Vitamin D: How Much is Enough?
While vitamin D’s role in strengthening bones is well established, its links to cancer and immune-system malfunctions have only recently emerged. At the Harvard School of Public Health, nutrition experts say large segments of the population don’t get enough vitamin D and are urging the U.S. Food and Drug Administration to raise the daily recommended dose, from 400 international units to 800. For an update on what’s known so far about this important nutrient, the Harvard Public Health Review spoke with HSPH Professor of Nutrition and Epidemiology Edward Giovannucci.
Q: What are the documented benefits of vitamin D?
A: Vitamin D’s best-known role is in building strong bones. We’ve seen plenty of advertising urging people to take calcium to strengthen bones, but people also need to know that calcium can’t do the job well if they’re low on vitamin D. The two micronutrients work synergistically, and a deficiency in either can lead to osteoporosis and associated bone fractures.
Q: What other benefits of vitamin D are researchers starting to uncover?
A: A growing number of human metabolic, epidemiologic, and animal studies suggest important new roles for vitamin D. Certain cancers and immune dysfunction are strongly associated with vitamin D deficiency.
In March, several HSPH and Harvard Medical School colleagues with the Physicians’ Health Study and I found a connection between prostate cancer and low blood levels of vitamin D (specifically, of proteins produced as vitamin D is metabolized). Men also unlucky enough to have certain variant genes involved in vitamin D processing were 2.5 times as likely to develop aggressive, deadly prostate cancers. Moreover, more than two-thirds of the nearly 15,000 men in this study were significantly deficient in the vitamin.
My own research with the large, long-term Health Professionals and Nurses’ Health studies found that populations with adequate vitamin D levels have about half the risk of colon cancer as people who don’t get enough. Other cancers have been linked to lower vitamin D levels, too, particularly those of the digestive tract.
Vitamin D deficiency also translates into a weakening of the immune system and an increased risk of so-called autoimmune diseases, including type 1 diabetes. A recent study led by Associate Professor of Epidemiology and Nutrition Alberto Ascherio found that the higher the vitamin D levels one has, the lower the risk of multiple sclerosis. Last year, HSPH Dean Barry Bloom and collaborators showed that the immune system’s response to some infections is critically dependent on vitamin D.
Their study seems to help explain why people have historically seen sunshine (a major source of vitamin D) as a useful therapy for patients with tuberculosis infections.
Q: How much vitamin D do people need daily?
A: The current daily recommended allowance varies with age from 200 to 600 international units (IU), a standard set by the Nutrition Board of the National Academy of Sciences in 1997. Randomized trials have since found that individuals receiving 800 IU per day had a lower risk of osteoporosis fractures, while 400 IU per day did not show this benefit. Most experts now believe 1,000 to 2,000 IU per day from all sources—sun, diet, supplements—may be what we need for optimum health.
The body is smart: It makes no more vitamin D than it needs. But a total intake greater than 2,000 IU per day has generally not been recommended. This upper limit, imposed for safety’s sake, is probably very conservative. Recent evidence suggests that even doses upwards of 10,000 IU a day aren’t toxic, though such high intakes are not recommended.
Q: What are the best sources of vitamin D?
A: The sun is the most potent source. When the sun’s ultraviolet rays hit the skin, the skin makes the vitamin, which is rapidly absorbed in the blood and can be stored for several months, mostly in the blood and fat tissue. This is why it’s hard to figure out how much supplemental vitamin D people might need.
If you spend a fair amount of time outdoors, you probably don’t need a vitamin D supplement. A light-skinned person living in Boston who takes walks in the summer with the face, neck, and arms exposed for 15 minutes gets enough. A 30-minute, full-body exposure to summer sun at noon without any sunscreen protection triggers the release of about 20,000 IU into the bloodstream. Most of that is stored. Someone dark-skinned would, with the same exposure, generate about half or less as much vitamin D.
For people who rarely get sun exposure or who live in the north, where the body can’t make vitamin D in the late autumn and winter months, even 800 IU per day during these months may be too low. Dark-skinned individuals, whom research suggests are more prone to colon cancer than whites, should probably take vitamin D supplements, considering that their colon cancer risk may be related to insufficient vitamin D.
Evidence shows modest exposure to sun can have long-term benefits. However, I don’t recommend excessive sun exposure because of the well-known risk of skin cancer.
Q: What are other good sources?
A: It’s hard to get vitamin D from diet. That’s why supplements are often a good idea. In the United States, some foods are fortified with vitamin D, such as milk. A glass is supposed to contain about 100 IU but in reality may contain as little as 50 IU on average. To get 800 IU from milk, one would have to drink at least 8 glasses of milk a day.
Fatty fish is the only natural source of vitamin D. A 3.5 oz serving of cooked salmon, for example, has 360 IU; 3 oz. of canned tuna has 200; and 13.4 oz. of canned sardines has 250.
Q: Who might benefit most from supplements?
A: Most people will benefit. The elderly; dark-skinned individuals; obese individuals, in whom fat cells extract vitamin D from blood and hoard it; and those who avoid the sun are all at risk for a less-than-adequate intake or a deficiency. People living in northern latitudes—for Americans, that means north of an imaginary line connecting San Francisco to Philadelphia—only make vitamin D from March through September. Although vitamin D stored from summer sun exposure lasts for some months, most of these individuals will be deficient by late winter.
Q: What’s the bottom line?
A: I suggest taking supplements of no more than 2,000 IU per day of vitamin D without specific medical reasons until researchers have more definitive data on the benefits and risks. Look for supplements that contain vitamin D3 (cholecalciferol), which is three to four times more potent than vitamin D2 (ergocalciferol).
Throughout human evolution, people were naturally and continually exposed to the sun. But today, many of us spend most of our day indoors. Researchers are not recommending massive doses of vitamin D. Instead, we’re saying that many people are vitamin D deficient—and therefore at higher risk for everything from bone fractures and certain cancers to immune-system-related disorders.