By Serena Gordon
HealthDay Reporter

FRIDAY, June 1 (HealthDay News) -- Too little vitamin D has been linked to a host of health problems, including obesity, diabetes and high blood pressure, and new research now suggests that a lack of this important nutrient may also contribute to mobility problems in old age.

The researchers followed more than 3,000 people between the ages of 70 and 79 for six years, and found that those with the lowest vitamin D levels at the start of the study had nearly a 30 percent increased risk of a mobility limitation at the end of the study and almost twice the risk of a mobility disability as people with the highest levels of vitamin D.

"In a growing older population, trying to find ways to reduce the risk of disability is really important," said study author Denise Houston, an assistant professor at the Sticht Center on Aging at Wake Forest School of Medicine in Winston-Salem, N.C. "Taking vitamin D could be one thing to do that might reduce the risk of disability, but until a randomized, controlled clinical trial is done, it's premature to say if it will definitely help prevent disability."

A randomized, controlled trial, in which study participants are randomly chosen to either receive a treatment or not, is the gold standard for medical research.

Results of the new study, which was funded by the U.S. National Institutes of Health, were published online in the May issue of the Journal of Gerontology: Medical Sciences.

The body produces vitamin D naturally when exposed to the sun's rays. However, because sun exposure increases the risk of skin cancer, many people limit their sun exposure or use sunscreen to protect their skin from the sun's potentially damaging light. As people get older, the skin doesn't absorb and process the sun's ultraviolet light as effectively as it does in younger folks.

Vitamin D plays an important role in muscle function, according to Houston. It's needed to get calcium into the muscles, which is needed for muscle contraction, and she noted that muscles also have their own vitamin D receptors.

In the current study, all of the volunteers were living on their own; none were in a nursing home or other long-term care facility. At the start of the study, the volunteers had no difficulty walking one-fourth of a mile, climbing 10 steps, or performing basic activities of daily living, the researchers noted. The participants were also free of any life-threatening illnesses. Just over half were male and about one-third were black.

The researchers measured vitamin D levels early in the study and found that 29 percent had vitamin D levels of less than 50 nanomoles per liter (nmol/L), while 36 percent had levels between 50 and 75 nmol/L. About 35 percent of the study volunteers had vitamin D levels over 75 nmol/L.

After six years of follow-up, 2,099 volunteers were still available to answer questions about their mobility. Persistent mobility limitation was defined as having two or more episodes of difficulty walking one-fourth of a mile or climbing 10 steps without resting due to a health or a physical problem. Persistent mobility disability was severe difficulty or an inability to perform these tasks, according to the study.

Compared to those with the highest vitamin D levels, people with less than 50 nmol/L had a 29 percent increased risk of having a mobility limitation, while those with between 50 and 75 nmol/L had a 27 percent increased risk. Those with the lowest levels of vitamin D had a 93 percent increased risk of a mobility disability, while those with levels between 50 and 75 nmol/L had a 30 percent increased risk, the investigators found.

"I wasn't surprised to see that after six years, some people in their 70s had developed mobility issues. But, even after adjusting for age and other variables, they found an association," said Jessica Shapiro, an associate wellness dietician at Montefiore Medical Center in New York City.

"But, there are so many variables that can affect mobility, I don't think vitamin D is the only, or even a large factor causing mobility issues," Shapiro said.

Houston said this was an observational study, and wasn't designed to prove a cause-and-effect relationship. She said a randomized, clinical trial where some people got vitamin D and others got an inactive placebo would be the only way to see if improving vitamin D levels could have an effect on mobility.

Right now, said Houston, it would be premature to recommend vitamin D solely for preventing mobility issues. But, many people take vitamin D supplements for other reasons.

"For most people, it's difficult to get enough vitamin D through diet alone," said Houston. "The current recommendations are for 800 international units a day for people over 70 years old, she said, adding that some experts think these levels should be even higher.

Shapiro said the vitamin is also found in fortified milks and juices, fatty fish (such as salmon or tuna), egg yolks, cheese and mushrooms. Houston said that fatty fish are the most significant dietary source of vitamin D.

More information

Learn more about vitamin D from the U.S. National Library of Medicine.

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