Vitamin D: Are You Getting Enough
By Rebecca Johnson
There has been a lot of recent research on vitamin D. Some has focused
on its known benefits, notably its crucial role in working with calcium
to keep bones strong. But many studies have looked at its potential to
reduce the risk of everything from some common cancers and multiple
sclerosis to diabetes, hypertension, and age-related muscle weakness.
The research is promising.
Much of this research has linked the potential benefits to
high blood levels of vitamin D-levels higher than most Americans and
Canadians have. Thus, some prominent researchers recommend that people
have their blood levels of D measured so that, if necessary, they can
take enough supplemental D to get their levels into the "desirable"
range. Some doctors have started testing many of their patients. Should
you be tested? First, some basics about this special vitamin.
D basics. Vitamin D is unique
in that your skin manufactures it just by being exposed to sun. The
amount made depends on the time of day, season, how far north you live,
skin pigmentation (darker skin makes less D), how much of your body is
exposed to the sun, and your age (older people produce less D from sun
exposure). Because D is fat soluble, the body can store it for the days
or even months when you don't get any sun or consume any D.
Few foods supply vitamin D.
Milk is fortified with D and is the major dietary source, with 100 IU
(international units) per cup. Some soy milks, orange juice,
margarines, and breakfast cereals are also fortified. Fatty fish, such
as salmon, tuna, and sardines, are naturally rich in D. But it's hard
to get adequate D from food alone. Thus, supplements are often
necessary (see below).
Many people are deficient in vitamin D, especially those who are
over 60, live at northern latitudes, have darker skin, or are rarely
outdoors. In the northern U.S. and in Canada, blood levels drop
markedly in the winter, when days are shorter, the sun is weaker, and
we wear more clothes and spend less time outside. Many young people
also have low blood levels of D, according to some recent studies.
Obesity is associated with reduced blood levels.
The case for testing. A recent
meta-analysis in Archives of Internal Medicine of 18 studies found that
people taking D supplements (usually 400 to 800 IU a day) had a 7%
reduction in total mortality rates. In an accompanying editorial, Dr.
Edward Giovannucci of Harvard concluded: "Given the high probability of
benefit for at least some of the many conditions that have been
associated with vitamin D deficiency, and the low likelihood of harm,
it seems prudent that physicians measure serum 25-hydroxyvitamin D in
their patients." That's the form of D in the blood measured by the
preferred test.
Among those advocating routine vitamin D testing is Dr. Bruce
Hollis of the Medical University of South Carolina, who has been doing
vitamin D research for 30 years. "Everyone needs to know his or her
level of 25-hydroxyvitamin D," says Dr. Hollis, because of its
potentially protective effect against chronic diseases. The amount of D
in a multivitamin (usually 400 IU) and/or from exposing your face and
arms to the sun for short periods may not be enough to reach desirable
blood levels. An intake of even 800 to 1,000 IU a day won't be enough
for some people, he says. The body's ability to make and utilize D
varies from person to person. That's why testing can be important.
Blood levels: what's desirable.
Most experts now agree that blood levels of at least 30 to 40 ng/ml
(nanograms per milliliter of blood) of 25-hydroxyvitamin D are
desirable. It's often hard to achieve such levels via current
recommended intakes of D (see box at left) and a little sun exposure.
Many people, especially those over 60 and during the winter, have
levels of 20 ng/ml or below. There is some debate, however, about what
the optimal blood levels are. Dr. Hollis believes that 50 to 60 ng/ml
is a better goal.
Exposing your arms and legs or your full body to the midday sun
without sunscreen (and not in winter in the northern half of the U.S.
or in Canada) can produce high blood levels of D, but can cause skin
cancer. That leaves supplements as the best option, unless you know
your blood level is in the desirable range.
Practical matters. If you ask
for the blood test for vitamin D, your doctor may well say it isn't
necessary. It costs about $100, and insurance may not pay for it unless
you have osteoporosis or another condition potentially related to
vitamin D deficiency. And if the test shows low D levels and you start
taking higher doses of D, you should repeat the test to make sure
you're getting into the desirable range. Keep in mind, too, that blood
levels of D vary markedly by season-with the lowest readings in late
winter and early spring. A result of 25 ng/ml may be okay for late
winter, for instance, but is low for late summer.
Bottom line: It's too early to
recommend vitamin D testing for everyone. The large, long-term clinical
studies that would justify routine screening have not yet been done.
But talk to your doctor about testing, especially if you are over 60,
for instance, or have low bone density. In any case, consider taking
800 to 1,000 IU of supplemental D a day. For most people, that should
raise blood levels to the desirable range (above 30 ng/ml) or at least
close to it.
How much D to take. The
official recommended daily intakes for vitamin D, devised by the
Institute of Medicine, are 200 IU (international units) for people 50
and younger, 400 IU for those 51 to 70, and 600 IU for those 71 and
older. Most people don't consume that much D, unless they drink lots of
milk and/or take a multivitamin. However, many researchers believe
those guidelines are too low, and that a better goal for everyone,
especially those over 60 and/or with darker skin, is 800 to 1,000 IU a
day. Some people with low blood levels of D may need even higher
intakes to reach the desirable range.
Thus, fifteen leading nutrition experts last year urged the
Institute of Medicine to increase its recommended intakes of D.
Meanwhile, the Canadian Cancer Society now advises all Canadian adults
to take 1,000 IU of D a day during fall and winter, and older and
darker-skinned people to take this much year round. That is good advice
for most Americans as well.
The official "Upper Limit" for D, set many years ago, is 2,000 IU a
day. It's possible to get that much, or more, if you consume milk,
other fortified foods, and fatty fish, and also take a multivitamin and
combined calcium/D supplement. However, a review article in the
American Journal of Clinical Nutrition last January concluded that D is
not toxic up to 10,000 IU a day. We don't recommend that much, but you
needn't worry if you get somewhat more than 2,000 IU.
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