Technically you can get fat on fewer calories. You just gotta nuke your thyroid.  Now being the "efficient machine" thats a whole new can of worms.  Known people with thyroid issues. The ones who did it to themselves usually also gave themselves the beetus.  Had one boss, like scar across his neck where they opened him up to access his thyroid. He was eating several fast food meals at a time, ending up in the bathroom and wasting away. So they "fixed" the glitch.  He didn't eat much after that. But he was always small fat. He'd also get really tired and couldn't lift very much after that. It was always funny to bring up his Columbian necktie in front of customers.