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My question is basically whether aerobic exercise training makes one more effective at utilizing a greater proportion of FFAs (free fatty acids) as opposed to glucose under resting conditions. So this excludes any increases in resting metabolic rate.

For example, if you take a sedentary individual and train that person with aerobic exercise for 6 weeks, will that individual be any quicker to lipolysis in response to normal sedentary activities (e.g. walking) after being aerobically trained? If we assumed the resting metabolic rate didn't change, should he or she rely on any greater percentage of FFAs?

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What do you mean with FFAs? –  Ivo Flipse Jun 15 '12 at 19:43
    
FFA stands for Free Fatty Acid. During the process of lipolysis, stored fat is broken down and FFAs are released into the bloodstream. The question at hand is how aerobic conditioning affects the process of lipolysis and, by extension, the amount of FFAs released into the bloodstream. –  Moses Jun 15 '12 at 23:48
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2 Answers 2

Since asking this I read chapter 2 of "Exercise Physiology Integrating Theory and Application" which explained a lot to me. e.g. "Endurance-trained individuals metabolize more triglyceride or fatty acids and less glycogen or glucose at the same absolute workload"

This question could be more succinctly restated as, "does aerobic exercise (endurance) training change the RQ or RER at rest?"

The answer appears yes,

Individual responsiveness to exercise-induced fat loss is associated with change in resting substrate utilization

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I'd have to reread my physiology book, but I don't think the body will prefer FFAs as a source of energy over 'simpler' forms of energy. I believe the body first uses up your regular store of energy before breaking down FFAs or other energy stores. Which I believe is also the reason why you have to work out at least for some amount of time before real FFAs 'burning' to occur –  Ivo Flipse Jun 16 '12 at 6:21
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In order to answer your question, it's best to review how the body's energy systems work. An article called "Death by Prowler" by Matt Reynolds has a good overview. It's an educational read, but I'll summarize a couple high points here:

  • Aerobic activity burns more fat in proportion to sugars.
  • Aerobic metabolism does not shut off when you push into higher demand energy systems (i.e. anaerobic activity).

Based on the Katch McCardle algorithm, resting metabolic rate is most affected by the lean mass you have. I.e. more lean mass, more calories burned at rest.

Now, the bottom line is this: you will burn more calories per given amount of time when you exercise at an anaerobic level. The major caveat is that you won't be able to keep up the anaerobic exercise as long as you can aerobic exercise. Even though the majority of the calories burned during exercise will come from blood sugars/glycogen in an anaerobic state, the body will have to free energy from your fat stores to make up the energy demands. This is what is commonly referred to as the "after burn" effect.

With aerobic exercise, your body is able to keep supplying energy at the rate it uses it. While this is sustainable, once you stop exercising, you no longer have any demand for more energy. Thus, there is no increase in resting metabolic rates.

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