What's really in your favourite fast food combo?

What's really in your favourite fast food combo?

Updated Sat. Nov. 15 2008 7:14 AM ET

Rebecca Ruiz, Forbes.com

You may want to reconsider getting that double cheeseburger with fries.

A study released earlier this month in the Proceedings of the National Academy of Sciences contains controversial claims about menu items served at McDonald's, Wendy's and Burger King.

Using a technique that identifies carbon and nitrogen isotopes in meat, co-authors A. Hope Jahren and Rebecca Kraft tried to determine the animals' diets and in what conditions they were raised. Based on the high levels of carbon and nitrogen isotopes found in the meat products, the authors claim that the cattle and poultry were predominantly fed corn, which makes them as fat as possible in as short a time as possible, and were raised in extreme confinement.

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In an interview, Jahren, who is a geobiologist and professor at the University of Hawaii, even suggested that the nitrogen isotopic signatures found in meat products were so high that they were consistent with environments where animals had consumed their own waste.

Burger King declined to comment on the study. A spokesman for Wendy's said the company has "very strict procedures in place" to protect animal welfare. A spokeswoman for McDonald's declined to comment and instead referred to a statement issues by the American Meat Institute, a trade association.

Janet M. Riley, senior vice president of public affairs for AMI, said that carbon and nitrogen isotopes are naturally occurring and are expected to be found in the environment and humans. She also said that while the study's authors had called for greater transparency regarding information about livestock feeding and production practices, consumers "appear satisfied" with the amount of information currently available.

Where's the beef?

Over a two-year period, the authors purchased 480 servings of hamburgers, chicken sandwiches and french fries at multiple chains in six cities across the country and tested them for carbon 13 and nitrogen 15 isotopes, both of which have been used by scientists for decades to reveal clues about eating habits in both humans and animals.

Farmers use nitrogen-enriched fertilizer to rapidly maximize their output. This is particularly true of corn crops; in 1940 a farmer could expect 70 to 80 bushels of corn per acre. Today, that number has reached 200 bushels. This corn, in turn, has been used to feed livestock and poultry for quick and efficient growth. In 2007, the U.S. produced 48.7 billion pounds of commercial red meat, 90.6 billion eggs, and 8.1 billion chickens, according to the USDA.

Based on the carbon isotopic signatures found in the sample meat, the study's authors argue that the cattle had been strictly confined and fed predominantly corn. Samples from Burger King showed the greatest variability, indicating that some cattle may have eaten natural vegetation or other feeds instead. The nitrogen isotopic ratios were much higher than ones found in grass-fed and free-range beef, and the authors attributed these findings to feed produced with nitrogen fertilizer and severe containment conditions.

The authors also found widespread homogeneity of chicken samples across chains. The chickens used were predominantly corn-fed. While the chicken samples had lower nitrogen levels than beef, the authors argue that they remained high enough to demonstrate that the poultry had been raised in extreme confinement.

Samples of french fries revealed that the restaurant chains are using one frying oil or a combination of oils despite claims stating otherwise. Wendy's, for example, claims their fries may contain one or more of canola, soy, cottonseed or corn oil. In fact, the study's findings pointed to a nationwide "corn-oil based protocol" for fries.

Marion Nestle, a professor in the department of nutrition, food studies and public health at New York University, says the study highlights what we already know about industrial food production: Most livestock and poultry live in cramped conditions and subsist on a fattening corn diet. There are a host of problems with industrial farming, including environmental pollution, says Nestle, but eating animals raised on corn diets is not one of them.

"It would be too bad if this adds to public fears about corn," she says.

Dr. Frank Monahan, a scientist at University College Dublin, has used the same technique to compare the eating habits of grass-fed and conventionally raised beef in Ireland. While he called Jahren's approach scientifically sound, he questioned the conclusiveness of the study's findings.

The levels of nitrogen, he says, may not be directly linked to confinement practices. Instead, they may reflect the fact that most conventionally raised beef and poultry are given feed that was grown with nitrogen-enriched fertilizer. And while the high levels of carbon 13 indicate a diet abundant with corn, Monahan says their values could have been much higher, which would provide convincing evidence that the animals had an exclusive corn diet.