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updated 12/8/2010 2:49:05 PM ET 2010-12-08T19:49:05

Taking large doses of vitamin D supplements may not reduce women's risk of frailty later in life, according to a new study.

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In fact, the study found that both low and high levels of vitamin D in the blood were associated with an increased likelihood of frailty among older women. The researchers considered women to be frail if they had symptoms such as a slow walking speed, weak hand grip or exhaustion.

While the link between high levels of vitamin D and frailty was not consistent over time, there was no evidence that higher levels were protective, the researchers say.

The results come on the heels of a report released last week by the Institute of Medicine issuing new guidelines for daily vitamin D intake. Older adults need about 800 international units (IU) a day — an amount achievable without taking supplements — and most Americans and Canadians get enough, the report said.

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The new findings underscore the need for more well-designed clinical trials in order to evaluate the health effects of vitamin D supplements, the researchers said.

Although vitamin D deficiency has been linked with adverse health effects, that doesn't mean that more vitamin D is beneficial, said study researcher Dr. Kristine Ensrud, a professor of medicine and epidemiology at the University of Minnesota.

"Sometimes people get so convinced that something's good for you," Ensrud said. "In a way, vitamin D supplementation sort of got put in the 'water supply' when the evidence wasn’t really wasn't behind that."

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Vitamin D deficiency can lead to weakness and slowness, both components of frailty. Some experts have recommended vitamin D supplements for older adults whose blood levels of vitamin D are below 30 nanograms per milliliter (ng/ml), the researchers said. This would likely require taking supplements of 1000 to 2000 IU of vitamin D per day.

Ensrud and her colleagues measured the vitamin D blood levels of 6,307 women 69 years and older, and also determined how frail the women were.

Women with vitamin D levels less than 20 ng/ml and greater than 30 ng/ml had higher odds of being frail than women whose vitamin D levels fell in between those marks. This association held even when the researchers included only the vitamin D in the blood produced by the body in response to sunlight. They performed this analysis to rule out the possibility that frail women might be prescribed vitamin D supplements.

The researchers also looked at the impact of vitamin D levels over time among 4,551 women who were not frail at the beginning of the study. They examined the frailty status of these women 4 1/2 years later. In this case, only vitamin D levels lower than 20 ng/ml were associated with increased risk of frailty or death later in life.

The researchers said that the fact that they didn't find a link between high vitamin D levels and frailty after the 4.5-year study period may mean the association between these two factors found in the earlier part of the study was not a true link. However, it might also be that there were too few women in the follow-up study to observe a link later on, Ensrud said.

Nonetheless, even over time, having high levels of vitamin D at the study's start didn't lessen a woman's chances of later developing frailty, Ensrud said.

The study will be published in an upcoming issue of the Journal of Clinical Endocrinology & Metabolism.

Follow MyHealthNewsDaily staff writer Rachael Rettner on Twitter @Rachael_MHND.

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