The term “aerobic” refers to cellular metabolism where ATP is produced “with oxygen” utilized in the process, and “anaerobic” refers to cellular metabolism where some or all of the ATP is produced “without oxygen.” When we are “aerobic,” our bodies are functioning at lower intensities where ATP can be produced with oxygen; or better stated, if our working muscles, organs, brain, etc. are getting all of the oxygen they need, then our body is producing energy aerobically. This can include what we think of in the fitness world “aerobic exercise” (moderate-to-vigorous intensity exercise), but also includes activities that are less strenuous such as reading, washing dishes, sitting at a computer and even sleeping. During all of these activities, your body is able to produce the ATP needed utilizing oxygen in the process. The fuel sources for aerobic cellular metabolism are fats and carbohydrates, with a greater percent of kcals coming from fats at lower intensities. As we start to move and increase the intensity, the percentage of kcals from fats begins to drop as the percentage of kcals from carbohydrates increases. The higher the intensity, the more we utilize carbohydrates as a fuel source. With the exception of very short duration, very high intensity anaerobic activities such as a 100 meter sprint, our bodies generally use a combination of carbohydrates and fats to produce the ATP.

Every time you exercise, your caloric expenditure goes up to meet the demands of the working muscles. This happens at ALL times of the day. This energy comes from fats and carbohydrates in proportions that vary based on the level of intensity as described above. Now, this does not mean that you burn more total kcals from fat at lower intensities than higher intensities. It generally works out to be the opposite, where your body burns more kcals from fat at higher intensities even though the percentage of kcals from fat is less. How can this be? Check out the following link to the Fitness Q&A section of the ACE website where you can read an example of how this works…

https://www.acefitness.org/fitfacts/f...spx?itemid=265

When we exercise, whether at 5:00 AM, 1:00 PM, or 10:00 PM, our bodies require increased kcals to meet the demands of the working muscles. Each individual has times of day that his or her body responds better to exercise for a variety of reasons (circadian rhythms, stress of the day, routines, meals, etc.), but there is no “set time” that is the “only time that aerobic exercise is effective in burning fat.”

Q: Will I lose body fat more efficiently by performing my aerobic workouts at a low, rather than a high, intensity?

A: Many aerobic exercise programs and videos feature low-intensity workouts which purport to maximize fat burning. The argument behind such an alleged theory is that low-intensity aerobic training will allow your body to use more fat as an energy source, thereby accelerating the loss of body fat.

While it is true that a higher proportion of calories burned during low-intensity exercise come from fat (about 60 percent as opposed to approximately 35 percent from high-intensity programs), high-intensity exercise still burns more calories from fat in the final analysis.

For example, if you perform 30 minutes of low-intensity aerobic exercise (i.e., at a level of 50 percent of maximal exercise capacity), you'll burn approximately 200 calories - about 120 of those come from fat (i.e., 60 percent).

However, exercising for the same amount of time at a high intensity (i.e., 75 percent of your maximal exercise capacity) will burn approximately 400 calories. Using a 35 percent fat utilization yardstick, 140 of the calories you've burned will have come from stored fat.

Although the more vigorous exercise burns both more total and more fat calories, the less intense form of exercise has its benefits as well. For example, because many overweight people tend to find that lower-intensity exercise is more comfortable, they may, therefore, be willing to engage in such workouts.

The point to remember is that low-intensity workouts do, in fact, promote weight and fat loss. You just have to do them for a longer period of time.

Low-intensity aerobic exercise, however, is not a better or more effective way to lose weight than more intense physical activity - the idea of a "fat-burning zone" is simply a myth.

Keep in mind that you lose weight and body fat when you expend more calories than you consume, not because you burn fat (or anything else) when you exercise.

Source: Bryant, Cedric X. 101 Frequently Asked Questions about "Health & Fitness" and "Nutrition & Weight Control". Sagamore Publishing, 1999.