How Do They Measure Calories?

Did McDonald’s have to torch its new Happy Meal?

Late last week, McDonald’s announced that it would start offering the Go Active! Adult Happy Meal to health-conscious fast-food consumers. According to the company, the meals—which include a salad, bottled water, and a health brochure penned by Oprah Winfrey’s personal trainer—contain between 130 and 550 calories. How do companies measure the calories in the food they sell?

Graduates of 9th-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 as 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 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. In a Kjeldahl analysis, for example, lab techs remove nitrogen from the food powder and then use it to calculate the amount of protein the sample contains. A hexane extraction can gauge the amount of fat. Carbohydrates are usually measured by difference—they’re what is left over when you remove everything else.

To determine the number of calories contained in these building blocks, however, food labs rely on conversion factors first assembled more than 100 years ago by 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. He used it, along with a larger device capable of measuring the heat output of an active person, to figure out how much usable energy different foods possess. The idea is that burning, say, a hamburger shows the total energy that hamburger contains, but it doesn’t account for what the human body cannot absorb, nor what is used in the digestive process. So Atwater derived a set of tables that specify the practical energy values of different foods, distinguishing, for example, among different sources of protein. The most recent update to the conversion tables was published by the U.S. Department of Agriculture in 1973.

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, a set of tomes that used to take up four feet on a bookshelf but is now available online. The database contains figures down to the calorie (and vitamin, and amino acid) for everything from Spam to foie gras—even fast-food items.

Even so, many food companies use labs to ensure their numbers are accurate enough to pass an FDA spot check. Although the USDA database contains several fast-food salads, for example, McDonald’s sent the ones in its new Happy Meal for independent testing.

Bonus Explainer: Food labs often gauge the accuracy of their tests by simultaneously analyzing government-issue reference materials that contain certified amounts of particular nutrients. The labs buy the materials—such as Meat Homogenate ($402 for four 85 g chunks) or Peanut Butter ($501 for three 170 g jars)—from the National Institute of Standards and Technology, and they’re available to anyone with a valid credit card.

Next question?

Explainer thanks Covance Laboratories, James Harnley at the USDA, and Heather Oldani at McDonald’s.