Vitamin D to ward off cancer? More study needed – Omaha World-Herald
Vitamin D’s role in helping build strong bones and teeth has been recognized for decades.
In recent years, a growing body of research has suggested other benefits of the “sunshine vitamin,” including helping to fend off some cancers.
But a definitive call regarding the vitamin’s place in cancer prevention apparently will have to wait a bit longer.
Confirmation has awaited the gold standard of science: a careful trial with participants randomly, and unknowingly, assigned to take D or a placebo. Several years ago a group led by a Creighton University researcher launched such a trial, following more than 2,300 rural Nebraska women who had gone through menopause.
The study, led by Joan Lappe, a Creighton professor of nursing and medicine, fell just short of showing a statistically significant reduction in risk for all types of cancer. The results were published last week in the prestigious Journal of the American Medical Association. The study was funded by the National Institutes of Health and Creighton.
Lappe said the study did find a 30 percent lower incidence of cancer in the women who took supplemental doses of vitamin D and calcium, which were well above currently recommended daily amounts. And further anal
ysis indicated that blood levels of vitamin D were significantly lower in women who developed cancer during the study than in those who remained healthy.
Lappe acknowledged, however, that more work is needed.
“I’m convinced that vitamin D does decrease the incidence of cancer,” she said, “but the scientific field won’t be that confident until they get that randomized trial.”
The study adds important information, noted Dr. JoAnn Manson of Brigham and Women’s Hospital and Harvard Medical School in Boston, in an editorial accompanying the study. But limitations, she wrote, “prevent it from delivering a clear verdict on the use of supplemental vitamin D or calcium for cancer prevention.”
Manson currently is leading a study of nearly 26,000 men and women across the United States examining the role of vitamin D supplementation in the prevention of cancer and cardiovascular disease. Another trial on a similar scale is underway in Australia.
“As ongoing large-scale trials report their findings, an improved understanding of these important relationships should come to light,” she wrote.
Epidemiologists began noticing decades ago that populations in northern latitudes, where the sun is too low from roughly mid-October to mid-March to allow skin to make D, had higher incidences of some cancers.
Some laboratory studies suggest a number of mechanisms by which D could work against cancer. An earlier trial in Nebraska women by Lappe and her colleagues also found a reduction in risk. But the study was aimed at bone health, not cancer, and was considered small. Manson noted that the results of other trials to date have been inconsistent.
Lappe and her co-authors wrote that one possible reason the most recent study missed the mark for significance was that the women started out with a relatively high average blood level of vitamin D — 33 nanograms per milliliter — compared with the rest of the U.S. population.
“With nutrients, you get to a threshold and more doesn’t do any good,” Lappe said. “These women were closer to the threshold. The more of the nutrient they have when you start supplementing them, the less of an effect you’re going to have.”
Additionally, Manson wrote that the study’s “null finding” may reflect the fact that most participants already were replete in D when the study started. “It may be that any potential cancer benefits of vitamin D supplementation are limited to or are most pronounced among individuals with relative vitamin D insufficiency,” she wrote.
Half of the women, who were 65 years old on average when the study started, randomly were assigned to take 2,000 International Units of vitamin D a day and 1,500 milligrams a day of calcium.
Over the four years, 45 women, or roughly 4 percent in the treatment group, were diagnosed with cancer. In the placebo group, 64, or a little less than 6 percent, were diagnosed. But the difference, the authors wrote, was not statistically significant.
Lappe said the study also involved mostly white women, which limits how broadly it can be applied to men and other races and ethnicities. Manson’s study includes a more racially and ethnically diverse group. Darker skin is less efficient at converting UV rays to vitamin D than lighter skin.
“I’m hoping Dr. Manson is able to find an effect,” Lappe said. “She’ll likely have people with lower levels (of vitamin D) than we did.”
Meantime, Lappe said, people should make sure they get adequate D. The wisest course is probably to take a supplement — specifically vitamin D3 — because it can be difficult to get enough D from food, and sunscreen blocks its conversion in skin.
The Recommended Dietary Allowances call for 600 International Units of vitamin D a day, or 800 IU for those 70 and older. Top food sources are fatty fish such as salmon, tuna and mackerel and fortified milk.
“We know that vitamin D is safe at the usual intake amounts,” she said. “It’s easy to take and inexpensive. It just seems (like) something everybody should be aware of.”