Our leptin, a powerful hormone which is created by our fat, governs aspects of our metabolism and hunger. When we eat more, our leptin levels increase and our brain finds us full and the leptin perks up our metabolism, when we eat little, our leptin levels drop, we start obsessing about food or feel hungry more often, and our metabolism (I believe we have a short term and long term metabolism though I cant expain the difference) starts to slow down. Once we eat more the leptin levels increase again and the metabolism perks up more etc. I'm not a scientist so I hope I explained that right, but you get the gist.