Corn is the key ingredient in U.S. fast food, but it isn't easy for consumers to find out what's in their burgers, sandwiches and fries, according to a U.S. study released Monday.

Researchers from the University of Hawaii sampled more than 480 servings of hamburgers, chicken sandwiches and fries from three leading chains in six cities, and concluded there was evidence that all of the chicken and 93 per cent of the beef had been fed corn exclusively.

Tests indicated that only 12 servings of beef had "an additional food source besides corn," the report published in Proceedings of the National Academy of Sciences said. (All 12 came from Burger Kings on the West Coast.)

The tests for the presence of corn used to prepare fries differed significantly, suggesting that the chains — Wendy’s, McDonald’s and Burger King — used different materials for deep-frying. "Wendy’s clearly used only corn oil, whereas McDonald’s and Burger King favoured other vegetable oils; this differed from ingredient reports," the study said.

"Of 160 food products we purchased at Wendy’s throughout the United States, not one item could be traced back to a non-corn source," A. Hope Jahren and Rebecca A. Kraft of the university's department of geology and geophysics wrote.

"Our results highlighted the overwhelming importance of corn agriculture within virtually every aspect of fast food manufacture."

They didn't study beverages, but said drinks are often sweetened with fructose corn syrup.

Not much information

The three fast food companies, which account for more than half the restaurants in the U.S., have sales of more than $100 billion US a year, the report said.

The top chains oppose ingredient reporting, the authors said. Distribution companies, including Martin–Brower L.L.C. for McDonald’s and Maines Paper and Food Service Inc. for Burger King and Wendy’s, "provide little information beyond their use of 'local farms' that feed 'mixed grains.'"

The authors said, "Ingredients matter for many reasons: U.S. corn agriculture has been criticized as environmentally unsustainable and conspicuously subsidized."

Because fast foods represent a "disproportionate" amount of meat and calories within the U.S. diet, "our work highlights the absence of adequate consumer information necessary to facilitate an ongoing evaluation of the American diet."

A consumer who ate one cheeseburger, chicken sandwich, and small order of fries would consume:

Half of the day’s recommended calories.

Four-fifths of the recommended carbohydrates.

Three-quarters of the recommended protein (90 per cent for a woman).

The full day’s limit of dietary fat.

The researchers used carbon and nitrogen stable isotopes to infer the source of feed to meat animals and the source of fat in fries.

They bought samples from the three chains in Los Angeles, San Francisco, Denver, Detroit, Boston, and Baltimore.