It all comes down to hormones and insulin. These concepts changed my life. I was always weight conscious: I used to work out all the time, try to eat "right" and would constantly hover between 10-12% bodyfat. I switched to eating "primally" (see the blog) and I feel better, am stronger than ever before, have near perfect vitals (yes, I've had blood work done since I started eating this way) and hover between 6-8% body fat now. I am not super diligent with it, so I don't have a full-out six pack, but I do have a four pack and those major veins coming up my biceps when I'm exerting myself . My clothing fits better than ever.
personally, I'm 145/ 6'1", eat 2500-3000 calories a day (high carb, low sugar and fat) and don't do any dedicated exercise. Aside from a high metabolism and being generally high strung (correlative imo), I also believe I have an extremely short digestive track. I've read a study out of Harvard by William Sheldon that was conducted in the 20's (?) that referenced autopsical evidence of the variation of adult male intestinal length reaching 50%. Does anyone with a scientific background know of any related findings? This is really the flip side of metabolic rate (digestive potential) and I'm amazed that it is never discussed.