The present invention relates to an agent for improving physical fitness which contains cystine as an active ingredient. The present invention provides an amino acid-containing nutritional composition that can obtain an improving effect that is superior to the prior art with respect to promoting the improvement of physical fitness associated with physical functions.本發明關於一種含有胱胺酸作為有效成分之行動體力提升劑,提供一種含有胺基酸的營養組成物,在關係到身體機能的行動體力的提升促進方面,與以往技術相比,能夠發揮較優異的改善效果。