Nowadays, the partial replacement of dietary protein energy (PE) by dietary non-protein energy (NPE, i.e. lipid and carbohydrate) is a common practice in the aquaculture sector. Marine and fresh water fish species is thought to have more protein energy available for growth when NPE is incorporated in the diets. Moreover, lower dietary PE implies less discharge of nitrogen compounds to the aquatic environment. The “sparing-action” of NPE over PE has, however, its limitations. As the NPE content of the diet increases so does the requirement for certain components (e.g. enzymes, hormones, cofactors) involved in the NPE metabolism. In particular, the role of several endogenous (reproduction, life cycle, tissue requirements) and/or exogenous (swimming activity, water temperature and salinity, crowding, fasting) has been shown to affect the mobilization and degree of essentiality of NPE substrates. This mini-review aimed to present an overview of recent work on the influence of swimming activity on the relative importance of the non-protein oxidative fuels in the energy partitioning of fish.