Vitamin D has impact on bone health

Frank Murray

Vitamin D Has Impact on Bone Health

Calcium isn’t the only nutrient necessary for strong bones. Vitamin D works synergistically with other minerals to keep bones healthy.

Because vitamin D regulates calcium and phosphorus metabolism, it is essential for the growth and maintenance of healthy bones. Unfortunately, many elderly people have trouble absorbing this vitamin from their diets and require a supplement.

Vitamin D increases calcium absorption from the small intestine, and a deficiency in the vitamin results in abnormal calcium losses in the feces. A sufficient source of vitamin D in the diet enhances the absorption of phosphorus through the intestinal wall, as well as the absorption of phosphates from the small tubes leading from the kidneys. With a vitamin D deficiency, urinary excretion of phosphate increases and blood levels of the mineral are lower.

A healthy phosphate level, as well as the balance between calcium and phosphorus in the blood, is imperative to the process of bone calcification and to the prevention of tetany, the abnormal contraction of muscles.

Vitamin D is found naturally in only a few food sources such as fish oils, liver, eggs and milk. It can be formed in the body after the skin is exposed to ultraviolet rays of the sun, hence its nickname, “the sunshine vitamin.”

The growth and proper mineralization of the bones and teeth require vitamin D. Lack of the vitamin or of exposure to sunlight in children causes weak bones and overgrowth of cartilage at the ends of the bones. The joints enlarge and bowed legs, knock knees, beaded ribs and skull deformities may result.

A vitamin D deficiency in adults can result in osteomalacia, a disorder which causes softening of the bones. The vitamin also prevents congenital malformations of the newborn and injury to the skeleton of the mother.

A deficiency of the vitamin will affect the enamel and dentin of the teeth, since both are composed mostly of calcium and phosphorus.

The withdrawal of calcium and phosphorus storage from bone [resorption] is stimulated by vitamin D. In this way, vitamin D helps maintain the blood levels of the two minerals.

An article in the Israeli Journal of Medical Science reported that 15 out of 82 elderly people were deficient in vitamin D and that 28 others had borderline levels. Since many of the elderly were farm workers and spent a great deal of time in the sun, the study suggests they had an impairment in vitamin D metabolism rather than an inadequate source of the vitamin.

In another study 109 randomly selected elderly women, aged 65 to 74 received either 15,000 IU of vitamin D2 (calciferol) weekly or a placebo. After two years, vitamin D therapy significantly reduced the rate of cortical bone loss as revealed by hand radiographs, according to the American Journal of Clinical Nutrition.

Since elderly patients often have low blood levels of vitamin D, many of them may need supplements, according to Medical Tribune.

The increased frequency of vitamin D deficiency among certain groups, together with the ease and safety with which normal vitamin D status can be restored, strongly favor supplementation for high risk patients, according to the report.

Vitamin D supplements and a vitamin D-rich diet may be warranted for all age groups in northern climates, according to Dr. Michael F. Holick, who heads the Vitamin D and Bone Metabolism Laboratory at Tufts University, Boston. Holick believes that people living in northern climates, especially in the winter, are wearing so many clothes that they do not benefit from the little sun that is available.

He added that many people, notably the elderly, do not get enough vitamin D from their diets, especially milk, which is fortified with vitamin D, or from the sun. In summer, exposure to the sun should be brief in order to avoid skin cancer, he said.

Dr. Holick added that a vitamin D deficiency in those with osteoporosis might significantly increase the risk of bone fractures. For example, in a study of 142 patients at Massachusetts General Hospital, Boston, all with fractured hips, 30 to 40 percent had little or no vitamin D circulating in their blood. Other studies in the United States and in Great Britain indicate that a similar percentage of men and women with hip fractures are deficient in the vitamin, Dr. Holick said.

“The major cause of age-related vitamin D deficiency is a decrease in milk consumption,” Dr. Holick said. “A substantial number of elderly develop an intolerance to milk and others believe that milk is only for children. Those who can’t tolerate milk should take a vitamin D supplement or spend more time in the summer sun. As people age, the ability of the sun to stimulate the manufacture of vitamin D in the skin progressively decreases. And, in northern latitudes, such as Boston, vitamin D synthesis stops completely in the winter. Dark skinned people require five to 10 times longer in the sun, depending on the amount of pigmentation, to get sufficient vitamin D. In laboratory tests, it took a dose of radiation that would produce a severe sunburn in Caucasians to increase vitamin D levels in dark skin.”

Dr. Holick said that four 8-ounce glasses of milk will provide the Recommended Dietary Allowance (400 IU) for vitamin D. And, depending on the fat content, 32 ounces of milk supplies about 1,200 mg of calcium which is one-third more than the RDA for calcium for men and women over 18 (800 mg).

“Vitamin D is an important nutrient for the elderly, especially in relation to calcium metabolism and osteoporosis,” said Mary Locniskar, Ph.D., assistant professor at the University of Texas, Austin. “Since there are two sources of vitamin D (diet and synthesis in the skin), dietary requirements are dependent on [internal] synthesis. When the RDA is set, the fact that vitamin D synthesis is decreased in the skin of elderly along with the fact that the elderly may not be exposed to as much sun as a younger person must be considered. There is also evidence that calcium absorption does not become more efficient when one is consuming a low-calcium diet, as is the case in younger individuals.”

Your health food store carries a variety of vitamin D supplements, either alone or in combination with other nutrients, especially vitamin A, since they are both fat-soluble. Vitamin D is also available in other forms, such as in cod liver oil.

COPYRIGHT 1989 PRIMEDIA Intertec, a PRIMEDIA Company. All Rights Reserved.

COPYRIGHT 2004 Gale Group

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