Fish has long been seen as a healthy food, high in protein and omega-3 fatty acids, which may be associated with lower risk for heart disease and better fetal and brain development.

Those adults currently at risk for cardiovascular disease may cut that risk by eating seafood regularly, especially fish with lots of omega-3 fatty acids.

The report offered guidelines that suggest a limited intake of fish -- two 3-ounce (85-gram) servings a week -- for women of child-bearing age and children aged 12 and under, but even these two groups can safely eat 12 ounces (340 grams) a week.

For healthy adolescent and adult males and for women who will not become pregnant, and for adults at risk for coronary heart disease, the guidelines list no upper limit.

Women of child-bearing age or who are breast-feeding and children age 12 or younger can benefit especially from eating fish such as salmon that are rich in omega-3 fatty acids -- "good fats," according to the report.

Dr. Dariush Mozaffarian, lead author of the Harvard study and cardiologist at the university's schools of public health and medicine, and colleagues concluded eating one or two servings a week of fish rich with omega-3 fatty acids reduces the risk of dying from a heart attack by 36 percent; it cuts the rate of death overall by 17 percent.

However, contamination with mercury, polychlorinated biphenyls (PCBs) and other chemicals can offset some of the benefits, the report found.

Large predatory fish like shark, swordfish, tilefish or king mackerel, which live longer, may absorb more toxins.

Healthy adolescent and adult men and women who will not become pregnant may reduce their risk for future cardiovascular disease by eating seafood regularly, but people who eat more than two servings of fish a week should make sure to eat a variety of different fish to avoid exposure to contaminants from a single source, the report said.

In addition, a Harvard study, to be published Wednesday in the Journal of the American Medical Association, also agreed the benefits of eating fish are greater than the health risks from contaminants.