In the world of health and nutrition, omega-3s rank among the superstars. These fatty acids -- found naturally in some foods, added to others, and available as supplements -- show so much promise in fighting disease that researchers have explored their effects on more than 30 conditions. In 2007 alone, investigators published 600-plus studies on omega-3s, covering everything from heart disease to depression to cancer. From the buzz, it would seem these fatty acids can cure just about anything. Delve a little deeper, though, and it's clear that solid scientific evidence in support of omega-3s doesn't pan out for more than a handful of diseases. "Knowing when to use omega-3s, and for what conditions, can be tricky," says Brent Bauer, M.D., director of the Complementary and Integrative Medicine Program at the Mayo Clinic. But with a little knowledge and advice from experts, you can figure out how to make these important nutrients work for you.

Making the Grade

A type of polyunsaturated fat, omega-3s -- eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and alpha-linolenic acid (ALA) -- aid in normal growth and development, reduce inflammation, and promote cognitive health. Most experts agree on the importance of omega-3s in the diet. What's less clear is how much they contribute to fighting specific diseases. To shed light on some answers, Natural Standard, a Cambridge, Massachusetts-based research firm, created a report on omega-3s that assigned letter grades to their various uses. More than 30 researchers analyzed hundreds of studies, looking at both omega-3s in the diet and supplement use, says company cofounder and doctor of pharmacy Catherine Ulbricht. To better understand the results (below), we asked several experts to share their opinions of the grades. Here's what we found.

Grade: A
According to research, your heart benefits the most from omega-3s. DHA and EPA can lower triglycerides, help prevent a second heart attack, and, if you have high blood pressure, may lower it slightly.

Experts say: William Harris, Ph.D., a professor of medicine at Sanford School of Medicine at the University of South Dakota and an omega-3 expert, agrees with the first two findings: lowering triglycerides and helping prevent a second heart attack. To achieve the first result, Harris notes, you'll need to take supplements (versus simply adding omega-3-rich foods to your diet). Harris is not as impressed with omega-3s' effect on high blood pressure, however, because the drop is small. "With high blood pressure, when you take omega-3 supplements in high doses, you see maybe a 5 percent drop max," he says.

Grade: B
Natural Standard gave this grade to omega-3s as a tool for preventing cardiovascular disease and improving rheumatoid arthritis. In its opinion, the supporting evidence is "good" and potential benefits exist, but it would like to see more, and, in some cases, better designed studies.

Experts say: While Harris concedes this use hasn't been studied as intensely, he thinks taking omega-3s to prevent a first heart attack makes just as much sense as taking them to ward off a second (which earned an A). Roberta Lee, M.D., medical director of the Continuum Center for Health and Healing, an integrative health program affiliated with Beth Israel Medical Center in New York City, also has a more favorable view of omega-3s and rheumatoid arthritis than a mere B grade. She says they're "important to maximize" treatment because it's an inflammatory condition.

Grade: C
For 20-plus other conditions that have been studied -- including depression, cancer, dementia, and stroke -- scientific evidence is unclear or conflicting. For now, using omega-3s for these issues isn't a sure bet.

Experts say: Some holistic practitioners may still recommend omega-3s for certain ailments based on their own results with patients. Lee, for instance, believes most chronic diseases with an inflammatory element will benefit from omega-3s. "I've seen amazing changes with eczema, asthma, psoriasis, and PMS," she says. Lee emphasizes that she considers the entire body when treating someone, and she makes other changes along with upping omega-3 intake.

Grade: D
Using omega-3s to help diabetes and lower high cholesterol were two among several conditions earning Ds from Natural Standard, meaning the scientific evidence didn't bear out. (No Fs were given.)

Experts say: Harris agrees with this grade. "I think some people do use fish oils in an attempt to lower cholesterol," he says, "but it's all based on very fuzzy logic. They have high cholesterol; fish oil is good for the heart; therefore, fish oil must lower cholesterol. But this is wrong thinking." What people might consider, he adds, is that omega-3s can help lower the risk for cardiovascular disease in people with high cholesterol or with diabetes.

Do fish oil supplements have too much mercury? See the research.