I'm not an exercise physiologist so I deal in concepts only, but I pulled out an article I have from a Sally Edwards' presentation I attended several years ago to get some numbers. For more precise info, read any good books on training or heart monitor books. www.heartzones.com

Here goes for zones 1-3 (of 5 and where you spend the most time):

Zone #/% Max HR/% Fat Cals/% Carb Cals/% Protein Cals/Cals burned in 30 mins

Zone 1 / 50-60 / 70-85 / 10-25 / 5 / >180
Zone 2 / 60-70 / 50-70 / 25-50 / 5 / >240
Zone 3 / 70-80 / 40-60 / 50-85 / 5 / >330

If you do the math, you will see that the number of fat calories burned is approximately the same and even a little higher in zone 3, but the total calories burned goes up. The calories of fat as a percentage go down but not the actual amount of fat burned.

The fat burning capacity of individuals varies widely, but these are some averages. Burning capacity is based on unique physiology, fitness, dietary habits, frequency and time of eating.