I'd also add psychologically. And the way our psychology impacts our physiology plays a large role, as well. The burgeoning field of psychoneuroimunnology helps to explain this, not just in the role it plays in our being overweight, but also the effect our minds, and stressors, have on disease. More of it starts in your head than you might think. It seems that, finally, Western science is making the mind/body connection of ancient Eastern medicine that we foolishly chose to sever long ago. Though, It's too bad we needed more science to figure out what was already known. Why Zebras Don't Get Ulcers by Doctor Robert Sapolsky is a great book for this.