"We found that women with type 2 diabetes who ate more fish had significantly lower risk of coronary heart disease and total death than those who rarely ate fish," says Frank B. Hu, M.D., lead author and associate professor of nutrition and epidemiology at the Harvard School of Public Health in Boston. "Previous studies have found that fish consumption reduces risk of heart disease in a largely healthy population. This is the first study to look at the relationship among diabetic patients, who have very high risk of heart disease." The American Heart Association recommends that adults, except pregnant women, eat two servings of fish a week. For those with, or at high risk of cardiovascular disease (CVD), supplementing fish in the diet with fish oil capsules may be advisable in consultation with a physician.
Also known as fish oil, omega-3 fatty acids from fish have been shown to reduce the risk of irregular heartbeats that can lead to sudden death, decrease blood triglyceride levels, improve the functions of blood vessels and reduce blood clot formation. These effects are particularly important for reducing risk for CVD among diabetics.
Even so, little data were available to confirm that diabetics who ate fish would receive the same benefits as people without diabetes who ate fish, Hu says. In addition, there was concern that fish oil might worsen control of blood sugar (glucose) among diabetic patients.
Hu and colleagues analyzed data from women with diabetes participating in the Nurses' Health Study, which was established in 1976 when 121,700 female registered nurses completed a questionnaire about their medical history and lifestyle. Every two years, follow-up questionnaires have been mailed to update information on risk factors and any new health problems. The current study includes 5,103 women who reported physician-diagnosed type 2 diabetes on any questionnaire from 1976-94. Women with a history of heart disease, stroke or cancer reported on the 1980 questionnaire (when diet was first assessed) or before were excluded.
The women were divided into five categories according to how often they ate fish: less than once a month, one to three times a month, once a week, two to four times a week, and five or more times a week.
Between 1980-96, the researchers documented 362 cases of heart disease (141 heart-related deaths and 221 nonfatal heart attacks). There were 468 deaths overall. Diabetic women who ate fish at least once a month were older, slightly heavier, typically didn't smoke, tended to have hypertension and high cholesterol, and took multivitamin and vitamin E supplements. Those who ate more fish also ate more fruits and vegetables but ate less red and processed meats.
Compared with diabetic women who seldom ate fish (less than once a month), the risk of developing heart disease was reduced on average by 30 percent in those who ate fish one to three times a month, 40 percent for those who ate it once a week, 36 percent in those who ate fish two to four times a week, and 64 percent in those who ate fish five or more times a week. Higher fish consumption was also associated with a significantly lower death rate.
Hu says that the association between higher fish consumption in diabetic women and better heart health can also be extended to diabetic men based on similar findings in studies of healthier men and women.
"One limitation of this study is that it is not a randomized clinical trial," Hu says. "Thus, the benefits we observed for fish may be due to other dietary and lifestyle factors related to fish intake." Even so, Hu says their findings are solid because of their "careful adjustment for many important cardiovascular risk factors.
"Regular fish consumption should be considered as part of a healthy diet for diabetes management," Hu says. "For individual patients, at least two servings of fish per week is recommended."
Fatty fish such as mackerel, lake trout, herring, sardines, albacore tuna and salmon are high in omega-3 fatty acids.
An accompanying editorial by Scott M. Grundy, M.D., director of the Center for Human Nutrition at the University of Texas Southwestern Medical Center at Dallas, provides a review of the current status of research on omega-3 fatty acids in fish, plants and supplements. Grundy says Hu's research supports previous prospective epidemiological studies that found omega-3 fatty acids offer protection against CVD. However, he urges that clinical trials of omega-3 fatty acids after a heart attack be conducted to determine if they can reduce coronary deaths in the short term.
Hu's co-authors are Eunyoung Cho, Sc.D.; Kathryn M. Rexrode, M.D.; Christine M. Albert, M.D.; and JoAnn E. Manson, M.D. The study was partly funded by the National Institutes of Health and the American Heart Association.
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