In all of weight loss, there may be no concept less aptly named than the “low-calorie” diet. That’s because the calorie unit we associate with food actually refers to kilo calories—meaning when we say, “2,000 calories a day,” we actually mean 2,000,000. A calorie is a unit of heat, or energy—specifically, the energy needed to raise the temperature of 1 gram of water by 1 degree Celsius. And if the number of calories we ingest is bad news, the upside is that we are burning them all the time.

A certain amount of calories—about 60–75% of the calories you burn each day—are needed to sustain the body's unconscious functions, such as breathing and circulation. Known as basal metabolic rate, the specific percentage depends on factors such as size and body composition, gender, and age (typically, as people get older, fat makes up a larger portion of body weight, causing calories to burn more slowly). Digestion makes up about another 10 percent of the calories burned, leaving physical activity to account for the rest.

During exercise, the muscles contract, causing the body's adenosine triphosphate (ATP) molecules to break down as the heart continues to pump faster and faster—increasing the body’s demand for more energy. Once the muscles have depleted the day’s caloric intake, they turn to other calorie sources to fuel the fire—making weight loss possible as the body begins to sacrifice fat cells to the god of the treadmill.