Why should calories be of concern?

People's energy needs vary depending on age, body size and activity level, but most of us require about 2,000 calories a day to go about our business. Stephen Prescott and Adam Cohen break down the calorie and how to burn it.
By Stephen Prescott and Adam Cohen Modified: February 25, 2013 at 3:04 pm •  Published: February 26, 2013
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Last week, federal health officials issued a report saying that Americans got 11 percent of the calories in their diets came from fast food restaurants like McDonald's, Burger King and Pizza Hut. With all the health risks associated with fast food and obesity, this new study could hardly be considered good news.

In addition to making me pass up Little Caesars for a home-cooked meal, this report also left me wondering about calories. I mean, we spend an awful lot of time talking about them, tracking them, avoiding them and, of course, feeling guilty when we consume too many of them. But what, exactly, are calories and what do they have to do with food?

Dr. Prescott prescribes

Most of us associate calories with food, but the calorie is actually a measure of heat energy. You know how your car generates heat — energy — by burning gasoline? Well, our bodies generate energy by burning food, and calories are the way we measure how much “fuel” a particular item on our plate can provide.

A food calorie (technically, a kilocalorie) is the amount of heat required to raise the temperature of one kilogram of water one degree Celsius. To figure out how many calories something possesses, scientists use a device known as a bomb calorimeter. They place a sample inside the device, use a small electric charge — the “bomb” — to ignite it, then see how much the water in a neighboring chamber heats up.

Lots of things that we'd never dream of putting in our mouths possess calories. For example, if our bodies could digest gasoline, a gallon would be worth about 31,000 food calories. That's about the same as 50 Big Macs. But when doctors and nutritionists talk about calories, they mean the ones generated by substances that our bodies actually can use to fuel our muscles and organs.