While exercising, your blood has to be pumping at the rate of 120 or whatever it is to burn fat. How long do you have to have to exercise at this rate in order to burn 1 gram of fat? What about calories? What's better, a bread with 70 calories with 1 gram of fat or a bread with 90 calories with 0 gram of fat?

sleeksiamese, hi, you ask a pretty good question.. and it got me thinking.. let's see ;D... now.. first things first.

To get any fair amount of blood pumping going, and depending on your age, resting heart rate, and fitness level, a person would need to exercise within the 60% to 85% of the maximum heart rate. For example, if you are 20 years old, 60% is 120 beats/min. At 85%, it would be 170 beats/min.

Now as far as losing that 1 gram of fat, this would have to depend on the kind of activity you're doing and at a determined amount of time, assuming you are doing the activity non-stop continuously within that amount of time.
But the calculation I am about to give is very general, because it does not take into account the speed at which you are doing the activity, your weight,and age. My calculation only takes into account your weight.

For example, if you are doing light activity, such as shopping, and you weigh, say, 120 lbs. TO burn off 1 gram of Fat or 9 KCAL (technically speaking, 1 gram of Fat is 8.98 KCAL to be exact), you would need
about 3.58 mintues. The general formula I used is:

0.021 KCAL/min./lb.

So, to get that 3.58 minutes, I did the following calculation:

(.021KCAL)/60 min./120 lbs.= 151.2 KCAl/60

min.= 2.52KCAL/ min. X 3.58 min. = 9.02 KCAL

To answer your 1 bread comparison, if you mean by "better" as in losing the KCAL from FAT, PROTEIN, and CARBOHYDRATE, 90 grams of 0 calories from FAT is more preferred, with everything else being equal.

Since FAT is a concentrated source of energy and 2.25 times as much energy per gram than PROTEIN and CARBOHYDRATE, FAT, thus, takes 2.25 times longer to digest in 1 bread of 70 calories with 1 gram of FAT. It must be remembered that insulin is required to stored FAT. What that means is it takes time to stored FAT and time for 9 KCAL in 1 gram to be completely oxidized. :) ;)