I just read a really interesting idea about fat buring during excersize. It's pretty well known that your body has two kinds of fuel to keep you going. The first is fat, the second is carbs. When you're excersizing below 75% of your maximum heart rate, in the aeorbic zone, you're buring mostly fats, a great way to loose weight. When your're above that 75% you burn carbs stored in your muscles as fuel anerobically. Then you eat and replace that fuel with little change in your fat stores.
The guy who was writing this said that he theorized that most people when starting to excersize were in such bad shape that basically any exertion put them into the anerobic heart range and they weren't burning fat at all. They actually had to get in shape FIRST before they could start to have meaningful (in regards to fat loss) excersize. This requires 1-3 months of cardio conditioning and then tailoring workouts with a heart rate monitor to ensure that most time is spent in the anerobic range. Needless to say, working out for 3 months and seeing no significant weight loss is pretty demoralizign to most people so they quit before they start to loose weight. This is something he wants to study clinically.
I though it was a really really interesting idea. One which has medical merit and makes sense antecdotally. Just thought I'd pass this along to help motivate anyone who is having trouble sticking with an excersize regimen. I just started biking and running agian about 3 months ago. I was in pretty good shape to start with but it was a full 6 weeks before my resting heart rate began to drop. Resting heart rate is a primary sign of cardiovascular fitness. It's really only since then that I've been able to feel myself getting stronger.

By John on Wednesday, October 22, 2003 - 11:07 am:

Some other things related to fat burning:

Stored glycogen(sugar) is stored in two places in the body:

Your liver
Your muscles

The body's glycogen stores are very limited. The average 160lb man has about 400-600g of stored glycogen or about 2400 calories.

When you eat carbohydrates, your blood insulin level rises, causing the body to replenish depleted glycogen stores.

However, once those stores are full, any additional consumed carbs are converted to tryglicerides by the liver and stored as fat.

A high insulin level (caused by eating a large amount of carbs for instance) causes the body to STOP burning fats for energy until the level drops to normal.

Glycogen is the "quick energy" fuel for the body. Therefore it IS the body's preferred fuel for quick intense exertion.

However, the body's muscles will easily begin burning ketones(the digestive product of stored fat) when glycogen is depleted.

In many ways, ketones are preferred by the muscles, providing greater energy output than stored glycogen without the cramping effect caused by glycogen metabolism (lactic acid burn).

A lower carbohydrate diet in combination with low intensity exercise is probably the best combination for building muscle and losing fat.

This is well known by body builders who sometimes strive for under 5% body fat.