NOAH ADAMS, host: 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.</s>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.</s>ANDY BOWERS: 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.</s>ANDY BOWERS: 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.</s>NOAH ADAMS, host: Andy Bowers is a Slate senior editor, and that Explainer was      compiled by Sam Schechner.</s>NOAH ADAMS, host: NPR's DAY TO DAY continues.  I'm Noah Adams.
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.