Chapter: Nutrition and Diet Therapy: Vitamins

Vitamin D

Vitamin D exists in two forms—D2 (ergocalciferol) and D3 (cholecalciferol).

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).


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