A protein synthesis promoter that exhibits a protein synthesis-promoting effect includes a whey protein hydrolyzate having a molecular weight distribution that is within a range of 10 kDa or less and has a main peak of 200 Da to 3 kDa, an average peptide length (APL) of 2 to 8, a free amino acid content of 20% or less, a branched-chain amino acid content of 20% or more, and an antigenicity equal to or less than 1/100,000th of that of ²-lactoglobulin.
