Requirements/Sources
We
think of vitamin D as the sunshine vitamin, but it's
deep within the confines of our bones that it does its
best work. Vitamin D is responsible for getting the
important bone builders--calcium and phosphorus--to
the places in the body where they can help bone grow
in children and remineralize in adults.
It does this first by making
certain that these minerals are absorbed in the intestines,
second by bringing calcium from bones into the blood,
and third by helping the kidneys reabsorb the two minerals,
says Binita R. Shah, MD, professor of clinical pediatrics
and director of pediatric emergency medicine at the
State University of New York Health Science Center at
Brooklyn.
Fortified milk is one ready source
of vitamin D, but you don't have to rely on diet alone
to give you the D you need. Ten minutes of summer sun
on your hands and face can provide enough, says Hector
F. DeLuca, PhD, professor and chairman of the Department
of Biochemistry at the University of Wisconsin. "During
the summer, you can store up quite a bit of vitamin
D in your fat cells," DeLuca says. "If your
diet is good, it will probably last you through the
winter." (Sitting next to a sun-filled picture
window or driving in a car doesn't count, however, because
glass filters out the rays you need, DeLuca says.)
As easy as vitamin D is to get
for youthful outdoor types, however, such is not the
case for the elderly. After evaluating the calcium and
vitamin D status of elderly people who were entering
nursing homes, researchers from Columbia University
in New York City determined that most had low vitamin
D levels and that nearly 85% had symptoms of osteoporosis.
"There is mounting evidence that vitamin D deficiency
in elderly people is a silent epidemic that results
in bone loss and fractures," reports Michael F.
Holick, MD, PhD, chief of the Section of Endocrinology,
Diabetes, and Metabolism at Boston University Medical
Center.
There is very little vitamin
D found naturally in the foods we eat (the best sources
are coldwater fish). In many countries, vitamin D is
added to milk and other foods like breakfast cereals
and margarine, contributing to our daily intake.
By far the best source of vitamin
D is sunlight. However, current recommendations which
stress sun avoidance and the use of sunblock may have
the unintended effect of increasing the prevalence of
vitamin D deficiency. Severe vitamin D deficiency was
common in England in the 1800s due to coal smoke obscuring
the sun. During that time, cod liver oil, which is high
in vitamin D, became popular as a supplement for children
to help prevent rickets. (Rickets is a disease caused
by vitamin D deficiency in which developing bones soften
and curve because they aren't receiving enough calcium.)
Vitamin D deficiency is known
to occur today in the elderly (who often receive less
sun exposure) as well as in people who live in northern
latitudes and don't drink vitamin D–enriched milk.5,7
The consequences of this deficiency may be increased
risk of hypertension, osteoporosis, and several forms
of cancer.
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