This is ultimately a result of interactions among genetics, diet and physical activity. Metabolism is the process by which the body converts what you consume into energy. Even when at rest the body needs energy for functions such as breathing, circulating blood and repairing cells. When “energy in” (food and drink) goes down, “energy out” (exercise expenditure) goes down to match it. You will burn fewer calories in response to eating less. This is how the body prevents unwanted weight loss and starvation. Likewise, when “energy in” goes up, “energy out” tends to go up too.