How Do We Estimate the Calories in Food?
By November, every chain restaurant or establishment with 20 or more locations that sells prepared food will be required by the Food and Drug Administration to post caloric content of their meals “clearly and conspicuously” on their menus. The idea is that consumers will think twice before downing daily Mocha Frappuccinos at Starbucks (410 calories, by the way), and restaurants might respond by creating healthier, lower-calorie options. But how do scientists actually measure the number of calories in our foods and beverages anyway?
“Basically, they use a bomb,” says Emily Dubyoski, a dietitian at the Johns Hopkins Weight Management Center. “An oxygen bomb inside an insulated container.”
Before we delve into explosions involving Chicken McNuggets and bacon double cheeseburgers, some basic science: A calorie, Dubyoski explains, is the amount of energy needed to raise one gram of water at 15 degrees Celsius by 1 degree. But because that amount of energy is too small to conveniently describe the energy content of food, nutritionists use kilocalories (1,000 calories) or Calories, with a capital “C.” Scientifically speaking, that’s really what you’re seeing on dietary labels and menu boards, the Calorie, or the amount of energy required to raise a kilogram of water by 1 degree. (Outside the lab, most people simply refer to Calories as calories.)
To measure the number of calories in food, Dubyoski says that for decades scientists have employed what’s called a bomb calorimeter, a desktop contraption composed of two sealed chambers. Researchers put a small, weighed sample of the food to be tested inside the interior chamber, which is full of oxygen, and then ignite it. Water contained in an insulated outer chamber rises in temperature due to the explosion within. “The heat released from combusting the [food] sample is actually absorbed by the water that surrounds the bomb, and that change in temperature is how the amount of calories contained in that food is determined,” says Dubyoski. “A higher calorie sample is going to release more heat upon combustion, and thus result in a greater change in water temperature.”
So a piece of carrot would cause a slight rise in temperature, whereas a sample of Subway’s double meatball sub with cheese would give off enough energy to practically power a light bulb.
Although bomb calorimeters have been around since French chemist Pierre Eugène Marcellin Berthelot invented the contraption more than 125 years ago, Dubyoski says contemporary research shows that calorie counting is not necessarily an exact science. “The calories you see on the nutrition label are a prediction, based on research, as to how much of that food your body will actually use,” she says. “While we know that each person has different energy needs based on age, gender, height, weight, muscle mass, and activity level, what is still being worked out is how additional factors potentially play a role in absorption of that energy, such as the types of bacterial colonies in a person’s gut.”
But don’t think your speedy metabolism means you’re ingesting fewer calories than the 467 in a Big Mac. It’s still a close estimate. “The take-home message is to pay attention to what you are eating and how your body responds, in this case by weight monitoring,” says Dubyoski. “Nutrition labels are a great resource, but it is also good to be aware that even with the best efforts of science, the calories listed on the label will be close but maybe not 100 percent accurate.”
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“Nutrition labels are a great resource, but it is also good to be aware that even with the best efforts of science, the calories listed on the label will be close but maybe not 100 percent accurate.”—Emily Dubyosk