PROBLEM TO BE SOLVED: To provide a composition which can increase or maintain muscle volume safely and efficiently.SOLUTION: Provided is a composition containing lactic acid and/or a salt thereof and caffeine, wherein the content ratio of lactic acid and/or a salt thereof and caffeine is 0.9:1-1000:1 as a weight ratio (but in the weight of lactate, the value converted into the weight as lactic acid is used).