A new study has revealed that the levels of trans fats in fastfoods worldwide vary from country to country, from city to city and even form one outlet to another.

"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," he told the Associated Press.

The researchers analysed the nutritional content of McDonald's chicken nuggets, KFC hot wings and the two chains' French fries in dozens of countries during 2004 and 2005 and their findings were reported in today's (13 April) edition of New England Journal of Medicine.

McDonald's French fries and nuggets purchased in New York City contained an average of 10.2 grams of trans fat, compared with three grams in Spain, Russia and the Czech Republic and only 0.33 grams in Denmark. And the differences are not just between nations - a large order of French fries in New York City contained 30% more trans fats than a large order of fries in Atlanta, Georgia.

A large order of hot wings and fries ordered in Poland and Hungary contained 19 grams of trans fats, while the same order in New York had 5.5 grams of trans fats and food purchased in Germany, Russia, Denmark and Scotland contained less than one gram.

McDonald's and KFC's parent corporation Yum! Brands said the differences arose from local taste preferences.

The researchers said that the type of frying oil used caused the variations. Partially hydrogenated vegetable oil, which has been injected with hydrogen to give it a longer shelf life, has a high trans fat content. The fastfood giants switched from this to other cooking oils in Denmark, where a 2004 law made it illegal for foods to contain more than a low level of trans fats.

Trans fats have been receiving increasing attention internationally because consumption carries with it serious health consequences - research has shown that eating just five grams a day increases the risk of heart disease by 25%.