Order french fries at a McDonald's in the United States and you're more likely to get a super-sized helping of artery-clogging trans fats.

A study of fast-food chains' products around the world found wide variations in trans-fat content from country to country, from city to city within the same nation, and from restaurant to restaurant in the same city.

The researchers said the differences had to do with the type of frying oil used, and the main culprit appeared to be partially hydrogenated vegetable oil, which is high in trans fats.

"I was very surprised to see a difference in trans fatty acids in these uniform products," said one of the researchers, Dr. Steen Stender, a cardiologist at Gentofte University Hospital in Hellerup, Denmark, and former head of the Danish Nutrition Council. "It's such an easy risk factor to remove."

McDonald's Corp., which promised in September 2002 to cut trans fat in half, and KFC parent Yum! Brands Inc. said the explanation is local taste preferences. But nutrition experts and consumer activists said it is about money: Frying oil high in trans-fats costs less.

The Danish researchers tested products from the chains' outlets in dozens of countries in 2004 and 2005, analyzing McDonald's chicken nuggets, KFC hot wings, and the two chains' fried potatoes. The findings are reported in today's New England Journal of Medicine.