As much as vitamins and minerals stay the same—oranges will always have Vitamin C, bones will always need calcium, zinc will always have an atomic number of thirty—our perception of them seems to change with a voguish flair. In the past few years, antioxidants have become mod and vitamin C and E supplements were bought and sold with a zealot-like fervor. They hogged the spotlight until the micronutrients selenium and zinc vied for attention (deserved or not) in the prevention of everything from colds to cancer. However, this year the focus has been on old standby vitamin D, whose role and function we thought was well understood. It was also somewhat boring: Vitamin D helps build strong bones. Drink your milk. End of story.
However, new research is making sure vitamin D gets its due time in the sun so to speak, raising questions about how much we need—and if its role is as straightforward as we think.
What Is It?
Vitamin D is a fat-soluble vitamin found in food and made endogenously from sunlight. It’s sometimes called the “sunshine vitamin” because ultraviolet rays from the sun help stimulate vitamin D synthesis by our bodies.
Vitamin D helps maintain normal levels of calcium and phosphorus in our blood. Because of its role in calcium absorption, vitamin D helps prevent brittle and malformed bones.
Too little vitamin D—either from limited exposure to sun or because of inadequate dietary absorption—can cause rickets, a disease characterized by soft bones and skeletal deformations. Together with calcium, vitamin D can protect older adults from osteoporosis. At the cellular level, vitamin D assists in neuromuscular and immune functioning.
Too much vitamin D is rare, but it can cause the body to absorb too much calcium. This can lead to kidney stones, vomiting, weakness, and weight loss. Diet and sunlight are unlikely to cause toxicity; it’s much more likely to be from high intakes of supplements.
Known and Unknown Role in Disease
While it’s known that vitamin D aids in bone health, how it interplays with other diseases, and just how much of it we need, is up for debate.
This past October, the American Academy of Pediatrics announced that it doubled the vitamin D recommendation for infants, children, and adolescents, from 200 international units (IU) to 400 IU per day. Many supplements and multivitamins already contain this amount as a minimum.
However, federal committees that establish recommended doses haven’t increased their levels. This is due in part to well-known adverse effects of too much vitamin D, as well as a believed lack of evidence to support benefits. The U.S. Department of Health and Human Services and the Institute of Medicine are currently revisiting the recommendations.
It is also unclear the role vitamin D may have in diseases such as cancer, diabetes, autoimmune diseases, and mortality in general.
In animal and laboratory studies, vitamin D has shown some promise in protecting against cancer. However, large-scale epidemiological studies haven’t been as clear. In one study, high vitamin D intakes were shown to be protective against colon cancers; in another, no effect was found from vitamin D supplementation. Further research is needed to assess whether the nutrient is protective or whether it has any effect at all.
Vitamin D may play a role in autoimmune disorders. This year, a research review indicated that extra vitamin D in childhood reduces adult type 1 diabetes risk. Promising, but the review was based on only five studies and the results are insufficient to come to a definitive conclusion. A 2006 article in the Journal of the American Medical Association found that low levels of vitamin D were associated with an increased risk of multiple sclerosis. Again, nothing conclusive can come out of this associational study.
Highlighting some of the problems with vitamin D research is an article recently published by the National Institutes of Health (NIH) entitled “Key Gaps Remain in Understanding Health Effects of Vitamin D.” These include the fact that research hasn’t identified the vitamin D levels needed to achieve desired health outcomes at various stages in life.
Where to Get It?
While it can be frustrating for consumers to hear the familiar phrase, “more research is needed,” the NIH assures us that most people meet their vitamin D needs through exposure to sunlight (which, of course, must be weighed against skin cancer risk). Sun exposure is the best source of vitamin D, although weather, smog, and sunscreen can affect one’s ability to get adequate amounts of the vitamin from the sun. Ten to fifteen minutes of sun exposure two times per week without sunscreen is about all we need to produce sufficient amounts of vitamin D. During the winter, and for those in sunless climates, food can provide adequate amounts of the vitamin.
Almost all milk in the U.S. is fortified with 10 micrograms of vitamin D per quart. According to the National Institutes of Health, children and adults aged nineteen to fifty years of age need 5 micrograms or 200 international units of Vitamin D; those aged fifty-one to seventy need 10 micrograms; and those aged over seventy-one years need 15 micrograms. That means two glasses of vitamin D-fortified milk provide half an adult’s vitamin D requirement. Other natural sources include fish (such as salmon, tuna, sardines, and mackerel), eggs, and milk products. Many cereals and orange juices are fortified as well. For those who can’t get their vitamin D from sun or food, supplementation is an option.
Which goes back to the question: how much? You can go by the NIH guidelines, your doctor’s, or your own. But for the answer to be completely clear, more research is needed.