Check out this article on how the body can adapt to exercise and diet, some thoughtful reading.
"Nature and evolution have made our bodies adaptive to all sorts of environmental variables. Just as with weight training exercises our bodies adapt to the stress of added weight resistance. Our bodies adapt to dietary stress also for example you can imagine how cultures have cropped up in desert isles such as Australia where little or no carbohydrates would have been consumed as a staple in that cultures diet. The aborigine were eating mostly meet and fish occasionally fruits, nuts or roots but not on a regular basis. Their bodies had to regulate blood sugar from proteins and fat. That's the adaptive nature of the human metabolism. Upon crawling out of the "Primordial ooze" of life, so to speak we humans were hunters; we nomadically traveled from plain to plain hunting, following the herds of mastodons, antelope etc. Some times going long periods of time with little food if any at all and certainly a diet high in carbohydrates could not have existed under those conditions, but our bodies adapted.
That's the amazing thing about the human body it WILL inevitably adapt to stress physical, dietary, mental etc. What I'm going to show you is how to use the adaptive nature of your body to your advantage, rather than your disadvantage. See our bodies use calories to maintain an operational status. If there is an excess of calories above what is needed it is either processed as fat or passed through the kidneys bowls liver and exerts a strain on your whole organ and soft tissue system. If there is a deficit then your body must use the tissue saved for such an emergency, as it would have 10,000 years ago. It burns fat, skeletal muscle, and in severe cases organ tissue. The trick is not to "Loose weight" but FAT obviously. Loosing fat and not muscle is what we are after in the battle of the bulge. On the other side of the spectrum, if you are skinny and underweight you should find ways to gaining weight and muscle. "