Buzz
The term calorie carries significant meaning. We're often told to keep them in check, but things get trickier from there. So, what exactly are calories, and how do I burn them?
THE SCIENCE BEHIND CALORIES
A calorie is a measure of heat energy that powers your body, enabling movement, breathing, thinking, sleeping—and even the digestion of food to create more energy.
Although there is some debate over who first introduced the term calorie, it is known that French chemist Antoine Lavoisier used it during experiments in the winter of 1782–1783. He employed a device called a calorimeter to measure how much ice melted in a metal container as a result of heat emitted by guinea pigs inside. Over time, other scientists refined this measurement to represent the energy required to raise the temperature of a kilogram of water by 1°C—this is called a kilocalorie.
The food calorie and a kilocalorie (kcal) are essentially the same, but we use the term calorie rather than kilocalorie because of an American chemist named Wilbur Olin Atwater. In the late 1880s, Atwater went to Germany to study at Carl Voit's laboratory, where Voit was investigating the nutritional value of food and animal feed. Inspired by this, Atwater took measurements of various foods using a bomb calorimeter—a device that measures the heat in food when burned—by having participants eat and then measuring and subtracting [PDF] the heat leaving their bodies through respiration and waste. He used a respiration calorimeter to track their breath and a bomb calorimeter to analyze their waste, calculating the calories left in their bodies for use. Atwater referred to his findings as calories (the term kcal would not be used in the U.S. until 1894, when it appeared in a physiology textbook).
From his experiments, Atwater developed a system to calculate how many calories human bodies can extract from food. There are three main nutrients that provide caloric energy: fats, proteins, and carbohydrates. Atwater found that a gram of fat contains nine calories, while both a gram of protein and a gram of carbohydrates each contain four. This system was revised [PDF] by USDA scientists in 1973, but it still serves as the foundation for how calories are calculated today.
WHAT HAPPENS TO A CALORIE IN YOUR BODY
When you eat, enzymes in the mouth, stomach, and intestines break down nutrients by converting fats into fatty acids, sugars into simple sugars, and proteins into amino acids. These components are then broken down into energy using oxygen cells throughout your body—a process known as metabolism.
The majority of the calories we burn daily are used simply to keep our body functioning, with roughly half directed toward fueling our vital organs—the brain, liver, kidneys, and heart. The remainder is used for physical activity and the process of converting food into energy. Any unused calories are stored, initially in the liver, and eventually as fat cells.
Certain foods, like honey (carbohydrates), are easily digestible, while others, such as nuts (a combination of carbohydrates, fat, and protein), can't be fully digested at all. Digestibility can even vary within the same type of food. For instance, older leaves on plants tend to be tougher (and thus harder to digest) and contain fewer calories than younger ones. Most importantly, especially in terms of human evolution, cooking or processing food allows the body to extract more calories than if the food were eaten raw. All of these factors influence the number of calories the body can actually use.
No food will directly speed up your calorie-burning rate (effects from foods like spicy peppers are fleeting), but factors such as age and extreme, rapid weight loss can slow it down.
Building more muscle can increase your metabolic rate (though the extent is debated), as muscle requires more energy to function than fat. While cardiovascular exercise may not permanently elevate your metabolism, it does burn calories; the exact amount depends on your weight and the intensity of your exercise.
Activities like cycling and running are great for burning calories, but virtually any activity burns something. In fact, you could burn more calories throughout the day by consistently engaging in low-energy activities like gardening or pacing during a conference call than you would during 30 minutes of intense cycling.
CALORIES: A CONSTANTLY EVOLVING SCIENCE
We still rely on the Atwater system to calculate food calories, but it's far from flawless. For example, a USDA study discovered that people absorbed fewer calories from nuts than Atwater’s system predicted—a serving of almonds, for instance, provided only 129 calories, not 170. There’s also evidence that individuals digest food at varying rates based on their unique gut bacteria, meaning that calorie absorption can differ from person to person.
Scientists now believe that the numbers on food labels are more of an approximation than an exact science. While food companies must provide calorie counts, the FDA doesn’t mandate how those numbers should be calculated. Some companies, like McDonald's, send their products to labs for testing, while others estimate totals by adding up the calorie values for each food component using the USDA’s extensive food composition database. As research advances, we’ll have a clearer understanding of the energy we can actually derive from different foods.
