Described herein are compositions suitable for the treatment of pathologicalconditions distinguished by ' insufficient or reduced mitochondrial function.The compositions comprise, as principal active ingredients, the amino acidsleucine, isoleucine and valine. Preferably envisaged, as further activeingredients, are the amino acids threonine and lysine, and possibly histidine,phenylalanine, methionine, tryptophan, as well as tyrosine and cyst(e)ine.