They say that timing is everything. Well, this old cliche applies to aerobic exercise as well. There are extremely fundamental factors regarding aerobic exercise timing and intensity that are constants and not to be argued — or are they?

Even in today’s age of modern research technology there remains some controversy on the issue of fat loss as it pertains to aerobic activity timing. NFPT has a long-standing position on this topic. Before we get down to the heart of the issue, let’s reestablish an understanding of the above mentioned fundamentals: timing and intensity.


Fat and Glucose Energy Use Relative To Timing and Intensity of Aerobic Exercise

Relative to total body function, at NO time is energy derived solely from fat or solely from glucose during the performance of aerobic exercise. Regardless of intensity, fuel used for exercise comes from a mixture of fatty acids and glucose.

  1. High intensity exercise calls upon greater glucose reserves and less upon fat reserves. Conversely, low intensity exercise calls upon greater fat reserves and less upon glucose reserves
  2. Activity performed in the presence of an abundant supply of blood glucose will draw more glucose energy than if performed in the absence of substantial blood glucose. Moreover, in the absence of significant amounts of circulating blood glucose, activity performed will draw upon a more significant amount of fatty acids and glycerol for energy.
  3. If the duration of activity is prolonged, fatty acids are the energy source of choice in the body, while short periods of activity generally requires less fatty acids and more glucose by comparison.

It is said by some that performing aerobic exercise during low blood sugar conditions will result in greater fat loss than that same exercise performance in the presence of higher levels of blood glucose. This is consistent with the NFPT position on the matter. On the other hand, there are those who say it does not matter when aerobic exercise is performed: In the course of a day, the energy coming from blood glucose sources during aerobic exercise performance are replaced by fatty acids thus resulting in the same amount of fat loss. Which theory is correct? For this second theory to be true, the carbohydrate-loaded subject would have to practically fast (not ingest any carbohydrate foods) for a prolonged period following his/her aerobic exercise performance in order for a significant amount of fatty acids and glycerol to be released during the post-exercise period, hence replacing glucose expenditures.

Keep in mind that one of the major differences between aerobic and anaerobic activity is that aerobic exercise does not accelerate the metabolism after exercise as anaerobic exercise does. The reason this is a significant recollection in this issue is that aerobics require energy during performance, but little in the way of recovery (recovery energy replacement is generally indicated by increased metabolism which does not occur after aerobic training).

What does all this mean to you? It means that it is best to perform aerobic exercise for fat loss during periods of low blood sugar levels (brief periods of ‘fasting’). Do not get confused however, for peak aerobic performance, it is not good to exercise ‘fasted.’ You will have no energy reserves and glucose exhaustion will lead to shortened performance. Keep in mind that when intense aerobic exercise is performed, there is a greater need for blood glucose. During intense aerobic exercise performance while in a somewhat ‘fasted’ state blood glucose reserves are not abundant and the body will undesirably resort to using amino acids, blood proteins, and even muscle tissue for energy. Fasted aerobic exercise needs to be low intensity to ensure the least amount of amino acid deamination and tissue cannibalism.

Fat Loss and A.M. Aerobic Activity Performance

What it all boils down to is that when you awake in the a.m., your blood glucose levels are generally at their lowest, as you have not ingested any calories for several hours. During the a.m. planned low intensity aerobic exercise, your body will have little choice but to draw primarily from its most abundant energy resource: fatty acids and glycerol from adipose tissue (fat), minimizing glucose uptake primarily due to its reduced presence in the blood (‘fasted’ state).

So, aerobics before breakfast, anyone?