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 or heart disease or having a stroke.

The study will be one of the first big nutrition experiments ever to target a specific racial group — blacks, who will be one fourth 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 revealed risks instead of benefits.