15.7: Summary

There are four fat-soluble vitamins—vitamins A, D, E, and K. Vitamin A is found naturally only in animal foods. Plants provide carotenoids, some of which can be converted to vitamin A in the body (e.g., beta-carotene).

Vitamin A is critical for night vision. It’s part of a chemical found in the eye called rhodopsin. When light strikes rhodopsin, vitamin A is released, causing an impulse to be sent along the optic nerve. Vitamin A must then be replaced in rhodopsin for the process to occur again. If there isn’t enough available, the impulses slow down and vision is impaired.

Vitamin A also maintains epithelial tissues (e.g., the outermost layer of the skin and the lining of the nose, lungs, and digestive tract), allowing them to secrete substances. Without this ability, we’re more susceptible to infection. The epithelium of the eye, for example, needs vitamin A, and when there’s a deficiency, eye tissue begins to dry out, causing a condition called xeropthalmia. The eye is then susceptible to infection, which can lead to blindness.

Animal products such as liver are rich sources of vitamin A. Excesses of vitamin A, commonly the result of regular supplementation, can be toxic. Large doses are especially dangerous to the fetus, causing malformations.

Yellow-orange and dark green plants provide us with pro-vitamin-A carotenoids, which aren’t toxic. The body doesn’t convert them to vitamin A in excessive amounts. Carotenoids can also act as antioxidants, and foods rich in them may be protective against cancer. A normal diet can supply all the body can use. In some developing countries, however, vitamin A deficiency is a serious problem, causing blindness and many deaths from infections.

Some vitamin A analogs (lab-produced drugs that resemble vitamin A, but don’t have vitamin activity) are used successfully by physicians to treat conditions such as severe acne and to prevent the recurrence of head and neck cancers.

Vitamin D deficiency causes rickets, a form of osteomalacia, which results in bone softening and skeletal defects. We get vitamin D from our diet and from exposure to sun. A cholesterol precursor in the skin is changed by ultraviolet light to inactive vitamin D, and moves to the blood.

The inactive vitamin D from the skin (or the diet) must then be chemically changed both in the liver and the kidney, becoming “activated vitamin D.” Activated vitamin D helps calcium and phosphorus to be absorbed in the intestine, and controls their use in bone and tooth formation.

Vitamin D requirements depend on sun exposure and skin pigment; many foods are fortified with D, and rickets and osteomalacia are rare for Americans. But it’s relatively easy to overdo consumption of vitamin D to toxic levels. Toxicity is usually from supplements; toxic levels aren’t reached by sun exposure alone.

Vitamin E works mainly as an antioxidant, preventing the oxidation of fats in various body tissues. It also prevents the oxidation of the fats in plants; therefore, plant oils are a good source of vitamin E. Vitamin E in large doses is relatively non-toxic. However, many people regularly take doses of E that far exceed the RDA and far exceed the amounts found naturally in food. The upper limit (UL) only applies to vitamin E supplements, and not the amounts found naturally in foods.

Vitamin K is provided in our diet, particularly by green leafy vegetables, but is also made by our own intestinal bacteria, and, thus, deficiency is rare. Vitamin K is critical for normal blood clotting. Clotting involves a series of chemical reactions, requiring both calcium and vitamin K.

With respect to all vitamins, it’s important to stress that they aren’t cures for diseases, unless the disease stems from a deficiency. It’s also important to remember that results of animal studies don’t necessarily apply to humans.

For continually updated fact sheets on individual vitamins, go to the website of our Office of Dietary Supplements. ods.od.nih.gov/factsheets