A lack of vitamin D may be put seniors at increased risk of developing mobility limitations and disability, U.S. researchers found. Lead author Denise Houston of the Wake Forest Baptist Medical Center and colleagues analyzed vitamin D and the onset of mobility limitation and disability over a six-year period. They used data from the National Institute on Aging Health, Aging and Body Composition study. Mobility limitation and disability were defined as any difficulty or inability to walk several blocks or climb a flight of stairs. The study involved 3,075 men and women ages 70-79. Vitamin D levels were measured in the blood at the beginning of the study. \"We observed about a 30 percent increased risk of mobility limitations for those older adults who had low levels of vitamin D, and almost a two-fold higher risk of mobility disability,\" Houston said in a statement. Vitamin D plays an important role in muscle function, so it is plausible that low levels of the vitamin could result in the onset of decreased lower muscle strength and physical performance, Houston said. People get vitamin D when it is naturally produced in the skin by sun exposure, by eating foods with vitamin D -- such as fortified milk, juice and cereals -- and by taking vitamin D supplements, Houston said. The findings were published in the Journal of Gerontology: Medical Sciences.