Trans fat raises bad cholesterol and lowers good cholesterol. Eating just 5 grams of it per day increases the risk of heart disease 25 percent, research shows. 'Per gram, it is more harmful than any other kind of fat. It's a metabolic poison,' says a Danish cardiologist.

A study of the fast-food chains' products around the world found remarkably 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."

Taste Preferences or Profit Motive?

McDonald's Corp., which promised in September 2002 to cut trans fat in half, and KFC's parent, Yum! Brands, 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 were reported in the New England Journal of Medicine.

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. But in Germany, Russia, Denmark and Aberdeen, Scotland, the same meal had less than a gram.

A large order of french fries at a New York City McDonald's contained 30 percent more trans fat than the same order from an Atlanta McDonald's.

'More Harmful Than Any Other Kind of Fat'

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 5 grams of it per day increases the risk of heart disease 25 percent, research shows.

"Per gram, it is more harmful than any other kind of fat," Stender said. "It's a metabolic poison."

McDonald's said it "continues to work diligently on ways to reduce" trans fat in its fries. It said most of its oils come from local suppliers, based on consumer preference, and that oil used in the United States is different from that in Europe and elsewhere.

Stender and other experts said many restaurants use partially hydrogenated vegetable oil to save money because it doesn't spoil and can be used over and over for frying.