Vitamin D in Elderly Said to Sequester Itself in Fatty Tissue

BOSTON, June 8 -- As elderly patients get fatter, their blood levels of vitamin D decline even if they continue to go out into the noonday sun, according to researchers here.
Instead, the most likely explanation is that the fatty tissue sequesters vitamin D and stops it from getting into the circulation, found Susan Harris, D.Sc., of the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University here.
The finding comes from a cross-sectional examination of baseline data from 381 volunteers talking part in a three-year trial of calcium and vitamin D supplementation to reduce bone loss in men and women 65 and older, Dr. Harris and co-author Bess Dawson-Hughes, M.D., also of Tufts, reported online in the Journal of Clinical Endocrinology & Metabolism.
Patients 65 and older with high body fat have lower levels of 25-hydroxyvitamin D, the storage form of vitamin D, compared with those who have lower body fat, Dr. Harris said.
There are several competing explanations, she said, but the two most likely are:
The fatter the patients, the more likely they are to stay indoors and not get enough sunlight to synthesize vitamin D.
The fatty tissue keeps vitamin D from entering the bloodstream.
To seek the more likely explanation, the researcher stratified the volunteers into quartiles, according to percentage of body fat, with the lowest being below 27.5% and the highest 40.3% or higher.
Volunteers in the study had not yet taken any vitamin D supplements and were excluded if they had recently traveled south of latitude 35 or were not Caucasian (because there is evidence that the link between body fat and vitamin D varies according to race).
Dietary vitamin D and sun exposure was estimated using a questionnaire and body fat was measured by dual-energy x-ray absorptiometry, the researchers said.
Analysis found:
There was a significant inverse correlation (at P=0.003) of fat percentage with vitamin D intake.
Circulating levels of 25-hydroxyvitamin D were about 20% lower in the highest quartile compared with the lowest - 26.7 ng/mL, compared with 33.2. The difference was significant at P=0.03.
Few of the volunteers reported sunscreen use (10% of men and 21% of women) and it did not vary across fat quartiles.
Men spent significantly more time outside than women (at P<0.001) and both sexes spent significantly more time outside from May to October than from December to April (also at P<0.001).
The time spent outside did not vary significantly across quartiles of body fat.
In contrast to 25-hydroxyvitamin D, the concentration of 1,25 (OH)2D did not differ across quartiles (P=0.654).
"Sunlight exposure could not account for low vitamin D stores in older people with high percent body fat," Dr. Harris said.
The finding is important because vitamin D plays an important role in bone health, according to Dr. Dawson-Hughes. '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," she said.
The finding also comes as researchers are reporting that high vitamin D levels can cut the risk of cancer in women by up to 60%.
The researchers noted that the results can't be extrapolated widely. "These results cannot be carried over to other populations, such as young people, or elderly living in different climates," Dr. Harris said.
But if low vitamin D stores aren't a result of low sun exposure in this population, she added, "it suggests that we should explore other possibilities."
The study was supported by the U.S. Department of Agriculture and the National Institutes of Health. The authors said they had no conflicts to disclose. Primary source: Journal of Clinical Endocrinology and Metabolism Source reference: Harris SS et al. "Reduced Sun Exposure Does Not Explain the Inverse Association of 25-Hydroxyvitamin D with Percent Body Fat in Older Adults." J Clin Endocrin Metab 2007;