Lack
of sun does not explain low Vitamin D in elderly
who are overweight
Newswise — 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.
Vitamin D is called the
“sunshine vitamin” because it is produced by the
body when the skin is exposed to ultraviolet
(UV) rays from the sun. Vitamin D can also be
obtained 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.
This is one of
several important forms of vitamin D, and is the
form that researchers and clinicians use 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, who is
also a professor at Tufts University School of
Medicine.
“These results cannot be
carried over to other populations, such as young
people, or elderly living in different climates.
However, if low vitamin D stores are not
attributed to low sunlight exposure in this
population, it suggests that we should explore
other possibilities,” says Harris. “The most
likely explanation seems to be that vitamin D is
sequestered in fat tissue, reducing its entry
into the blood.”
This study was supported by
the Agricultural Research Service of the U.S.
Department of Agriculture and by a grant from
the National Institutes of Health.
