We talk, or at least think, about calories almost daily. You cut calories. You count them. You burn them. But do you know what a calorie is?
Simply put, a calorie is a unit of energy. When you eat, you're fueling your body with potential energy in the form of calories, which your body uses not just for exercise but for vital functions like breathing. Basically, a calorie is the amount of energy it takes for the temperature of one gram of water to increase by one degree Celsius, or 1.8 degrees Fahrenheit. However, when we talk about calories burned or calories consumed we are technically talking about kilocalories, each of which equals 1,000 calories. Kilocalories are most often referred to as calories for short. Scientifically speaking, the term is capitalized, but in most literature about food and exercise "calorie" is written in all lowercase and it's assumed we are all talking about the same unit of measurement.
Foods are made out of mixtures of macronutrients: protein, carbs, and fat. Protein and carbs each contain four calories per gram, while fat contains nine calories per gram. These caloric amounts represent the amount of potential energy these macronutrients provide.
To see how the body burns calories, just read more.
The body uses calories for three distinct categories of activities. One is your basal metabolic rate (BMR), or how many calories your body burns just to keep the basic operations going — heart beating, kidneys functioning, lungs breathing. Sixty to 70 percent of all calories go toward just keeping your body operational, kind of like overhead expenses. The body also needs calories for physical activity, be it washing your dishes, walking your dog, or running a marathon. The third component of calories burned is the thermic effect of food, or how much energy it takes to digest your food to turn it into energy. Your body burns 10 percent of overall calories consumed to fuel the digestion process of those calories you just consumed.
Food is energy, or calories, that you put into your body to fuel all your activities for the day. If you do not use all that fuel, your body stores the fuel just in case you need it during a famine, and you gain weight. If you use more fuel than you have consumed, by burning more calories than you have eaten, you will lose weight.
The Health Guide has some pretty cool tools to help you understand your personal relationship with calories. To see how many calories your body requires to maintain your weight, check out this Nutritional Needs Calculator. To see roughly how many calories you're burning during a variety of activities, visit the Calorie Burner Calculator.