Burning Muscle (protein)?

I asked this question in another thread back in August but didn't get a reply from anyone and the subject has come up again so I thought I'd give it another shot. It has to do with how the body metabolizes carbs, fats, and proteins.

Ok, I've got plenty of fat on this body that I'd like to burn off. So, during exercise, my body will burn the available carbs and then start to burn fat stores...am I right so far? So at what point does the body start to burn muscle (protein)? And why would it do that when there is still so much body fat available? I'm fairly sure that the burning of muscle has taken place from time to time because my workout clothes will sometimes smell like ammonia.

You are right so far. Look at the next to last paragraph here . Are you eating too much protein? Are your liver and kidney function normal?

Ahhh, I think I understand. At the times I smelled "ammonia" I was on a 1200 calorie diet. The article states, "Furthermore, fats can only be metabolized when there is a certain amount of glucose present to produce oxaloacetate that condenses with acetyl CoA in the citric acid cycle." If I'm understanding this correctly this means when there isn't enough glucose present to metabolize fat that's when you run into problems.

Thanks Zen.

First off, remember that everything "burns" on a continuum. You don't switch from one source to another like flipping a lightswitch on & off.
Next, you should realize that breaking down protein & using it as fuel is not popular with your body. Unless you're starving (literally), or you're doing ultra long duration aerobic exercise (think 4+ hours of running or riding at an effort that's not incredibly easy), only about 8% of what you break down for fuel will be protein (it can creep up to 15-20% in the aforementioned situations).

There is some truth to those charts at the gym that tell you to exercise at a lower intensity to burn fat, because it takes more available oxygen to break down and mobilize fat. Working at lower intensity levels fits this bill. Also, at very high intensities, a low (acidic) Ph in the blood can prevent fat from getting mobilized from the adipose tissue to the muscle in need.

However, don't forget the continuum- when you're exercising at a moderate to high intensity, your energy demands are much greater, so you're likely to burn more of both fat and carbohydrate for fuel, even if you are going a little heavier on the carb side than you were at low intensities.
And yes, you do need some carbohydrates to burn the fat- this just means that during extended durations (50+ min) of moderate to high intensity exercise, you should supplement with some sort of carbohydrate- drink, gel, food, whatever works for you. At those intensities, there is a risk of getting low on carbohydrates if you don't keep them coming in.