updated 6/22/2009 2:21:32 PM ET 2009-06-22T18:21:32

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 getting cancer, heart disease or having a stroke.

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The study will be one of the first big nutrition experiments ever to target a specific racial group — blacks, who will comprise one quarter of the participants.

People with dark skin are unable to make much vitamin D from sunlight, and researchers think this deficiency 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. She and Dr. Julie Buring, of Harvard-affiliated 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 they were tested in big studies that sometimes revealed risks instead of benefits.

In October, the government stopped a big study of vitamin E and selenium pills for prostate cancer prevention after seeing no evidence of benefit and hints of harm.

Vitamin D is one of the last major nutrients to be put to a rigorous test.

Cancer rates higher where sun is weaker
For years, evidence has been building that many people are deficient in "the sunshine vitamin." It is tough to get enough from dietary sources like milk and oily fish. Cancer rates are higher in many northern regions where sunlight is weak in the 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. However, studies of it so far have mostly involved people who already 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.

"Vitamin D and omega-3s have powerful anti-inflammatory effects that may be key factors in preventing many diseases. They may also work through other pathways that influence cancer and cardiovascular risk," Manson said.

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.

20,000 people to be studied
The new study, which will start later this year, will enroll 20,000 people with no history of heart attacks, stroke or a major cancer — women 65 or older and men 60 or older. They will be randomly assigned to take vitamin D, fish oil, both nutrients or dummy pills for five years.

The daily dose of vitamin D will be about 2,000 international units of D-3, also known as cholecalciferol, the most active form. For fish oil, the daily dose will be about one gram — five to 10 times what the average American gets.

Participants' health will be monitored through questionnaires, medical records and in some cases, periodic in-person exams.

"We're hoping to see a result during the trial, that we won't have to wait five years" to find out if supplements help, Manson said.

Researchers also plan to study whether these nutrients help prevent memory loss, depression, diabetes, osteoporosis and other problems, Buring said.

The $20 million study will be sponsored by the National Cancer Institute, with the National Heart, Lung and Blood Institute and other federal agencies. Pharmavite LLC of Northridge, Calif., is providing the vitamin D pills, and Ocean Nutrition Canada Ltd. of Dartmouth, Nova Scotia, is providing the omega-3 fish oil capsules.

Copyright 2009 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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