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Definition of a calorie: A calorie is a measure of energy, especially heat energy. In engineering terms, one calorie is the energy required to raise the temperature of one gram of water by one degree Celsius (1.8 degrees Fahrenheit). One gallon of gasoline contains about 31,000,000 calories. One calorie is equal to 4.184 joules.
Most people associate the word "calorie" in reference to food energy.
The calories listed on food packages are actually kilocalories (1,000 calories = 1 kilocalorie). A food item listing 200 food calories actually contains 200,000 regular calories, or 200 kilocalories.
The same applies when you burn calories from exercising. If you burn 100 calories for every half hour of exercise, it means 100 kilocalories.
Caloric Breakdown
Foods contain potential energy. The amount of calories in a given food tells us how much potential energy that food has. We eat food to get the energy that we need to live.
Foods are comprised of three main food categories; carbohydrates, protein and fat. A gram of carbohydrates contains 4 calories, a gram of protein contains 4 calories, and a gram of fat contains 9 calories. Our bodies "burn" calories through metabolic processes in which enzymes break down the food into glucose, glycerol and amino acids which are used by our body.
Tracking Calories (calories consumed and calories burned)
If you eat more calories than you need your body will store the extra energy as fat. If you eat fewer calories than you need, your body will burn fat to provide the extra energy.
How many calories should I eat each day to lose weight?
If you're trying to lose weight, many nutritionists recommend a
calorie range between 1,200 and 1,800 for optimum weight-loss.