Aerobic metabolism is a way for your cells to convert fat, carbohydrate and sometimes protein into energy, but only in the presence of oxygen. Aerobic metabolism is slow, so it is useful for sustained activities like jogging or dancing rather than short bursts of effort like sprinting or weightlifting.
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Aerobic metabolism converts fat and carbohydrate into units of cellular energy called ATP. Aerobic metabolism is very efficient, producing 34 molecules of ATP from every molecule of glucose, compared to the two ATP molecules anaerobic metabolism generates. Aerobic metabolism is also the only means by which your cells can use fat for fuel, which is part of the reason why aerobic activities like swimming or cycling are such an effective means to lose weight.
Your body performs aerobic metabolism all day long to provide energy for activities of daily living. Because aerobic metabolism requires water and oxygen, it is the reason that humans breathe constantly and part of the reason why you need to drink water. Carbohydrates are a necessary ingredient for aerobic metabolism -- even to burn fat. Because the muscles and liver can only store a limited amount of carbohydrate between meals, aerobic metabolism is also the reason why you need to eat carbohydrates every day. Although your body can metabolize protein for energy in the absence of carbohydrates, it is a slow and inefficient process. Aerobic metabolism in the absence of carbohydrate can only keep up with activities with a very low energy cost, leaving you feeling sluggish.
Aerobic metabolism produces carbon dioxide and water as byproducts. The extra carbon dioxide production from keeping up with the demands of exercise is what makes you breathe hard when you exercise. According to the American College of Sports Medicine, at their maximum exercise capacity, humans only use 75 percent of the oxygen they inhale, meaning that there is no shortage of air coming in. Instead, it is the extra carbon dioxide buildup in the blood and the need to expel it that makes you short of breath.
Activities aren’t solely aerobic or anaerobic. In fact, anaerobic metabolism is the first step in aerobic respiration. Both types of metabolism carry on simultaneously in different proportions depending on your effort level. Even as you sit reading, your cells are still performing anaerobic metabolism. The reason that you aren’t out of breath at rest is that your cells can easily clear anaerobic metabolism’s byproducts as quickly as you produce them.
Aerobic metabolism can only sustain a certain level of activity before it can’t process oxygen fast enough to keep up with the cells’ demands. Anaerobic metabolism supports any additional activity beyond this point, producing lactic acid as a byproduct. As the Sports Fitness Advisor explains, eventually your body reaches a point where it is producing lactic acid faster than it can clear it, forcing you to slow down. The amount of time it takes for a person to hit that “wall” depends on the intensity of exercise and her individual fitness level.