Calorie surplusCaloric surplus · Energy surplus

A calorie surplus is intake above total daily energy expenditure. The excess supports the construction of new tissue and is otherwise stored, mostly as fat.

It matters because building appreciable muscle is easier in a surplus, particularly for people who are already reasonably lean and reasonably trained. Novices and people carrying more body fat can often add muscle at maintenance or in a small deficit; that window narrows as training age rises.

In practice the size of the surplus determines the ratio of muscle to fat gained, and the rate at which muscle can be built is capped by biology rather than by food. A small surplus is enough; a large one simply adds fat that has to be dieted off later.