Several studies have demonstrated low rates of cardiovascular disease in groups with a high dietary intake of fish. Some secondary-prevention trials showed that increased fish or fish-oil consumption was beneficial in persons with pre-existing coronary disease. However, significant studies were not conducted in women. Hu and colleagues conducted this analysis of the Nurses' Health Study to determine whether there is an association between risk of coronary heart disease (CHD) and intake of fish and long-chain omega-3 fatty acids.

The Nurses' Health Study enrolled 121,700 female nurses in 1976 and gathered information from questionnaires administered every two years. Among the questions were those about diet and cardiac diseases and/or events. In 1980, there was one question about fish intake (specifically, it asked how often the women ate a 6- to 8-oz portion of fish). In subsequent years, four questions about fish intake were added. These questions were designed to collect information about the quantity and type of fish consumed: dark-meat fish (mackerel, salmon, sardines, bluefish, swordfish); canned tuna or other fish; and shrimp, lobster, or scallops. The amount of omega-3 fatty acids was calculated respective to the type of fish or seafood. Incidence of CHD, including CHD deaths and nonfatal myocardial infarctions (MIs), was the end point for this study. Diagnostic evidence was sought and confirmed when possible.

There were 1,513 cases of CHD (484 deaths from CHD and 1,029 nonfatal MIs) during the 16-year follow-up period. The women who ate more fish tended to be slightly older, nonsmokers, more likely to take aspirin and multivitamins regularly, and more physically active. Their rates of obesity and hypertension, however, were higher. After adjusting for age and cardiovascular risk factors, intake of fish was inversely associated with incidence of CHD. Specifically, those with higher intakes of omega-3 fatty acids had significantly less risk of CHD and all-cause mortality. This beneficial association occurred independent of other cardiovascular or dietary risk factors for CHD (such as trans-fatty acid and saturated fat intake).

The authors conclude that the inverse relationship between fish intake and risk of CHD in women cannot be interpreted as a causative relationship, but that the evidence is strong enough to back the American Heart Association's recommendation that fish be eaten twice weekly for CHD prevention.