If you thought you were eating mostly grass-fed beef when you bit into a Big Mac, think again: The bulk of a fast-food hamburger from McDonald's, Burger King or Wendy's is made from cows that eat primarily corn, or so says a new study of the chemical composition of more than 480 fast-food burgers from across the nation.

And it isn't only cows that are eating corn. There is also evidence of a corn diet in chicken sandwiches, and even French fries get a good slathering of the fat that makes them so tasty from being fried in corn oil.

"Corn has been criticized as being unsustainable based on the unusual amount of fertilizer, water and machinery required to bring it to harvest," says geobiologist Hope Jahren of the University of Hawaii at Manoa's School of Ocean and Earth Science and Technology, who led the research. "We are getting a picture of the American diet on a national scale by using chemistry, which is quite objective."

Eating a diet of meat from corn-fed animals hasn't been linked to any specific health effects in humans. But it has resulted in widespread environmental degradation, including drained water supplies, degraded soils, and reliance on fossil fuels for fertilizer, pesticides and farm machinery fuel, says preventive medicine physician Bob Lawrence, director of the Center for a Livable Future at the Johns Hopkins Bloomberg School of Public Health, who was not involved in the study.

It's also hard on cows, whose stomachs are specially designed to break down the cellulose in grass, leading to an epidemic of antibiotic use. Also, humans may lose out on beneficial omega-3 fatty acids—important for development of the nervous system and heart health—when they consume corn-fed as opposed to grass-fed beef.

"Instead of eating a predominantly whole grains, fruits and vegetables, we are diverting the grain supply to feeding the animals," Lawrence says, arguing for a diet that treats meat as a garnish rather than the main course and corn for human consumption rather than cows. "Corn-finished beef does add to what has become a preferred taste for the American palate. We've acquired that taste at our own peril."

Jahren and her colleague Rebecca Kraft collected hamburgers, chicken sandwiches and fries from three separate Burger King, McDonald's and Wendy's locations in six U.S. cities: Baltimore, Boston, Detroit, Denver, Los Angeles and San Francisco. The scientists were looking for the amount of carbon 13 (13C), a variety of carbon with an extra neutron (known as an isotope) that makes its atom heavier.

Corn tends to have more of this 13C than other plants. That telltale signature persists as the corn travels through the complex system that turns it into feed, which is consumed and processed by cattle to grow tissue. It continues after the animals are slaughtered and the meat is cooked. The result: 93 percent of the tissue that comprised the hamburger meat was derived from corn.

In fact, only 12 samples from the entire country did not show this unique corn signature: all from a Burger King on the west coast. "My best guess is that it represents meat from another country," Jahren says.

And all of the chicken, in addition to being sourced from just one company, Tyson Foods, Inc., had been fed an entirely corn diet, resulting in a chemical composition that was almost exactly the same from coast to coast. Jahren notes that the isotopic composition of this chicken meat varied from restaurant to restaurant and state to state less than if a sample were taken from just one farmyard-raised chicken.

Further, by studying the levels of a particular heavier isotope of nitrogen, the researchers found that this corn-fed beef was relying on heavy applications of fertilizer as well as, potentially, animals surrounded by their own waste. "As metabolism proceeds, the nitrogen products become heavier and heavier," Jahrens explains. "Nitrogen is just cycling through the animal, including potentially ingestion of that waste or respiration. Our results are consistent with that."

Researchers at Johns Hopkins are now completing a study measuring the levels of carbon 13 in human blood, in an effort to understand how much of the corn in our meat and in the sweeteners (high fructose corn syrup) in our food and drink ends up in our bodies. The fast-food outlets did not return calls for comment.

As Jahren notes, Americans spend more than $100 billion a year on such fast food, making it a significant part of the diet. "Diet related disease is causing more and more suffering in this country and the information you can get is either vague or nonexistent," says Jahren, who spent the last two years trying to get information about what specifically goes into fast food at these chains and how it is made, with no success. "You shouldn't have to use stable isotopes to get the answer to what's in something I just spent my money on and am about to put in my body."