts, soya beans and sunflower seeds.

Deficiency

A dietary deficiency of vitamin E is extremely rare. However, the signs and symptoms to be aware of include problems in the nervous, vascular and reproductive systems. Vitamin E may also exacerbate the effects of any vitamin K deficiency, thus affecting blood clotting.

Toxicity

Vitamin E has low toxicity and it's unlikely that you'll take in too much from your diet. Very high doses of vitamin E from supplements can cause headaches, fatigue, gastrointestinal distress, double vision and muscle weakness.

Vitamin K

Role

This is an essential component in the body’s normal blood-clotting process and plays an important role in maintaining bone health.

Sources

Most vitamin K is produced by micro-organisms in the intestine, and is stored in the liver. Dietary vitamin K is largely obtained from green leafy vegetables such as spinach, Brussels sprouts, broccoli and cabbage, and some vegetable oils including soybean and rapeseed.

Deficiency

Newborn infants are at risk of haemorrhagic disease, a bleeding disorder that may occur because vitamin K production is not established in the intestine until after the first week of life. To tackle this problem, newborn infants are given a vitamin K injection at birth. Otherwise, vitamin K deficiency is rare.

Many studies report that low vitamin K intake increases the risk of developing osteoporosis. Vitamin K is essential for the normal formation of a protein in bones called osteocalcin. This protein binds to calcium and helps to hold the calcium in the bones. Too few studies have been carried out to determine the range of adult vitamin K requirements. One microgram for every kilogram of body weight per day is recommended to maintain the normal blood-clotting process.

Toxicity

Vitamin K has low toxicity from dietary sources.