I read once that think of carbs like sugar. So when you eat big bowl rice it becomes sugar in your blood. Then your pancreas release the hormone insulin to tell cells to open up and let in energy. But if you are not exercising after you eat the rice the muscle cells don't need it so your fat cells take it. But when that happens your sugar drops quickly and you lose all energy and feel bad. But you gain weight too.

I think its hard to gain weight and feel good too.