I've been on both ends of this. I was lean throughout college, gradually gained some weight after starting at Google and after a few years I was having trouble functioning after not having eaten for a while. I got some blood work done and everything was normal.

For me personally, I think it was completely driven by my mind. I was stressed while there and became further stressed when my willpower inevitably lost to my biology. Consistent, hard cardio; lowering my stress levels; and (most importantly) acknowledging and accepting that I don't have perfect control over the outcome were key to fixing it.