Professor of Nutrition and Epidemiology, Departments of Nutrition and Epidemiology, Harvard School of Public Health
1. There’s been a lotof news lately about vitamin D’s role in prevention of cancer, heart disease,and other chronic diseases. What do we know today—and what questions remain?
Vitamin D has a well-established role in maintaining calciumlevels in the body and in building strong bones. That’s why vitamin Ddeficiency could increase the risk of osteoporosis. In addition, elderly whodon’t get enough vitamin D have weaker muscles and are more prone to falls,which could further increase the risk of fractures.
Research over the past few decades has uncovered several newpotential roles of vitamin D. Many types of cells in the body can use vitamin Dto help regulate critical cellular functions. Vitamin D deficiency could thuslead to several potential problems, such as a weakened immune system and anincreased risk of colon and other cancers. Immune dysfunctions may lead to anincreased risk of autoimmune diseases including type 1 diabetes and multiplesclerosis, and perhaps to some infectious diseases such as tuberculosis.
Our latest research in the Health Professionals Follow UpStudy suggests that vitamin D deficiency may also be linked to heart disease. Wechecked the vitamin D blood levels in men who were healthy, and then followedthem for 10 years. Men who were deficient in vitamin D were twice as likely tohave a heart attack as men who had adequate levels of vitamin D. Otherscientists have found evidence that vitamin D plays a role in controlling bloodpressure and preventing artery damage, and this may explain our findings.
Vitamin D’s role in many of these diseases has not beendefinitively proven through a randomized trial. Nonetheless, an increasing bodyof human metabolic studies, epidemiologic investigations, and animal studiessupport an important role of vitamin D that goes far beyond bone health.Vitamin D’s complex role is currently the focus of intense scientific scrutiny,and more definitive answers should become available over time.
2. How much vitamin D do I need to get a day to help prevent osteoporosis and perhaps prevent otherchronic diseases? How much is too much?
Several randomized trials found that individuals whoreceived vitamin D supplements of 800 IU per day lowered their risk ofosteoporosis; trials that provided only 400 IU per day did not show thisbenefit. It is reasonable to postulate that more than 800 IU per day wouldprovide even more benefit, but this is not proven. A promising analysis ofmultiple studies suggests that taking 400 to 800 IU of vitamin D each day maylower mortality rates by 7 percent; there are some limitations to thisanalysis, however, and more research needs to be done before any broad claimscan be made about vitamin D supplements and mortality.
Vitamin D intakes greater than 2,000 IU per day havegenerally not been recommended, and this is probably a conservative upper limitfor safety. In fact, recent evidence suggests that doses up to 10,000 IU a daydo not cause toxicity—but this does not mean that people should start taking10,000 IU of vitamin D every day.
A complicating factor in determining how much vitamin D weneed to get from diet or supplements is that exposure to sunlight producesvitamin D in the skin, which is then rapidly absorbed in the blood. Thus, avitamin D intake of 800 IU per day may be too low for a person who rarely getssun exposure (or for someone living in the north, where vitamin D cannot bemade in the late-autumn and winter months). Yet that level of intake may noteven be necessary for a person who receives ample sun exposure.
3. Since milk isfortified with vitamin D, should I drink lots of milk to get the benefits ofvitamin D? If not, what other foods are good sources of vitamin D? Or can Ijust get enough vitamin D from the sun?
Milk alone is unlikely to be an adequate source of vitaminD. True, milk is fortified in the U.S. (it is not fortified in mostcountries). Each glass of fortified milk should contain about 100 IU of vitaminD (but on average, it may contain only 50 IU). So someone would have to drinkat least 8 glasses of milk per day to get 800 IU of vitamin D. Moreover, mostexperts now conclude that 1,000 to 2,000 IU per day of vitamin D may be what weneed for optimum health.
Fatty fish is the only good natural source of vitamin D. A3.5 oz serving of cooked salmon, for example, has 360 IU of vitamin D; 3 oz ofcanned tuna has 200 IU; and 1 3/4 oz of canned sardines has 250 IU.
Sun exposure can be a good source of vitamin D for somepeople: For example, in a light-skinned person, one, 30-minute, full bodyexposure to summer sun at noon triggers the release of about 20,000 IU ofvitamin D into the circulation; in a dark-skinned person, that amount of summersun would create about half as much vitamin D. But one must be very cautiousabout making recommendations to increase sun exposure solely to increasevitamin D intake. There are downsides to receiving excessive sun exposure, suchas skin cancer and premature skin aging.
4. Should peopleconsider taking vitamin D supplements? If so, who might benefit most fromsupplements?
Because we may need more vitamin D than most people get intypical diets, and because of the potential downsides of excessive sunexposure, supplementation may be warranted. It may turn out that most peoplemay benefit from supplements.
Several groups are at risk for vitamin D deficiency orless-than-adequate intakes-in particular, the elderly, dark-skinnedindividuals, obese individuals, and those who avoid the sun. People who live inmore northern latitudes (such as Boston, Mass., Milwaukee, Wis., or Portland,Ore. ) can only make vitamin D from March through September; supplies storedfrom summer sun exposure must last for many months, and by late winter, most ofthese individuals may be deficient.
Although definitive evidence is not available currently,supplements of at least 1,000 to 2,000 IU per day of vitamin D may be warranted;look for supplements that contain vitamin D3 (cholecalciferol), rather thanvitamin D2 (ergocalciferol), since vitamin D3 is three to four times morepotent than vitamin D2. As always, it is important to discuss use ofsupplements with your personal health care provider. I suggest not taking morethan 2,000 IU per day of vitamin D in supplement form without specific medicalreasons until more definitive data are available concerning the benefits andrisks. If you fall into one of the groups that are at a higher risk of vitaminD deficiency, ask your doctor to order a blood test for vitamin D, since yourdoctor may find that you need a larger daily supplement dose, on the order of3,000-4,000 IU, to achieve adequate blood levels.
The aim of the Harvard School of Public Health Nutrition Source is to provide timely information on diet and nutrition for clinicians, allied health professionals, and the public. The contents of this Web site are not intended to offer personal medical advice. You should seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this Web site. The information does not mention brand names, nor does it endorse any particular products.