Omega-3s are good for you in the long term, but a new Guelph study has found they have the immediate effect of inhibiting the body’s ability to remove blood clots.

The research by Prof. Lindsay Robinson, Human Health and Nutritional Sciences, revealed that the fish oil fatty acids increased blood fat and clotting factor activity in men who had a combination of high blood pressure, obesity and elevated blood-fat levels.

“We were surprised to find that the acute response has some potentially negative effects in comparison with what you might expect from chronic long-term intake,” says Robinson. “But it’s possible that in these individuals, there may be a different response to omega-3 fatty acids.”

The study was published in the January issue of  the Journal of Nutrition and has received widespread media attention.

She cautions, however, that the study results should not affect the current recommendation to eat more oily fish to get the omega-3 polyunsaturated acids that reduce the risk of blood clots, which can cause heart attacks and stroke.

“The recommendation to increase intake is very well-studied, and this doesn’t change it.”

In the study, eight men had controlled intake of three regimens: a high dose of omega-3 fatty acids, a low dose and plain water. Robinson and her colleagues then 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 and that the increase in clotting factor was greater for the higher doses of omega-3 fatty acids than the lower intakes.

These results indicate that further research needs to be done on the immediate effects of omega-3 fatty acid intake, says Robinson.

“We need to look at the mechanisms such as why clotting factors are increased. It’s possible there are important differences between the short-term and long-term responses to many dietary fats.”