THURSDAY, Dec. 17, 2009 (HealthDay News) -- In a surprise finding, Canadian researchers report that the immediate effect of the fish oil fatty acids that are good for the heart is a short-term increase in blood fats and the molecules that help them form clots.
"We were surprised to find that the acute response has some potentially negative effects in comparison to what you might expect from chronic, long-term intake," said Lindsay E. Robinson, an associate professor of nutrition at the University of Guelph, and leader of the group reporting the finding in the January issue of the Journal of Nutrition.
However, the study results shouldn't affect the current recommendation for eating more oily fish to get the omega-3 polyunsaturated acids that reduce the risk of blood clots that can cause heart attacks and stroke, Robinson said.
"The recommendation to increase intake is very well-studied, and this doesn't change it," she said.
And the effects were seen in a selected group of middle-aged men with metabolic syndrome, a combination of high blood pressure, obesity and elevated blood fat levels, Robinson noted.
"We don't have any reference to a healthy control group, which the study didn't have," she said. "It's possible that in these individuals, there may be a different response to omega-3 fatty acids."
Still, it does indicate that further study is warranted of the effects of omega-3 fatty acids in the postprandial period, the hours immediately following a meal, Robinson said.
"We spend up to 18 hours a day in the postprandial period," she said.
In the study, eight men had controlled intake of three regimens: high doses of omega-3 fatty acids, low doses of them and just plain water. Robinson and her colleagues measured several blood components involved in clotting, including fats and clotting factors such as plasminogen-activator inhibitor-1 (PAI-1) for the following eight hours.
PAI-1 inhibits the destruction of blood clots, so high levels of it in the blood increase the risk of artery-blocking clots.
The researchers found that both omega-3 fatty acid regimens increased blood fat and clotting factor activity. But the increase in clotting factor was greater for the higher doses of omega-3 fatty acids than for the lower intakes.
Robinson said her group hopes to do further studies of the immediate effects of omega-3 fatty acid intake. "We need to look at the mechanisms, why blood lipid levels go up," she said. It's possible that there are important differences between the short-term and long-term responses to many dietary fats, she said.
"My quick read on it is that they are looking at a one-time treatment of these patients," said Donald B. Jump, a professor of nutrition at Oregon State University.
"This may be a reflection on the experiment design," Jump said. "From a clinical perspective, most patients take these compounds over periods of weeks or months. There is probably some adaption that occurs. That metabolic adaption probably requires some time. If they treated the patients for a couple of weeks and did the experiment again, they might get a different response."
Learn about the benefits of oily fish and omega-3 fatty acids from the American Heart Association.