A study of two leading restaurants 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 differences were 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.
Estimates a few years ago indicated trans fats prematurely killed 30,000 to 75,000 Americans a year.
Dr Steen Stender, a cardiologist at Gentofte University Hospital in Hellerup, Denmark, and former head of the Danish Nutrition Council, who worked on the research, said: "I was very surprised to see a difference in trans fatty acids in these uniform products. It's such an easy risk factor to remove."
McDonald's, which promised in September 2002 to cut trans fat in half, and KFC parent Yum! Brands said the explanation was local taste preferences. But nutrition experts and consumer activists say it is about money: Oil high in trans fats costs less.
The researchers tested products from the two chains in dozens of countries in 2004 and 2005, analysing McDonald's chicken nuggets, KFC hot wings, and fried potatoes. The findings were reported in Thursday's New England Journal of Medicine.
At a New York City McDonald's, a large fries-and-chicken-nuggets combo had 10.2 grams of trans fat, compared with 0.33 grams in Denmark and about 3 grams in Spain, Russia and the Czech Republic.
McDonald's said most of its oils
come from local suppliers
At KFCs in Poland and Hungary, a large hot wings-and-fries order had 19g of trans fats or more, versus 5.5g for wings and fried potato wedges in New York. In Germany, Russia, Denmark and Aberdeen, Scotland, the meal had less than a gram.
A large order of fries at a New York City McDonald's contained 30% more trans fat than the same order from an Atlanta McDonald's.
Partially hydrogenated vegetable oil is cooking oil that has been injected with hydrogen to harden it and give it a longer shelf life. Switching to liquid vegetable oils such as canola, corn, olive or soy eliminates the trans fat, as has been done in
Denmark under a 2004 law allowing only a minuscule amount of trans fat in foods.
Trans fat raises bad cholesterol and lowers good cholesterol. Eating just 5g of it per day increases the risk of heart disease by 25%, research shows.
"Per gram, it is more harmful than any other kind of fat," Stender said. "It's a metabolic poison."
McDonald's said that most of its oils come from local suppliers, based on consumer preference, and that the oil used in the US is different from that in Europe and elsewhere.
Stender said many restaurants still use partially hydrogenated vegetable oil to save money because it does not spoil and can be reused.
Michael Jacobson, executive director of the Centre for Science in the Public Interest, said his group has petitioned the Food and Drug Administration to limit the use of trans fats and require restaurant menus to note foods containing trans fat. He said FDA is still reviewing the petitions, "even though they agree it's killing thousands of people a year".
In January, the FDA began requiring package labels to list trans fat content. KFC and McDonald's list the trans fat and other components of their foods on their websites and in stores, on such things as tray liners and brochures.
Jacobsen's centre estimated a few years ago that trans fats prematurely killed between 30,000 and 75,000 Americans a year. That number has probably fallen, he said, because many packaged-food companies have switched to healthier oils.
But many processed foods - including pies, tortilla chips, margarine and microwave popcorn - still contain partially hydrogenated vegetable oil.