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).
D & sun exposure was estimated using a questionnaire & body fat was measured by dual-energy x-ray
absorptiometry, the researchers said.
- 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
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."