A controversy is brewing over vitamin D. Will an extra dose protect you from cancer and infections?
Vitamin D builds strong bones. That's what everyone knows. The discovery of how to isolate this nutrient that helps the body absorb calcium led to a Nobel Prize in chemistry in 1928. Once authorities started adding the vitamin to milk, rickets, a previously common bone deformity, virtually disappeared. More recently some trials have shown that supplements can boost bone density or reduce fractures and falls in the elderly.
But now a vocal band of researchers are touting a far larger role for this once obscure vitamin. They cite a flurry of intriguing, if preliminary, epidemiological and lab studies hinting that vitamin D may play a role in staving off a wide range of diseases, including colon cancer, infections, multiple sclerosis and other autoimmune ailments and possibly even heart disease. "Up until now we looked at vitamin D the way we look at an iceberg. Eighty-five percent of its function has been hidden, and we had no idea until two or three years ago," says an excited Robert Heaney, an endocrinologist at Creighton University in Omaha. "The field has just exploded." Adds Medical University of South Carolina biochemist Bruce Hollis: "I often say its skeletal effects are the least interesting thing we know about it."
Vitamin D is naturally found in few foods other than oily fish. Most of what we get is synthesized in the skin as it absorbs the sun's ultraviolet light. Just 15 short minutes in the summer sun produces a blast of 10,000 international units. Ancient humans spent all their time outdoors and got such doses. By contrast, the government recommends a mere 200 to 600 IU a day, depending on age--what you get in two to six cups of fortified milk.
Vitamin D researchers estimate that up to half of Americans might be getting inadequate amounts, especially in the winter months when the sun is low. Dark-skinned people living in northern locales are most at risk, as they absorb sunlight more poorly than light-skinned folks. The wide use of sunscreen to prevent skin cancer has also hastened the decline in vitamin D levels.
In Pictures: New Benefits From Your Favorite Vitamins?
"Most people in this world are vitamin D deficient. It is a major health issue," proclaims Boston University's Michael Holick, a doctor and biochemist who has studied vitamin D since 1969.
Robert Heaney, who sat on the panel that made the dose recommendations in 1997, is now agitating to get them raised, calling them "grossly inadequate." Many vitamin D researchers take supplements with 2,000 IU a day or more.
Holick has a solution--limited direct exposure to the sun--that infuriates many dermatologists worried about the skin cancer risk. (A far better idea, they say: take a supplement.) In 2004 he published his book The UV Advantage, touting the benefits of sun exposure, and was kicked out of the bu dermatology department, where he previously held a joint appointment. Department Chairman Barbara Gilchrest says she asked him to resign because of his ties to the tanning salon industry. Holick gives speeches to salon owners and receives some research funding from the industry-funded UV Foundation.
A wide range of vitamin D benefits beyond bone growth is certainly plausible. Vitamin D works inside the cell nucleus as a basic building block to help turn genes on or off. Starting in the 1970s, researchers began finding the receptor for vitamin D in a huge variety of cells that have nothing to do with bone growth, including breast, colon, lung, brain, prostate and white blood cells. More recently studies using DNA chips have found that vitamin D can raise or lower the activity of at least 1,000 genes, says McGill University molecular biologist John White.
One analysis looked at the effects of vitamin D supplements on mortality in 18 randomized trials of 57,311 people (originally performed to assess vitamin D's bone effects). It found that the supplements reduced the overall death rate by a statistically significant 7%, according to results published in the Sept. 10, 2007 Archives of Internal Medicine. This might "translate into one or two more years of life" for a regular supplement taker, says lead author Philippe Autier of the International Agency for Research on Cancer. By contrast, high doses of antioxidants such as beta carotene or vitamin E slightly boosted the death rate in trials, a giant Danish study found last year.
Many epidemiological studies link low vitamin D levels to a high risk of getting or dying from various cancers down the road, especially colon cancer. In 1980 researchers at Johns Hopkins University noticed that northern states in the U.S. had higher colon cancer death rates than southern ones and theorized that vitamin D might be responsible.
Intrigued by the concept, Harvard epidemiologist Edward Giovannucci in the mid-1990s began examining data from a continuing study of 33,000 female nurses who submitted blood samples in 1989 and another following 18,000 male health workers who gave blood samples in 1993. He had to wait a while for data to roll in. But in 2004 he found that the nurses with the highest initial vitamin D levels had a 47% lower risk of colon cancer over the next decade; in 2007 his researchers reported that the male health workers with the best D levels had a 54% lower colon cancer risk.
In Pictures: New Benefits From Your Favorite Vitamins?
This doesn't prove cause and effect, Giovannucci admits. But if there is that connection, he says, "you could prevent 30% to 50% of colon cancer by getting everyone to the top levels." Muddying matters, a 36,000-patient government-sponsored trial in 2006 compared modest doses of vitamin D with a placebo and found the supplements had no effect on colon cancer rates. It could have been because the dose was too low, the trial researchers admitted.
How vitamin D might ward off cancer is murky. But it's known that vitamin D stimulates white blood cells to produce a powerful natural antibiotic called cathelicidin. In the Mar. 24, 2006 issue of Science, scientists led by ucla dermatologist Robert Modlin found that when white blood cells were mixed with blood serum samples from African-Americans (who are prone to low vitamin D levels), they produce 63% less of this antibiotic than if the cells were mixed with blood samples from Caucasians. So, says Georgetown University immunologist Michael Zasloff, "Vitamin D has the capacity to turn on powerful antimicrobial genes." He predicts there will be new ways of staving off infections by modulating vitamin D levels.
None of this proves that taking extra vitamin D will help healthy people. "The links beyond bone get quite speculative," says Brown University dermatologist Martin Weinstock. "It is an article of faith [to say that] if you took a completely healthy person and gave them vitamin D supplements that they would be healthier ten years later," says Boston University's Gilchrest, who calls the evidence linking vitamin D to nonskeletal diseases "extremely weak" and "inconclusive." For every study showing a link, "there are studies showing the reverse that the vitamin D advocates don't talk about." She worries that the theoretical benefits of vitamin D are being "kidnapped" by the tanning industry to stave off regulation and boost business--putting people at risk of getting melanoma. If you believe in the case for vitamin D, she advises, take supplements and stay pale.
Proving vitamin D prevents nonbone diseases will require big human trials comparing vitamin D supplements with dummy pills. Such trials are largely in the planning stages, and it is not clear how fast they will get done. "If this were a patentable drug you would see tremendous push and hype on this," says Creighton's Heaney.
No comments:
Post a Comment