Dark beer, it appears, may be better for you than light beer, according to research released this week.

A comparison of Guinness Stout, a dark beer, and Heineken, a light beer, presented to the American Heart Association, showed that the darker beer had substantially more anti-clotting activity.

The research, by John Folts, a professor of medicine and nutritional director of the University of Wisconsin Coronary Thrombosis Research and Vascular Biology Laboratory, suggested that Guinness was about twice as effective at preventing the blood platelets from clumping and forming clot, which can cause a heart attack.

The beneficial effect comes from flavonoids in the beer.