There is provided a method comprising orally administering a composition before and/or during exercise. The composition contains a casein hydrolysate containing a free amino acid and a peptide, or a mixture of the free amino acid and the peptide. The casein hydrolysate is produced by hydrolyzing an animal milk casein, and has an average chain length of 2.1 or less in terms of the number of amino acid residues. There is further provided an oral composition for use in the method. The oral composition contains, as an active ingredient, a casein hydrolysate containing a free amino acid and a peptide, or a mixture of the free amino acid and the peptide. The casein hydrolysate is produced by hydrolyzing an animal milk casein, and has an average chain length of 2.1 or less in terms of the number of amino acid residues. There is still further provided the use of the oral composition for inhibiting increase in heart rate during exercise.
