Scientists Say: Calorie

This unit helps measure the energy we get from food and the energy we use doing various activities

three chocolate chip cookies sit atop a page that reads "Chocolate Chip Cookie Nutrition Facts," which lists nutrition facts including the number of calories per serving of cookies (180)

According to this food label, these chocolate chip cookies contain about 180 nutritional calories per serving.

Peter Dazeley/Getty Images

Calorie (noun, “KAL-or-ee”)

A calorie is a unit for measuring energy.

The word calorie can have two slightly different meanings. Both meanings relate to energy. “Big” calories appear on nutrition labels. These measure the amount of energy provided by food. “Small” calories are used in chemistry. They provide an estimate of the energy needed to heat water.

Before the mid-1900s, scientists defined one “small” calorie as the energy needed to heat one gram of water by one degree Celsius (1.8 degrees Fahrenheit) at the air pressure found at sea level. But then scientists realized that this amount of energy varies, depending on the water’s original temperature. So, today, a “small” calorie is simply defined as 4.2 joules. A joule is another unit of energy. Chemists might use these calories when describing how much heat gets released from burning a particular substance.

Each “big” calorie is equal to 1,000 “small” calories. We see such “big” calories on food labels. Imagine a typical can of soup. The nutrition label might read “250 calories per serving.” This number estimates how much energy our body will get from eating that soup. Each serving gives a person’s body about 250 “big” calories — equal to about 250,000 “small” calories.

Most food companies determine the calories in a food by how much protein, fat and carbohydrates that food contains. For instance, each gram of fat is thought to give a person’s body about nine calories. Each gram of carbohydrate and protein is expected to give about four calories.

The way we cook our food can change how much energy we can get from it. In 2011, for instance, researchers found that the heat applied in cooking increased the energy mice got from meat and sweet potatoes. So the calories on food labels are only estimates.

People’s bodies also need different amounts of calories to keep them healthy. A 12-year-old, for instance, might need around 1,800 to 2,200 calories to power them through the day. But other animals need a lot more energy. A blue whale gulps between 10 million to 20 million calories daily!

In a sentence

Because blood is low in calories, vampire bats must drink up to 1.4 times their body weight during each feeding in order to live.

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Katie Grace Carpenter is a science writer and curriculum developer, with degrees in biology and biogeochemistry. She also writes science fiction and creates science videos. Katie lives in the U.S. but also spends time in Sweden with her husband, who’s a chef.

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