ABSTRACT
The concept of energy balance is a simple one. An adult animal in energy balance is neither losing nor gaining weight. The concept does not apply to immature, growing animals nor does it apply to pregnant or lactating females. By denition, an animal (including humans) in energy balance is consuming enough energy to offset that which is expended. Mathematically, it is expressed as follows:
Σ Σ kcals consumed kcals expended=
Within the framework of energy balance, there are day-to-day variations in both intake and expenditure, but these are of little importance when the long-term energetic efciency of the individual is maintained. Should long-term perturbations occur on either side of the equation, then energy balance will shift such that weight is either gained or lost and a new balance is established. If weight is lost, the individual is in negative energy balance, while if weight is gained, the individual is in positive energy balance. If weight is lost or gained with no change in energy expenditure, the weight loss or gain will affect the adipose tissue fat store. If expenditure is also changed (adding or reducing physical activity), then the body composition will change as well. Increased physical activity will stimulate muscle development, while a reduction in activity will result in reduced muscle mass. Body composition change affects the energy balance because muscle and adipose tissue consume fuel sources differently.
