Calories are the energy you get from eating and drinking. Eating more calories than your body burns increases your weight, eating less decreases your weight.
From the SA Forums Watch and Weight FAQ:
Diet is the most important single factor in your health, body composition and overall appearance.
Food determines how big you are. If you consume more calories than you expend, you will get bigger. If you consume fewer calories than you expend, you will get smaller. If you meet your maintenance needs, you will stay the same. Regardless of your metabolism, body composition, genetics, or whatever, at some point your body must obey the laws of thermodynamics. At the end of the day, everything is calories in and calories out.
You can’t get big if you don’t eat big. That goes for muscle, fat, whatever. You can lift huge weights 10,000 times a day, and if you don’t eat more calories than you expend, you won’t gain a milligram of mass. Conversely, if you burn 10,000 calories a day and eat 11,000 calories a day you will gain weight. Exercise plays a big role in what that extra weight becomes (fat or muscle), but the weight comes from food.
Your body creates one pound of weight from the energy in 3500 calories. By the same token, burning 3500 calories destroys one pound of weight because all of the energy stored in that pound is used up.
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