Energy balance is the difference between energy intake (food and drink) and energy expenditure (basal metabolism, physical activity, digestion, and non-exercise movement). When intake exceeds expenditure, the body stores the surplus; when expenditure exceeds intake, stored energy is drawn down.
This is a thermodynamic reality — but “calories in, calories out” is often presented as simpler than it is. The body is not a passive calculator. Metabolic rate adapts to prolonged restriction (adaptive thermogenesis), suppressing expenditure and making a deficit progressively harder to sustain. Hormones governing hunger and satiety (leptin, ghrelin) shift unfavourably during weight loss, increasing appetite. These adaptations are part of why maintaining weight loss is harder than achieving it.
The quality of food also affects energy balance indirectly: protein has the highest thermic effect of feeding; fibre-rich foods promote satiety; ultra-processed foods appear to drive passive overconsumption in controlled trials.
Energy balance is the mechanism; it is not a complete weight management strategy on its own. See calorie deficit and set-point theory for related concepts.