I have a question. .. How do you know if you're in the fat-burning mode while you're exercising, or if you've switched to burning muscle? Is there something you feel when that happens? I eat a few blocks, then go out and power-walk, and I want to make sure that I'm working off the fat, not anything else. How can I be sure? Also.....theoretically, if one is in the fat-burning mode when one is in the Zone, can't one just eat..exercise...eat...exercise..eat...exercise....etc?

From everything that I've read, fat is the body's preferred fuel source if you keep your heart beating between 60%-80% of your maximum heart rate. A good way to tell whether or not your in this range is by paying attention to how you're breathing. If you're breathing too fast then you're more than likely over 80%. Not breathing hard enough and you're probably under 60%. Hope this helps. Mike

Hi! Personally, I can tell I'm in fat-burning mode if... well... if I pee a lot. You can definitely tell if you're 'peeing out fat cells' as I like to call it. By the way, if you're in exercise mode, try this... whenever you think of it, engage your abs. By this I mean, pretend there's a string that goes from your navel through your body and comes out to the middle of your back. Pull the string and hold it. You can do this while you're driving, walking, whatever. It really does work to tighten up your abs and, of course, now that you've been losing weight around your middle, you do need to tighten up those muscles. :P

It's true that low-intensity exercise burns a higher PERCENTAGE of fat than high-intensity exercise, but the ABSOLUTE amount of fat burned per unit of time is actually LESS during low-intensity exercise. There is no such thing as an optimal "fat burning zone" corresponding to heart rate- that's fitness mythology. All of the muscle fibers in your body use a certain percentage of aerobic and anearobic energy EVERY time they contract, and because slow-twitch fibers have a greater aerobic capacity than the fast-twtich fibers, they generally burn more fat. Now, when you jog slowly, you obviously use mostly ST fibers and therefore burn mostly fat. However, as you start to jog a little faster, you start to recruit more fast-twitch fibers in addition to the ST fibers. However, those fat-burning ST fibers are still working in addition to the FT fibers, so even though the relative percentage of fat being used is lower, the absolute amount is actually greater because the FT fibers also use a certain amount of fat.