Sun worshiping may be out of fashion -- even a health hazard -- but unless older people get enough sun, they run a serious risk of being deficient in vitamin D.

A year-long study by the Boston University School of Medicine of 50 residents of the Hebrew Rehabilitation Center for the Aged has shown that nearly 80 percent of them were deficient in vitamin D by the end of winter.

The body requires vitamin D to help absorb calcium and ensure bone strength, which is important in preventing falls, a leading cause of injury and death among the elderly. Vitamin D deficiency can also cause rickets in children and a rickets-like disease in adults called osteomalacia, which increases the risk of fractures.

Vitamin D is found in foods, principally fortified milk, along with fatty fish and fish oils. But older people tend to find milk unpalatable and hard to digest. Sunshine, which causes the vitamin to be synthesized in the skin, often becomes the main source of vitamin D in older people.

In addition to studying 50 nursing home residents, the Boston group, headed by Michael F. Holick, took blood samples from about 200 other volunteers. The researchers found that without supplemental vitamin D, residents did not get enough sunlight to synthesize an adequate amount of the vitamin, especially during the winter.

Holick recommends that elderly people spend five to 15 minutes two or three times per week outside during sunny weather, without using a sunscreen. Longer exposure than that, he says, does not increase the production of vitamin D. He also recommends that those who do not routinely spend time outside take a multi-vitamin containing 400 units of vitamin D, especially in the winter.