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Vitamin D in Elderly Said to Sequester Itself in Fatty Tissue
 
By Michael Smith, Senior Staff Writer, MedPage Today
Reviewed by
Zalman S. Agus, MD; Emeritus Professor at the University of Pennsylvania School of Medicine.
June 08, 2007
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 & 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 3 year trial of calcium & vitamin D supplementation to reduce bone loss in men & women 65 & 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 & 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 & 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% & the highest 40.3% or higher.

Volunteers in the study had not yet taken any vitamin D supplements & were excluded if they'd recently traveled south of latitude 35 or were not Caucasian (because there's evidence that the link between body fat & vitamin D varies according to race).

Dietary vitamin D & sun exposure was estimated using a questionnaire & 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 & 21% of women) & it didn't vary across fat quartiles.
  • Men spent significantly more time outside than women (at P<0.001) & 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 didn't vary significantly across quartiles of body fat.
  • In contrast to 25-hydroxyvitamin D, the concentration of 1,25 (OH)2D didn't differ across quartiles (P=0.654).

"Sunlight exposure couldn't 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's evidence that many older Americans have low blood levels of vitamin D, which can put them at risk for bone fractures & 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 can't 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."

source: Medpage

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