Many people maintain their bodies in a state of energy balance within quite narrow limits, as evidenced by a constant body weight over a prolonged period of time. However, everyone has days, even weeks, of substantial energy imbalance. Over time, short periods of energy excess are balanced by energy decit. This compensation is difcult to detect and occurs subconsciously. When there is energy imbalance sufcient to cause weight gain or loss, this is usually a very small imbalance, of the order of 1%–2% over a long period of time. Again this usually goes unnoticed, and the mismatch is too small to be measured.