It’s a unit of measure for the energy in food. First defined in France in the early 1800s, a food calorie (kilocalorie) is the amount of heat (energy) needed to raise the temperature of 1,000 grams (a liter) of water 1° Celsius. Researchers measure calories by using a “bomb calorimeter,” a small chamber in which a food is burned to heat water; the hotter the water, the higher the calorie count. Food companies today don’t need to burn their products to figure out the calories in them—they simply add up the calories of the ingredients, based on standard databases.
The components of food that provide calories include carbohydrates (4 calories per gram), protein (4 calories), fat (9 calories) and alcohol (7 calories). Fat provides the most calories, which is a good thing when food is scarce, but not so good for people trying to lose weight, or at least not gain it. Vitamins, minerals and indigestible fiber have no calories.
Obviously, calories themselves are not bad, since you need them to live—the problem is their overconsumption. A calorie is a calorie, whether it comes from fat, carbs or protein. But the source of calories does matter for health. For instance, while the 100 calories in a big bowl of broccoli come with lots of nutrients and a satisfying bulk that will help fill you up, the 100 calories in one-third of a doughnut have neither advantage. Calories accompanied by few nutrients are often called “empty” calories.
A 190-pound person can consume far more calories than a 120-pound person without gaining weight. A larger body simply needs more energy (calories) for basic functions and to move about. Thus, if those two people are moderately active, the heavier one may burn 3,000 calories or more a day, while the lighter one probably burns less than 2,000, depending on their age, sex, fitness level, how much muscle and fat they have and genetic factors.
That’s the common “rule” for weight loss, meaning if you achieve a 3,500-calorie deficit by eating less and/or exercising more, you’ll lose a pound of body weight. That figure is simplified, however, coming from 1950s research that focused primarily on very overweight women. Many factors besides weight come into play, including body composition, sex, age, metabolic rate and activity level. Lean people, for example, usually need to burn fewer calories to lose a pound (an example of “unfair!”). Overall, though, the 3,500-calorie estimate works fairly well for obese people.
Slow and steady is best for weight loss, as opposed to crash dieting. You can cut 3,500 calories by eliminating 250 calories (e.g., that afternoon candy bar) every day for two weeks, or by walking for an extra hour every day for two weeks. Better yet, combine the two. That said, it’s not just simple arithmetic. The body tends to adapt to the calorie deficit and weight loss, notably by lowering its metabolic rate—making it harder to continue losing weight or even maintain your lower weight. Long-term weight loss isn't easy. But “calories in, calories out” is still key.