The amino acids can be used as free amino acids as well as salts, such as hydrochloride or alkali, earth-alkali or ammonium salts.Further, oligopeptides having an average of 2-3 amino acids, which have a high percentage (>50%, especially >70%) of the specified amino acid, can also be used according to the invention.Especially preferred as per the invention are arginine, asparagine, asparaginic aci