Calories are a measure of how much potential energy food contains. One calorie is the amount of energy it takes to raise the temperature of one kilogram of water by one degree Celsius.
Different food types contain different levels of energy and as a result varying amounts of calories. One gram of carbohydrate has four calories as does one gram of protein. One gram of fat has more than twice this amount at nine calories. Eating too many calories is unhealthy because when you cannot use all the energy stored in the food you eat, it is eventually stored as fat.
However, eating fewer calories than your your body needs to function effectively, is just as damaging, if not more so. This can lead to a variety of diseases and conditions from poor skin to debilitating bone disease.
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