Vitamins, Fat-Soluble Health Article

Deficiency.

Due to the widespread use of vegetable oils, primary vitamin-E deficiency is rare. Most deficiencies occur in people with fat malabsorption syndrome. Smokers and adults on very low-fat diets are at increased risk of developing vitamin-E deficiency. Preterm infants are particularly susceptible to hemolytic anemia (anemia caused by the destruction of red blood cells) due to vitamin-E deficiency. These infants are born with limited stores of vitamin E, which are exhausted by rapid growth, and they are inefficient in absorbing vitamin E from the intestinal tract. Without vitamin E to protect against oxidation, the destruction of cell membranes causes red blood cells to burst. To prevent hemolytic anemia, special formulas and supplements containing vitamin E are prescribed for preterm infants.

Toxicity.

Large doses of vitamin E can counter the actions of vitamin K and decrease the production of vitamin K–dependent clotting factors, thus promoting serious hemorrhaging effects in adults. Individuals who are vitamin-K deficient or who are taking anticoagulant medications such as warfarin or aspirin are especially at risk from megadoses of vitamin E.

SEE ALSO VITAMINS, WATER-SOLUBLE.

Kiran B. Misra

Vitamin D (Calciferol)

In the seventeenth century, vitamin-D deficiency was so common in British children that it came to be known as "children's disease of the English." In the mid-1800s, cod liver oil became well known for treating this disease. In 1925, Elmer McCollum and coworkers determined that the "antirachitic" (antirickets) substance in cod liver oil was vitamin D. Because vitamin D is relatively stable in foods, many countries fortify milk with vitamin D to help prevent rickets. However, significant losses may result from fortified milk exposed to light.

Vitamin D from foods is absorbed from the upper part of the small intestine, along with dietary fat, and transported to the liver. In the skin, ultraviolet (UV) radiation from the sun converts a cholesterol derivative to cholecalciferol, which enters the blood stream and is transported to the liver. In the liver, vitamin D is converted to calcidiol, an inactive form that circulates in blood. Kidneys take up calcidiol and convert it to an active hormone form of vitamin D called calcitriol. People with chronic kidney failure have very low levels of calcitriol and must be routinely treated with this form of the vitamin.

The best-known function of active vitamin D is to help regulate blood levels of calcium and phosphorous. Vitamin D increases absorption of these minerals from the gastrointestinal (GI) tract. In combination with parathyroid hormone, it enhances their reabsorption from the kidneys and their mobilization from bones into the blood. Vitamin D helps maintain calcium levels even if dietary intakes are not optimal. Calcitriol affects growth of normal cells and some cancer cells. Adequate vitamin-D status has been linked to a reduced risk of developing breast, colon, and prostrate cancers.

Vitamin K

In 1929, the Danish researcher Henrik Dam first noted that vitamin K played a critical role in blood clotting, and he named it vitamin "K" for "Koagulation." Vitamin K comprises a family of compounds known as quinones. These include phylloquinone from plants and the menaquinones from animal sources. Phylloquinone is the most biologically active form. Menaquinones are also synthesized by bacteria in the colon and absorbed, contributing about 10 percent of total vitamin-K needs. Vitamin-K absorption depends on normal consumption and digestion of dietary fat. It is primarily stored in the liver.

Vitamin K helps in the activation of seven blood-clotting-factor proteins that participate in a series of reactions to form a clot that eventually stops the flow of blood. Vitamin K also participates in the activation of bone proteins, which greatly enhances their calcium-binding properties. Low levels of circulating vitamin K have been associated with low bone-mineral density. Thus, an adequate intake of vitamin K may help protect against hip fractures.