We all know people who eat like horses yet are as skinny as rakes, while others seem to pile on the pounds just thinking about food. At the heart of the difference is metabolism, the sum of the chemical reactions occurring in the body at any given time. Clearly there is more to it than a simple balance between calories eaten and calories burned through exercise. Regardless of whether your metabolism is fast or slow, our bodies are designed to store excess energy in fat cells. So if you eat and drink more calories (energy “intake”) than your body expends (energy “output”) you will gain weight. On the other hand, if you eat and drink fewer calories than are burned through everyday activities (including exercise, rest and sleep), you will lose weight.