Counting calories today is as easy as checking the label in a grocery store, or perusing the menu in a restaurant. But how accurate are these numbers, and how do food manufacturers and restaurants come up with them in the first place?

In a study published July 19 in the Journal of the American Medical Association, researchers from the Nutrition Research Center at Tufts University revealed that when they compared their own tests of restaurant meals' caloric contents with the calories on the menu, the numbers didn't add up. Although the Tufts team found that food items only have an average of 10 more calories than the restaurants suggested, almost 20 percent of the dishes tested contained at least 100 calories or more than the listed number. Reading the news coverage of this discovery raises a scientific question: How did the researchers collect their data?

The Tufts team tested the dishes' calorie counts using a method called bomb calorimetry, which involves burning the food to discover how much heat it releases, which tells scientists how much energy it contains. Calories are simply units that measure energy. Specifically, one calorie on a menu is actually one kilocalorie, the amount of energy required to heat one kilogram of water by one degree Celsius.

But the process of calorie testing is more rigorous than just lighting a match. First, the Tufts researchers gathered 269 different food items from restaurants, making sure to choose dishes for which nutrition information was available. In the lab, the food went from palatable to pellet. After weighing each dish, researchers put it in a food processor with water and blended it to create a smooth puree, mixing all the components of the meal together. In order to render this soupy mix flammable, they freeze-dried a sample of it. Finally, the scientists ground it into powder, which they pressed into a pellet.

The researchers placed each pellet in a tiny cup less than five centimeters wide, and draped a current-carrying wire over it. Then, they surrounded it with as many layers as a Russian nesting doll: The bomb calorimeter machine held a bucket of water, which surrounded an airtight cylinder in which the pellet-carrying cup hung suspended.

The bomb calorimeter sent electricity through the wire on the pellet, creating a spark that set the pellet ablaze. The heat that it produced warmed the air in the cylinder, which heated the water surrounding it, a change detected by probes in the bomb calorimeter. "The temperature probe measures the change in temperature of the water that the cylinder is sitting in," says Lorien Urban, a nutrition researcher on the Tufts team. The calorimeter translates the change in temperature into calories per gram of food.

Knowing the weight of the food before its transformation into a pellet allowed the researchers to work backwards to find how many calories were in the restaurant's original offering. But their work didn't stop there. "The number you get out of the bomb calorimeter will be the total food energy, but we can't actually absorb all the calories," Urban says. The body only absorbs the metabolizable energy, and this, not the gross energy the calorimeter measures, is the number that restaurants report.

The metabolizable energy in a food item depends on its ratio of fat, carbohydrates and protein (which can be determined using chemical analysis). Using the nutrition facts that restaurants posted about their food, the Tufts team translated the calories listed on the menus—the metabolizable energy—into the gross energy, and compared this number to the gross energy that they had measured with the bomb calorimeter.

There are other ways to calculate a food's calorie content, as this 2003 Ask the Experts feature in Scientific American explains. However, the Tufts team chose the bomb calorimetry method because they had previously tested it on mixtures with known caloric values, and found it to be accurate. Whether their finding will affect the food choices that diners make remains to be seen.

Photo courtesy of Wikimedia Commons / Arthur Jan Fija kowski