Last Updated Nov 11, 2008 7:56 PM EST
By looking at carbon and nitrogen isotopes, co-authors A. Hope Jahren and Rebecca Kraft concluded that out of the 162 fast food burgers they tested, only 12 could have been from cows fed anything other than corn. And all of the chicken sandwiches were made from corn-fed chickens. Their meat samples came from McDonald's, Burger King and Wendy's franchises in different cities across the U.S. -- and all the non-corn-fed samples came from West Coast Burger Kings.
The authors also claim that the presence of particular isotopes proves these animals were kept in cramped confinement. But Dr. Frank Monahan, who has performed similar studies in Ireland, told Forbes that the findings didn't necessarily prove confinement -- if the animals were given feed grown with nitrogen-rich fertilizer, that could lead to similar levels of the nitrogen 15 isotope.
As for oils used in deep-frying, the study concluded: "Wendy's clearly used only corn oil, whereas McDonald's and Burger King favored other vegetable oils; this differed from ingredient reports."
Both Burger King and McDonald's declined to comment on the study, while a Wendy's spokesman told Forbes that the company has "very strict procedures in place" on animal welfare.