Among severely obese people, vitamin D may make the difference between an active and a more sedentary lifestyle, according to a new study published in the Endocrine Society's Journal of Clinical Endocrinology & Metabolism (JCEM).

The study found severely obese people who also were vitamin D-deficient walked slower and were less active overall than their counterparts who had healthy vitamin D levels. Poor physical functioning can reduce quality of life and even shorten lifespans.

Severe obesity occurs when a person's body mass index (BMI) exceeds 40. About 6.5 percent of American adults are severely obese.

"People with severe obesity already are eight times more likely to have poor physical function than people with a healthy BMI," said one of the study's authors, Tomás Ahern, MB, BCh, BAO, of St. Columcille's Hospital and St. Vincent's University Hospital in Dublin, Ireland. "Poor vitamin D status contributes to the deterioration of physical function in this population. Among those with severe obesity, 43 percent are at risk of vitamin D deficiency."

The cross-sectional study examined physical functioning and vitamin D levels in 252 severely obese people. Participants were timed as they walked 500 meters and climbed up and down a single step 50 times. They also provided estimates of their physical activity.

Researchers took a blood sample to measure each participant's vitamin D levels. For analysis, the study population was divided into three groups based on vitamin D levels.

The study found the group with the highest vitamin D levels had the fastest walking times and highest amount of self-reported physical activity. This group also had the lowest average BMI of the study participants.

"Improving vitamin D status should improve quality of life and may decrease the risk of early death in people with severe obesity," Ahern said. "This could be a simple matter of spending more time outside, since sun exposure can boost the body's natural vitamin D production."