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Vitamin D: Are You Getting Enough?

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.

UC Berkeley Wellness Letter, February 2008