Energy is in reality heat output. Whenever muscles contract to start movement, your body is constantly using energy, creating heat so calories are always being burned. You burn energy just by breathing. If you check your body temperature this instant, unless you're not feeling too well, your temperature should read 98.6 degrees. You radiate heat constantly, and since a calorie is a measure of heat yield, which we can also use to define how many calories a food item has.
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