Middle-aged and older men who eat fish every day are less likely than infrequent fish eaters to develop a collection of risk factors for heart disease, diabetes and stroke, a new study suggests.
Whether a fishy diet itself is the reason for the benefit is not clear from the findings.
But, the researchers say, the results are in line with studies showing that omega-3 fatty acids -- found most abundantly in oily fish like salmon, mackerel and albacore tuna -- may have heart benefits.
Clinical trials have shown, for instance, that omega-3s can lower triglycerides (a type of blood fat), and a prescription medication containing the fats -- sold as Lovaza -- is sometimes used to treat very high triglyceride levels.
Research has also suggested that fish oil supplements can help lower blood pressure and may reduce the risk of death among people with established heart disease, according to the National Institutes of Health.
The new study, of 3,500 Korean adults ages 40 to 69, found that men who had a serving of fish each day were 57 percent less likely than those who dined on fish less than once per week to develop metabolic syndrome over three years.
Metabolic syndrome refers to a collection of risk factors for diabetes, heart disease and stroke -- including abdominal obesity, high blood pressure, high blood sugar, low HDL cholesterol and high triglycerides. The syndrome is typically diagnosed when a person has three or more of those traits, and the National Health and Nutrition Examination Survey, a major study, found that it can double the risk of heart attack and stroke.
Past research had linked higher fish intake to a lower risk of some individual components of metabolic syndrome. But the current study is the first to show an association with the collection of risk factors, according to the researchers, led by Dr. Inkyung Baik of Kookmin University in Seoul.
They found that of 232 men who said they ate fish every day at the study's outset, 29 -- or about 12 percent -- developed metabolic syndrome over the next three years. Of the 190 men who said they ate fish less than weekly, 16 percent developed metabolic syndrome.
When Baik's team considered other factors -- including the men's income and education, body weight and lifestyle habits such as smoking and exercise -- daily fish consumption was linked to a 57 percent lower risk of metabolic syndrome versus less-than-weekly consumption.
There was no such association seen among women, however.
The reason for the gender discrepancy is not clear. The researchers speculate that, as a group, women may not have gotten enough omega-3 fatty acids from fish to show a reduction in the risk of metabolic syndrome.
Men in the top 10 percent for omega-3 intake from fish typically got 786 milligrams (mg) per day, while their female counterparts got 563 mg.
Men in that top omega-3 group also showed a lower risk of metabolic syndrome than those in the bottom 10 percent -- which, according to Baik's team, supports the theory that it is the fats that explain the connection between fish and the risk of metabolic syndrome.
However, this type of study cannot by itself prove cause-and-effect.
"Our findings need to be confirmed by other investigators," Dr. Chol Shin, one of the researchers on the study and a professor at Korea University Ansan Hospital, told Reuters Health in an email.
And in general, it is not yet clear what the optimal daily intake of omega-3 fatty acids might be, Shin noted.
In the U.S., there is no recommended daily allowance set for EPA and DHA, the two major omega-3 fatty acids, writes registered dietitian Gretchen K. Vannice, in an editorial published with the study.
However, she adds, the American Dietetic Association and other groups have suggested a daily allowance of anywhere from 250 to 500 mg of EPA and DHA, based on the overall body of research into omega-3s and heart health.
Two servings of fatty fish per week would be enough to achieve those levels, Vannice notes.
That also fits in with the current advice from the American Heart Association that adults strive to eat two servings of fish, preferably fatty varieties, each week.
However, even if eating fish regularly does help reduce the risk of metabolic syndrome or its components, it would be only one of many factors involved. A healthy weight, not smoking, regular exercise, and a balanced diet rich in fruits, vegetables, high-fiber foods and "good" unsaturated fats are all considered important.
The current study was funded by Kookmin University and a Korean government grant. Vannice is a nutrition research consultant who has consulted for companies that market omega-3 supplements.
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