Sun exposure unrelated to excess fat and vitamin D
NEW YORK (Reuters Health) - A lack of sunshine doesn't appear to be responsible for the lower vitamin D levels that are observed in older, heavier, adults, a new study shows.
Instead, it's likely that the excess fat itself prevents the nutrient from reaching the bloodstream, Dr. Susan S. Harris of Tufts University in Boston, one of the study's authors, told Reuters Health.
While fatter people are known to have less of the vitamin in their blood, the reasons why have not been clear, Harris and her Tufts colleague Dr. Bess Dawson-Hughes note in their report in The Journal of Clinical Endocrinology & Metabolism. One possible explanation has been that heavier people spend less time in the sun or cover themselves up more.
To investigate, the researchers analyzed information on 381 men and women age 65 or older participating in a study of calcium and vitamin D supplementation to prevent bone loss.
When people were divided into four groups based on their body fat percentage, the researchers found no difference among the groups in time spent outdoors, percent of skin exposed to the sun, or sunscreen use.
But they did find that the people with the highest percentage of body fat had 20-percent lower blood levels of vitamin D than those with the least body fat. "That's quite a big difference," Harris said.
Low vitamin D levels are common in the U.S., and are known to increase the risk of osteoporosis, she added. And an increasing number of chronic conditions are being linked to deficiency in the vitamin.
"The idea is that somehow vitamin D is getting trapped in fat tissue, so it's not available to the blood for circulation," Harris explained, and this would occur if vitamin D is consumed through diet and supplements, as well as vitamin D made in the skin during sun exposure.
"In general, it seems that heavier people may need more vitamin D, whether it's from more sun exposure or getting more in the diet, because a given intake is not going to lead to the same blood level," she concluded.
SOURCE: The Journal of Clinical Endocrinology & Metabolism, August 2007.
