Originally Posted by MsMittens
Last I heard the theory on working out on an empty stomach wasn't true. But that seems to depend on who you talk to. I did find
this article. I personally don't do it (I used to) because I'll be concentrating more on what I'd like to eat than the workout (and with so many bakeries along my route it's rather distracting)
It's true, with qualifications. The fact that you have an empty stomach is not critical, the reason there is greater potential for your body to convert fat into it's energy source in the morning (before breakfast) is that it's preferred source of very available energy, glycogen, is depleted.
Your body stores a certain about of glycogen in the liver and that comprises the energy source that will typically supply energy for the first 20-30 minutes of aerobic exercise. Because your body continues to consume energy overnight, but you do not provide it with any, you wake up glycogen-depleted.
It's not all good news though. The body will break down muscle fiber aswell as fat in order to supply it's energy needs. There is good reason for this, which is basically that your body does not *want* muscle. Muscle is a liability to your body, as it increases it's calorific needs, making survival more difficult, and the body wants nothing more than survival. Of course today, supplying your body with calories is not difficult, but we have spent 499,800 years with periods of famine and only a couple of hundred with continuous plenty, so your body isn't quite convinced it should make any changes to it's tried and tested system just yet
Having said that, make sure to provide adequate nutrition and rest post-exercise and your body will reciprocate by repairing and even reinforcing the muscle fibers you stimulated, and damaged, earlier. After all, it needs to be able to survive the next attack.
As a bodybuilder, I would always choose to do my aerobic work first thing in the morning, because the only other way you can begin to access fat stores for energy is by exercising for a significant time first.
Last thing to mention is that the more aerobically fit you are, the more efficiently your body will be able to process fat for energy.