The researchers bought French fries from 68 of the 101 national fast-food restaurants represented on the island of Oahu, including McDonald's, Wendy's, Burger King and others, as well as from 66 small businesses. Then they measured carbon isotope composition of the oil used to fry the food.
Almost seven out of 10 of the national chains but only 20 percent of the small businesses sold fries dipped in corn oil. Eleven percent of small businesses and 7 percent of chains used blends containing more than 50 percent corn oil, the researchers found.
The authors speculated that larger conglomerates are able to negotiate economical deals to purchase large quantities of oil from suppliers.
Corn oil content ranged from 16 percent on the low end for McDonald's, to 36 percent and even 50 percent and up in other eating establishments.
Danya Proud, a spokeswoman for McDonald's USA stated that, in May 2008, the corporation "completed the transition to a new canola-blend cooking oil in our 14,000 U.S. restaurants. This blend of canola, corn and soybean oil allows us to serve fried menu items with reduced levels of trans fat and saturated fat, while delivering the same great taste our customers expect from McDonald's."
Even if dipped in relatively healthy oils, though, French fries aren't high on any nutritionist's list of preferred foods.
"French fries aren't exactly the most healthy food ... and we all know that eating too much of anything is not a good thing," said Marianne Grant, a registered dietician and certified diabetes health educator at Texas A&M Health Science Center Coastal Bend Health Education Center in Corpus Christi.
"We try to steer people away from foods like this," Congro added.
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