It has long been known that fast-foods don’t do you any good but still you can’t help yourself from ordering that double cheeseburger. Fast-foods have been linked with a high rate of obesity in the United States over the last three decades and consequently with a high rate of type 2 diabetes and heart disease. Maybe it’s time for you to learn more about what is really served in you fast-food. You might be surprised to find out that many of the menus served at McDonald’s, Burger King and Wendy’s contain a large quantity of corn.

Researcher A. Hope Jahren, a geobiologist and professor at the University of Hawaii and colleague Rebecca Kraft analyzed 480 fast food products at three restaurants from each of the three major chains in Los Angeles, San Francisco, Detroit, Denver, Boston and Baltimore. Using a technique that identifies carbon and nitrogen isotopes in meat, the researchers tried to determine the animals’ diet and in what conditions they were raised. The analysis indicated high levels of carbon and nitrogen, which can mean only one thing: the cattle and poultry were predominantly fed corn, which makes them very fat in a very short period of time. Making a quick analogy, it is no wonder fast food products result in a high rate of obese people. Even when analyzing fries at the three restaurants, the researchers found that Wendy’s used only corn oil, while McDonald’s and Burger King used other vegetables oils.

“Out of hundreds and hundreds of samples, only 12 (beef) patties didn’t go straight corn (as a food source),” said Jahren.

What’s more, the animals were found to be raised in harsh confinement.

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. McDonald's declined to comment.

The findings were published in the Proceedings of the National Academy of Sciences.