Two of the most popular and promising dietary supplements — vitamin D and fish oil — will be tested in a large, government-sponsored study to see whether either nutrient can lower a healthy person's risk of cancer, heart disease or stroke.
The study, to start later this year, will be one of the first big nutrition studies to target a specific racial group — blacks, who will comprise a quarter of participants.
People with dark skin are unable to make much vitamin D from sunlight, and researchers think this may help explain why blacks have higher rates of cancer, stroke and heart disease.
"If something as simple as taking a vitamin D pill could help lower these risks and eliminate these health disparities, that would be extraordinarily exciting," said Dr. JoAnn Manson, who with Dr. Julie Buring, of Brigham and Women's Hospital in Boston, will co-lead the study.
"But we should be cautious before jumping on the bandwagon to take mega-doses of these supplements," Manson warned. "We know from history that many of these nutrients that looked promising in observational studies didn't pan out."
Vitamins C, E, folic acid, beta carotene, selenium and even menopause hormone pills once seemed to lower the risk of cancer or heart disease — until big studies sometimes revealed risks instead of benefits.
Vitamin D is one of the last key nutrients put to a rigorous test.
Evidence has been building that many people are deficient in "the sunshine vitamin." It's hard to get enough from dietary sources like milk and oily fish. Cancer rates are higher in many northern regions where sunlight is weak in winter, and some studies have found that people with lower blood levels of vitamin D are more likely to develop cancer.
Fish oil, or omega-3 fatty acid, is widely recommended for heart health. But studies have mostly involved people who have heart problems or who eat a lot of fish, such as in Japan. Foods also increasingly are fortified with omega-3, so it is important to establish its safety and benefit.
However, getting nutrients from a pill is different than getting them from foods, and correcting a deficiency is not the same as healthy people taking large doses from a supplement.
The new study will enroll 20,000 people with no history of heart attack, stroke or major cancer — women 65 or older and men 60 or older. They'll be randomly assigned to take vitamin D, fish oil, both or placebos for five years.
"We're hoping to see a result during the trial, that we won't have to wait five years" to learn if supplements help, Manson said.