Order french fries or hot wings at a McDonald’s or a KFC in the United States and you’re more likely to get a super-sized helping of artery-clogging trans fats than you would be at their restaurants in some other countries.

A study of the 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.

One of the researchers, Dr. Steen Stender, a cardiologist at Gentofte University Hospital in Hellerup, Denmark, and former head of the Danish Nutrition Council, said the results surprised him. “It’s such an easy risk factor to remove,” he said.

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.

Researchers tested chicken and potato products from the chains’ outlets in dozens of countries in 2004 and 2005.

At a New York City McDonald’s, a large fries-and-chicken-nuggets combo was found to contain 10.2 grams of the trans fat, compared with 0.33 grams in Denmark and about 3 grams in Spain, Russia and the Czech Republic.

At KFCs in Poland and Hungary, a large hot wings-and-fries order had 19 grams of trans fats or more, versus 5.5 grams for wings and fried potato wedges in New York.