Fish oil capsules won’t help boost weight loss if you’re already dieting and exercising, new research in the American Journal of Clinical Nutrition shows.

Among a group of overweight and obese adults enrolled in a diet and exercise program, those who took omega-3 fatty acids didn’t lose any more weight than those given placebo capsules, Dr. Laura F. DeFina of The Cooper Institute in Dallas and her colleagues found.

There is evidence from animal studies that omega-3 fatty acids promote weight loss, DeFina and her team note, while studies in people have had mixed results. Because fish oil has many other potential health benefits, including cutting cholesterol, improving insulin sensitivity, and reducing blood pressure, “weight-loss programs associated with the use of omega-3 polyunsaturated fatty acids seemed appropriate,” they write.

To investigate whether fish oil enhanced the results of a diet and exercise regime, the researchers randomly assigned 128 sedentary overweight or obese men and women to take five fish oil capsules (providing a total of three grams of omega-3 fatty acids) or five placebo capsules every day for 24 weeks.

Participants were also instructed to do 150 minutes a week of aerobic exercise and 20 to 30 minutes of strength exercises at least twice a week.

The people in the omega-3 group lost 5.2 kilograms, or about 11.5 pounds, on average, compared to 5.8 kilograms (nearly 13 pounds) for the placebo group, not a statistically significant difference. People in both groups lost more than five percent of their body weight, enough to produce health benefits.

At the end of the study, there was no difference between the groups in measures of heart disease risk, such as blood pressure and cholesterol levels. However, omega-3 blood levels in the fish oil group increased to a level “previously found to have a positive cardiovascular effect,” the researchers note.

Not everyone actually completed the 24-week program; there were only 81 participants at the end. But the results were similar when the researchers looked only at those 81 people.

“Whereas one may not enhance weight loss by taking supplements with this level of omega-3 fatty acids,” they write, “the protective cardiovascular effect should still be realized because of the sheer increase in blood concentrations of the fatty acids.”

SOURCE: American Journal of Clinical Nutrition, online December 15, 2010.