Vitamin D is an essential human nutrient. It helps our bodies absorb calcium and turn it into bones and teeth. What's unique about the nutrient is that most of the vitamin D we need is manufactured by the human body as a result of exposure to the sun. Just fifteen minutes of sunshine on the arms and hands is enough for most people. But as people age, they produce less vitamin D in response to sunlight.

Denise Houston is a nutrition researcher who teaches at the Wake Forest University School of Medicine in Winston-Salem, North Carolina. She says deficiencies of vitamin D could accelerate physical decline in older adults.

"There have been a few other studies that have looked at this and found an association between vitamin D measured in the blood and physical performance and muscle strength," Houston says.

Houston took data from a large study done in Italy of about a thousand adults over the age of 65. She compared the subjects' physical performance to the levels of vitamin D in their blood. Houston says seniors with too little vitamin D were weaker overall. "We found differences in both a physical performance battery, which includes things such as walking speed, chair stands and a balance test, as well as differences in grip strength -- which is a good predictor of disability and future mortality."

Houston says that even accounting for other factors that might affect human strength, the data still points to an important role for Vitamin D in the mobility and strength of older adults. "We did try to control for other variables that might impact disability and physical function," she explains, "including age, or physical activity, body mass index, their kidney function -- since that actually converts the vitamin D that we get from the food and the sun into the active form that's used by the body. And we still found an association between their vitamin D levels and their physical function and strength."

Houston notes there are few sources of vitamin D in food - usually it's contained only in fortified milk and other foods such as oily fish. So, Houston says, if seniors are not consuming enough vitamin D and they're not getting it from the sun, they're at risk. Her research appears in the Journal of Gerontology: Medical Sciences.