Vitamin D

Vitamin D

Vitamin D exists in two forms—D₂ (ergocalciferol) and D₃ (cholecalciferol). Each is formed from a provitamin when irradiated with (exposed to) ultraviolet light. They are equally effective in human nutrition, but D₃ is the one that is formed in humans from cholesterol in the skin. D₂ is formed in plants. Vitamin D is consid-ered a prohormone because it is converted to a hormone in the human body.

Vitamin D is heat-stable and not easily oxidized, so it is not harmed by storage, food processing, or cooking.

Functions.The major function of vitamin D is the promotion of calciumand phosphorus absorption in the body. By contributing to the absorption of these minerals, it helps to raise their concentration in the blood so that nor-mal bone and tooth mineralization can occur and tetany (involuntary muscle movement) can be prevented. (Tetany can occur when there is too little cal-cium in the blood. This condition is called hypocalcemia.)

Vitamin D is absorbed in the intestines and is chemically changed in the liver and kidneys. Excess amounts of vitamin D are stored in the liver and in adipose tissue.

Sources.The best source of vitamin D is sunlight, which changes a pro-vitamin to vitamin D₃ in humans. It is sometimes referred to as the sunshine vitamin. The amount of vitamin D that is formed depends on the individual’s pigmentation (coloring matter in the skin) and the amount of sunlight avail-able. The best food sources of vitamin D are milk, fish liver oils, egg yolk, butter, and fortified margarine. Because of the rather limited number of food sources of vitamin D and the unpredictability of sunshine, health authorities decided that the vitamin should be added to a common food. Milk was selected.

Consequently, most milk available in the United States today has had 10μg of vitamin D concentrate added per quart.

Requirements.Under the DRIs, there are several reference values in-cluded. Vitamin D levels are given as Adequate Intake levels, or AI (Table 7-4).

People who are seldom outdoors, those who use sunscreens, and those who live in areas where there is little sunlight for 3 to 4 months a year should be especially careful that their diets provide their AI levels of vitamin D. Drink-ing 2 cups of vitamin D–fortified fat-free milk each day will provide sufficient vitamin D to those between birth and 50 years of age. Between the ages of 51 and 70, 1 quart of such milk will be needed each day to fulfill the AI. After 70, 1¹ ⁄2 quarts will be needed daily. In this last age-group, a vitamin D supplement may be needed.

Vitamin D or, specifically, cholecalciferol values are given in micrograms (μg or mcg.) or in international units; 5μg equals 200 international units.

Hypervitaminosis.Hypervitaminosis D must be avoided because it cancause deposits of calcium and phosphorus in soft tissues, kidney and heart damage, and bone fragility.

Deficiency.The deficiency of vitamin D inhibits the absorption of calciumand phosphorus in the small intestine and results in poor bone and tooth forma-tion. Young children suffering vitamin D deficiency may develop rickets, which causes malformed bones and pain, and their teeth may be poorly formed, late in appearing, and particularly subject to decay. Adults lacking sufficient vitamin D may develop osteomalacia, softening of bones. A deficiency of vitamin D con-tributes to osteoporosis (brittle, porous bones).