Vitamin D Not Calcium For Strong Bones

Calcium has been the focus of nutritional research for the prevention of postmenopausal osteoporosis as many clinical trials of calcium supplementation have found that it can indeed reduce bone loss and lower the risk of bone fractures.

However, the trials often include a simultaneous treatment with vitamin D, which makes it difficult to attribute the benefits to calcium alone. Also, the bone density increase found during the first year or two of calcium supplementation may not substantially increase in the long term.

In contrast to most clinical data, most observational studies did not find a significant association between calcium intake and fracture risk or bone loss. Despite this, and based largely on the clinical evidence, the Food and Nutrition Board of the National Academy of Sciences raised the recommended daily dose of calcium for women over 50 years of age to 1200 mg.

Milk is often thought to decrease osteoporotic bone loss and fracture risk since it is a primary source of calcium and vitamin D. However, while milk, and other dairy food, intake has been found beneficial in women under the age of 30 years, there has been no clear benefit documented for women over the age of 50 years.

It is known that calcium, at low or moderate doses, is largely dependent on the action of 1,25-dihydroxyvitamin D for active transport. Sufficient amounts of vitamin D are important in the prevention of postmenopausal bone loss.

For instance, one 18-year study, which examined calcium and vitamin D intakes, milk consumption, and use of calcium supplements among postmenopausal women, found that those with higher vitamin D intakes, either from food alone or from food plus supplements, had a significantly lower risk of hip fracture. However, calcium intake did not appear to be associated with fracture risk.

Insufficient vitamin D leads to less calcium absorption, elevated blood concentrations of parathyroid hormone, and increased rates of bone resorption, which may eventually lead to bone fracture. In line with this are several studies that have found that older people who experience a hip fracture have lower serum concentrations of 25-hydroxyvitamin D than do those without a fracture.

Some 25 percent to 50 percent of the elderly have vitamin D deficiency, which can lead to loss of muscle strength and an increased likelihood of falling that, in turn, increases the risk of hip fracture.

Sunlight exposure is a major source of vitamin D, however older adults may not spend enough time in the sun to get adequate amounts of vitamin D. Also, sunscreens, of which usage has increased, significantly reduce skin production of vitamin D.

Another problem with obtaining vitamin D from sunlight is that synthesis of vitamin D is absent during the winter months in higher latitudes where nearly all of the United States is located.

As we age, vitamin D status may be further affected by a decreased capacity of the skin to manufacture vitamin D, a reduced ability of the liver or kidney to hydroxylate vitamin D to its metabolically active form, or a lower consumption of dairy foods or diminished intestinal absorption of vitamin D.

American Journal Clinical Nutrition February, 2003;77:495-503