CSPI testers purchased large orders of fries from five McDonald's, Burger King, and Wendy's outlets in Manhattan -- which requires the use of trans-fat-free frying oils -- and asked an independent laboratory to analyze them.
McDonald's had the least trans fat at 0.2 grams per serving. Wendy's had 3.7 grams per serving, and Burger King had 3.3 grams per serving, although officials noted Wendy's serving size was 25 percent larger than Burger King's.
Dietary guidelines recommend people consume no more than 2 grams of trans fat daily -- about as much as occurs naturally in milk and meat, leaving virtually no room for artificial trans fat from partially hydrogenated vegetable oil, the CSPI said.
"French fries are fried twice, once in the factory and once in the restaurant," said CSPI Executive Director Michael Jacobson. "If these chains want to claim they are switching to trans-fat-free frying oil, they need to switch it at the supplier as well as at the restaurant."