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Exercise triggers chemical reactions that burn calories.
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Your body requires a certain number of calories to perform its basic functions each day. This is known as your basal metabolic rate. But exercising requires additional energy, above the basal metabolic rate. Therefore, when your muscles work hard, your body sends messages to its fuel sources, which deliver extra energy to those busy muscles. It's like calling a local deli and having it deliver a sandwich to your door, except your muscles don't have to worry about tipping.
In fitness terms, calories are discussed as if they are solid objects that you can literally burn. In fact, a calorie is a measure of energy. Specifically, 1 calorie equals the amount of heat necessary to raise the temperature of 1 gram of water from 14.5 to 15.5 Celsius. What is commonly referred to as a calorie is actually a вЂњCalorieвЂќ -- with a capital C -- in scientific terms, which equals 1,000 calories, or 1 kilocalorie. For the sake of convenience, however, 1 kilocalorie is commonly referred to a 1 calorie. On a food label, for example, a 500-kilocalorie item is simply listed as 500 calories. In layman's terms, if you eat 500 calories of food, you must do exercise that equals 500 calories of effort to avoid gaining weight.
It's All About ATP
When you exercise, your muscle contractions are mainly fueled by adenosine triphosphate, or ATP. As the name indicates, ATP consists of an adenosine molecule plus three phosphorus molecules. When chemical reactions in the mitochondria of muscle cells break down the bonds between the phosphorus molecules in ATP, energy is released to fuel the muscles performing the exercise. This energy release can be measured in calories -- which are, in fact, the calories you burn during exercise. ATP also powers a variety of bodily functions when you're at rest, such as breathing and cell repair.
Using Food as Energy
Your body is constantly breaking down the food you eat and transforming almost half of the nutrients into usable energy. The beginning of the process is the same, whether you're exercising or at rest, as food elements such as proteins, fats and carbohydrates are broken down into smaller organic matter by the enzymes of your digestive system. If you're at rest, some nutrients will go into storage. But if you're exercising, most nutrients travel through the bloodstream to your muscles, where chemical reactions create the ATP, which is then broken down to produce energy. The chemical reactions in your muscle cells also produce heat and water.
Drawing on Fat Stores
Depending on the amount of food you eat before a workout, exercising may require more energy than you have available from your food intake. As a result, your body sends messages to some fat cells, where triglycerides are broken down into glycerol and several fatty acids, which then enter your bloodstream. Much of the glycerol is absorbed in the liver, while most of the fatty acids travel to your working muscles. In the muscles, the fatty acids go through the same types of chemical breakdowns that your recently digested food encounters, eventually producing ATP and then releasing the required calories of energy.