Our muscles are primed to burn fat, and it is our most concentrated form of energy. When we begin to exercise, we burn glucose and glycogen, but it only takes a matter of minutes to shift to fat. It is not an abrupt switch, more like a smooth transition that works better as the body becomes more fit. Think about what is going on when we are at rest.
A minimum amount of fats are in the bloodstream, and our breathing and heartbeat are relaxed, taking in just enough oxygen and pumping blood at a rate that fuels the needs of the moment.
Now think about what happens if we were to start to exercise. The muscles respond instantly, but soon after, the pace of our heartbeat picks up and our breathing gets deeper and more rapid. If, by chance, we happen to be out of shape and push too hard, we become "out of breath" and have to pause.
Now let's go behind the scenes to discuss what is actually happening. If you think about it, it is essential that our muscles can respond even when there is insufficient fat or oxygen to fuel the effort.
Otherwise, we would be unable to confront or escape danger. At first, glucose and glycogen are the main energy sources. The brain immediately sends a signal to the fat storage areas to send some fuel, and to the lungs to start breathing faster and deeper to supply more oxygen and start removing carbon dioxide, the exhaust gas from muscular work.
If the muscles are worked too hard, the available glucose is used up before the fats and oxygen arrive on the scene, and the muscle runs out of gas. The actual speed and ease of the shift from anaerobic to aerobic reflects the level of fitness. Warming up or starting slowly also helps the process, because it gets the signal sent earlier and helps shunt blood flow to the working muscles.
Even after we have made the shift to fat burning, we continue to burn some glucose. After your body has used up the carbohydrates from a meal or snack, your cells begin to break down glycogen , which is glucose stored in your muscles and liver. Glycogen stores vary from person to person, but are typically depleted within 24 hours , at which point your body has to begin breaking down other compounds for energy.
When glucose and glycogen are not available, your body preferentially breaks down fatty compounds known as triacylglycerols , which are present in adipose or fat tissue. Because fat is a high-energy source with 9 calories per gram, fat is an efficient fuel source.
Additionally, your body metabolically prefers to preserve lean body mass and, when possible, breaks down fat stores for fuel. Only when your fat stores are extremely low or depleted does your body then have to break down protein.
Read more: What Is Fat Metabolism? When glucose and fat stores are depleted, your body will turn to muscle to break it into individual amino acids for energy. Unlike carbohydrates and fat, your body does not store amino acids, which is why muscle breakdown is the only way to release amino acids for fuel. Under normal conditions when you're eating on a regular basis, your body doesn't use muscle for energy. Typically, protein is used for fuel only when you're in starvation mode.
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