Most people do not take very large amounts of supplemental vitamin D but make sufficient amounts through the skin from exposure to the sun. There is more concern with toxicity from the fortified vitamin D, especially in milk. This synthetic, irradiated ergocalciferol (D2) has decreased the incidence of rickets, but it may be contributing to calcification of the arteries, or atherosclerosis, from infancy through old age. The added 400 IUs per quart of milk is about 15 times the amount normally found in milk and may increase the amount of calcium in the circulation, which could be a problem.

Deficiency of vitamin D has not been a major problem of late. Older people are more prone to vitamin D deficiency (a blood level can be measured) since their skin production is lower, their digestion and absorption may be diminished, and their liver function may be reduced. Vitamin D may be deficient in people with gastrointestinal disease, such as ulcerative colitis. The sun’s action on the skin to produce vitamin D is inhibited by pollution, clouds, clothing, window glass, skin pigmentation, and sunscreens. The occurrence of several of these factors together may make the development of the symptoms of rickets more likely.

The decreased absorption of calcium, along with the retention of phosphorus that usually accompanies it, leads to poor mineralization of bone and the inability of the bones to handle stress. This problem, called osteomalacia, is manifested by poor calcification and soft bones. Vitamin D deficiency in the elderly increases general bone loss and osteoporosis. Supplementing this vitamin improves calcium absorption and reduces bone loss. In children, the bone disorder from vitamin D deficiency is rickets. It is characterized by soft skull bones and fragility of other bones, with bowing of the legs, spinal curvature, and an increase in the size of the joints, such as the wrists, ankles, and knees. Muscular development may be diminished as well. Because of low calcium availability, the teeth may have poor structure, and there may be muscle spasms from a problem called tetany, which also causes tingling and weakness of the areas affected. Nearsightedness and loss of hearing may also develop from vitamin D deficiency because of the vitamin’s influence on the eye muscles and from loss of calcium in the ear bones. Furthermore, one of the current theories of multiple sclerosis is that it may be influenced by low vitamin D levels in puberty.

Requirements: Vitamin D is best utilized with vitamin A. Most of our calciferol needs are met with some vitamin D in foods and regular sunlight exposure. If we live in smoggy cities or where tall buildings block the sunlight, we may need more vitamin D. Those who have darkly pigmented skin, work nights, or cover their bodies with lots of clothes, as do members of some religious orders, probably need more vitamin D than the avid sunbather. In winter, we usually require more D from supplements or from our foods.

The RDA for vitamin D is 400 IUs, or roughly 10 mcg., per day. Infants and growing children probably need more vitamin D relative to body size than do adults. During pregnancy and lactation, more D is needed than the 400 IUs. Therapeutic doses for problems treated with vitamin D are about 1,000–1,500 IUs maximum per day, though some doctors may prescribe even more, mainly of the natural vitamin D3. In general, however, it is wise for adults to limit any supplemented vitamin D to the 400 IUs per day commonly found in multivitamins and to limit use of vitamin-D-fortified milk for a variety of reasons.