If you are into fitness, chances are you have heard the terms "anaerobic" and "aerobic" being thrown around by people who want to seem real gurus of exercise. Such "gurus" will often tell you that only one of these two burns fat (opinions differ on which one, though). Don't accept such claims on blind faith - learn more of the science behind them!
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Two types of metabolism
Our cells have to constantly convert nutrients into energy, stored in body in the form of a complex molecule called ATP (adenosine-triphosphate). The primary source of energy is glucose, but there are two main ways to convert it into energy: aerobic (with oxygen) and anaerobic (without oxygen). Normally, our cells prefer the aerobic method (we get up to 90 per cent of our energy this way), since it produces almost 40 ATP molecules out of one glucose molecule, as opposed to just 4-6 with anaerobic metabolism. However, aerobic breakdown of glucose is only possible when cells get enough oxygen. For example, when you are walking, jogging, or doing some light exercise, your cells can keep producing energy for long periods of time, and you don’t get tired. The downside is that you don’t burn fat so easily, either.
What about anaerobic metabolism?
Whenever we have to make a huge physical effort fast, we start breathing heavily… but soon even that oxygen is not enough, and anaerobic breakdown of glucose kicks in (usually it happens after 1 minute or so). It uses different metabolic reactions and can be performed very fast thanks to the help of special enzymes. However, it is inefficient and creates waste products (the infamous lactic acid). The body quickly uses up the available glucose, and we feel fatigue; waste products accumulate, and we experience pain (remember the last time you had to go up several hundred stairs!). This anaerobic activity cannot be sustained for long, and we need to slow down, so that our cells can use aerobic reactions again.
So how do I burn fat?
When Usain Bolt runs his 100 meters, he uses almost exclusively anaerobic metabolism. All fast, explosive physical activities (weight lifting, bouldering, sprinting, etc.) rely on anaerobic reactions. However, our general fitness routines are a mix of the two: say, riding a bike on plain surface is mostly aerobic, but riding it uphill starts to use anaerobic reactions more.
Both kinds of training burn fat, but aerobic activities do it slower (as you may know, walking burns up to 150 calories an hour, while rock climbing burns up to 800). However, aerobic training is extremely beneficial for your heart, lungs, and general endurance. Besides, anaerobic training, being much more intense and heavy on the muscles, tendons, and heart, is simply not accessible to many people. It would be unwise to make an overweight, unfit person start with weight-lifting or a sprint right away!
Studies prove that anaerobic, intense training burns fat more efficiently. However, it is not just about fat: remember that cardiovascular disease is the prime cause of death in the West, and it is a balanced combination of both kinds of activities that can promote heart health (see this study, for example). Therefore, make sure you don’t just focus on one: combine bike-riding with rock-climbing, or jogging with jumping lunges and squats, or martial arts with hiking, and you will get the best of both worlds!