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.
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 vitamin 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.
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 vitamin D, according to some recent studies. Obesity is associated with reduced blood levels.
The case for testing
A recent meta-analysis inArchives of Internal Medicineof 18 studies found that people taking D supplements (usually 400 to 800 IU a day) had a 7 percent 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 vitamin 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.