Older women with low blood levels of vitamin D may have an increased risk of frailty - but the high vitamin D levels that some experts recommend may offer no special protection, a new study suggests.
Researchers found that among 6,300 elderly women followed for 4.5 years, those with relatively low blood levels of vitamin D at the outset - below 20 nanograms per milliliter (ng/mL) - were somewhat more likely to become frail than those with higher circulating levels of the vitamin.
However, there was no evidence of any extra benefit from having vitamin D levels at or above 30 ng/mL - a level that some researchers have advocated for optimal health.
“People tend to assume that more is better,” said lead researcher Dr. Kristine E. Ensrud, of the University of Minnesota and VA Medical Center in Minneapolis.
But as far as frailty risk, she told Reuters Health, “We found no evidence that this ‘magic level’ of vitamin D was protective.”
The findings come on the heels of a long-awaited report on vitamin D from the Institute of Medicine (IOM), an independent scientific body that advises the federal government.
Released in November, the IOM report raised the recommended intake of vitamin D for children and adults, but also stressed that the average American or Canadian already has vitamin D blood levels at or above 20 ng/mL (equivalent to 50 nanomoles per liter, the more common measure used in Canada).
And that level, the IOM said, is all that is needed for good bone health. Vitamin D has long been known to help the body use calcium to build bone. The report found insufficient evidence that vitamin D, of any level, has health benefits beyond building and maintaining strong bones.
Ensrud said her group’s current findings, reported in the Journal of Clinical Endocrinology & Metabolism, fit in with the IOM conclusions.
“Society has sort of gotten ahead of the evidence,” Ensrud said, referring to a craze for vitamin D in recent years, spurred by studies linking the vitamin to everything from immune system function, to depression and heart disease.
But whether vitamin D deficiency produces excess risks in such diverse aspects of health - and whether taking supplements can curb those risks - has yet to be shown.
When it comes to frailty, Ensrud explained, some studies have also linked lower levels of the vitamin to muscle weakness and slowed movement, but evidence was sparse and conflicting.
To test whether low vitamin D levels increased women’s risk of becoming frail, Ensrud and her colleagues followed 6,307 U.S. women age 69 and older for an average of 4.5 years.
They found that, at the outset, women with either relatively low vitamin D levels (below 20 ng/mL) or high levels (above 30 ng/mL) were somewhat more likely than women with moderate levels (20 - 29.9 ng/ml) to be frail - meaning they had limitations like muscle weakness, exhaustion and slow walking speed.
However, when the researchers looked at the women’s risk of becoming frail over time, only low vitamin D levels were linked to increased risk.
Of the 4,500-plus women who started the study in good physical condition, 16 percent became frail and almost 10 percent died over the follow-up period. The risk of becoming frail or dying was 21 percent higher among women whose initial vitamin D levels were below 20 ng/mL, versus their peers with moderate levels.
Women who started the study with levels above 30 ng/mL showed neither an increased nor decreased risk of becoming frail over time, versus women with moderate levels.
That finding, Ensrud said, offers some “reassurance” that the link between high vitamin D levels and frailty at the study’s start does not reflect a true effect. It is possible, for example, that some of the women with high D levels who were considered frail at the beginning of the study period were already supplementing their vitamin D intake.
For now, Ensrud said that the best bet for older women - and everyone else - is to follow the vitamin D intakes recommended by the IOM. That means 800 IU per day for adults in their 70s and up, and 600 IU per day for everyone else older than 12 months.
What’s needed now, Ensrud said, are clinical trials that actually test whether giving older adults vitamin D supplements curbs their risk of becoming frail.
“You can make all the observations about vitamin D you want,” she said, “but if you really want to know if vitamin D is helpful, you have to do a clinical trial.”
Dr. Heike A. Bischoff-Ferrari, a researcher at the Center for Aging and Mobility at the University of Zurich, Switzerland, agreed on the need for trials to test the vitamin’s effects.
But she also noted that there is already evidence from clinical trials that vitamin D might help the elderly avoid falls.
In an analysis of eight such trials, Bischoff-Ferrari and her colleagues found that vitamin D supplements trimmed elderly adults’ risk of falling by 19 percent over one to three years.
The researchers speculate that improved muscle strength could be one explanation.
The vitamin D dose needed to curb fall risk? In these trials, it ranged between 700 and 1,000 IU - which includes the dose now recommended for elderly adults.
Vitamin D is manufactured in the skin in response to sunlight and occurs naturally in some foods, as well as being added to others such as milk. Over-the-counter supplements costing about the same as other vitamin products, less than $100 for a month’s supply, may be used to ensure adequate daily intake but the IOM recommends that adults take no more than 4,000 IU daily.
SOURCE: Journal of Clinical Endocrinology & Metabolism, December 2010