3500 calories = 1lb of ....

Ok... do 3500 calories = 1lb of fat or just 1lb of a mixture of things?

I ask that to ask this... which is better? Low intensity cardio that stays in your fat burning zone and thus you burn FAT, or high intensity cardio that elevates your heart rate out of the fat burning zone and ends up burning carbs/sugars?? I understand that it's all about the calorie deficit, but if you end up burning 3500 calories, perhaps it's not all fat, whereas it would be if one did low intensity cardio to stay in the fat burning zone? Confused-- please help. Thanks!

If you are trying to burn fat, low intensity for a longer period of time.  You can also intermix it with short bursts of intense cardio.

The 3500 calories question, I don't know, I always thought it was fat.  However when you burn calories thru diet and exercise, it's never all fat.  However I think if you do some strength training you can reduce  muscle loss.

While it may be true that you burn more fat calories (percentage wise) at lower intensity, most of the stuff that I have read says that you will burn way more overall calories at higher intensity-including fat calories.  Be sure to add some weight training and get plenty of protein to help prevent muscle loss.

It's fat.  1 lb of muscle is a lot less than 3500 cal.  As far as I recall, it is appx. 1700 cal = 1 lb of muscle or something like that.  So if you find someone saying s/he lost 2 lbs by creating 5000 cal deficit, it would probably be the mixture of fat and muscle (and possibly some water too).  To burn pure 2 lbs of fat, it does require 7000 cal deficit.