A team of Danish researchers found that the content of fatty acids varied from less than 1g in Denmark and Germany to 10g in New York (McDonald's) and 24g in Hungary (KFC). Some 50 percent of the 43 servings analysed contained more than 5g per serving.
"The daily intake of about 5g of trans fat is associated with a 25 percent increase in the risk of ischemic heart disease," wrote Steen Stender and Jorn Dyerberg from Gentofte University Hospital in Hellerup, Denmark.
"For this reason, it is recommended that the consumption of trans fat be as low as possible."
The conclusion reached by the team was that these discrepancies were caused primarily by the type of frying oil used - with partially hydrogenated vegetable oil the most heavily laced with trans fatty acids.
The cooking oil used for French fries in McDonald's outlets in the US and Peru contained 23 percent and 24 percent trans fatty acids, respectively, whereas the oils used from French fries in many European countries contained only about 10 percent trans fatty acids, with some countries as low as 5 percent (Spain) and 1 percent (Denmark).
Within the same chain in the same country, large variations in these values were also observed. This was noticeable, for example, between KFC outlets in Hamburg and Wiesebaden in Germany and between Aberdeen and London in the UK.
The study could help strengthen pressure on the food industry to use oils that are low in trans fatty acids. Last year a Marin County judge finalized an $8.5 million settlement of a suit filed against McDonald's in 2003 for reneging on its promise to reduce the amount of trans fat oils a year previously.
And within Europe, there has been growing demand for member states to follow Denmark's example and force food makers to clearly label the presence of trans fats.
For the study, the scientists determined the content of industrially produced trans fatty acids in 43 servings of fast foods bought in 20 counties between November2004 and September 2005. Visits to particular cities were based on previously arranged trips for other purposes.
The foods - chicken nuggets and French fries - were all purchased from McDonald's and KFC outlets.
The foods were homogenized, and the fatty acid content analyzed by capillary gas chromatography according to a method accredited by the International Organization fro Standardization.