It's not yet clear why overweight elderly adults have low levels of vitamin D in their blood. However, researchers at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University (USDA HNRCA) have found that lack of sun exposure may not account for low levels of vitamin D in elders who are overweight.

"People aged 65 and over with high percent body fat have lower levels of 25-hydroxyvitamin D, the storage form of vitamin D, compared to those who have lower percent body fat," says corresponding author Susan Harris, DSc, epidemiologist in the Bone Metabolism Laboratory at the USDA HNRCA.

Harris and co-author Bess Dawson-Hughes, MD, director of the Bone Metabolism Laboratory at the USDA HNRCA, interviewed 381 Caucasian men and women aged 65 and over about their sun exposure over a previous three-month period. Individuals reported how much time they spent outdoors, how much skin was exposed while outdoors, and whether or not they wore sunscreen. Seasonality, or when the individual entered the study, was also taken into account, because in Boston , where the study was conducted, sun rays are weak in winter compared with summer months.

The researchers measured participants' percent body fat using dual-energy x-ray absorptiometry (DXA), a precise method for determining body composition. Individuals were grouped into quartiles of percent body fat: less than 28 percent, 28 percent to 33 percent, 34 percent to 40 percent, and greater than 40 percent. Blood levels of 25-hydroxyvitamin D were measured and participants were asked to fill out a dietary questionnaire to measure the amount of vitamin D they obtained from food.

Harris and Dawson-Hughes found that when adjusted for sex, age, seasonality and dietary vitamin D intake, 25-hydroxyvitamin D significantly decreased as body fat increased, (P<0.024). When the researchers further adjusted for sunlight exposure variables, 25-hydroxyvitamin D values still significantly decreased as body fat increased. "Sunlight exposure could not account for low vitamin D stores in older people with high percent body fat," explains Harris.

The human body produces Vitamin D, the "sunshine vitamin," when the skin is exposed to the sun’s ultraviolet (UV) rays.   People can get Vitamin D from foods such as fish and fortified milk and from supplements. When this fat-soluble vitamin enters the body it is converted in the liver to 25-hydroxyvitamin D.  Researchers use this important form of vitamin D, as an indicator of vitamin D status in individuals. "Vitamin D is especially critical in maintaining bone health, and there is evidence that many older Americans have low blood levels of vitamin D, which can put them at risk for bone fractures and osteoporosis," says Dawson-Hughes, a professor at Tufts University School of Medicine.

Scientists think that “vitamin D is sequestered in fat tissue, reducing its entry into the blood.”