Slate's Explainer: How McDonald's Counts Calories

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McDonald's executives announced the fast-food restaurant chain will begin listing nutritional information on its product packaging within the next year. Slate senior editor Andy Bowers explains how companies measure calories in the food they sell.


The food at McDonald's may not be vegetarian, but at least you will soon have an easier time trying to figure out what is in it. The fast-food giant announced yesterday that starting next year, it will place nutritional information right on the packaging for most of the food items. That got the Explainer team at the online magazine, Slate, wondering how does McDonald's calculate that a Big Mac has 560 calories. Here with the answer is Slate's Andy Bowers.

ANDY BOWERS: Graduates of ninth-grade science may remember a very simple answer: Burn the food to see how much heat it gives off. That energy can be measured in calories. Nutritionally speaking, one calorie is defined at 1,000 times the energy it takes to heat a gram of water from 14.5 to 15.5 degrees Celsius.

But instead of burning anything, food laboratories these days often freeze their samples in liquid nitrogen and then blend them into a fine monochromatic powder that can then be used in a variety of chemical analyses. Food labs still rely on conversion formulas first assembled more than a hundred years ago but the agricultural chemist Wilbur O. Atwater, who literally did burn things like beef and corn in a device called the bomb calorimeter. While today's calorimeters look a lot more sophisticated, Atwater's was more or less a fireproof container sheathed in water and hooked up to a thermometer.

These days, the FDA requires that companies print accurate calorie data on food labels, but doesn't say how they must gather the information. It's permissible, for example, to guesstimate from the USDA's published nutritional data for thousands of foods. The database contains figures down to the calorie for everything from Spam to foie gras. Even so, many companies like McDonald's use labs to ensure their numbers are accurate enough to pass an FDA spot check.

ADAMS: Andy Bowers is a Slate senior editor, and that Explainer was compiled by Sam Schechner.

NPR's DAY TO DAY continues. I'm Noah Adams.

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