Many athletes take protein or amino acid supplements before or after weight training, to boost muscle mass and strength. The amino acids from protein act as building blocks for protein synthesis and muscle growth. Also, key amino acids such as leucine turn on chemical pathways that promote protein synthesis. Casein is the main protein found in milk and cheese. Hydrolyzed casein protein contains protein hydrolysates, which are small packets of amino acids produced by breaking down milk proteins with acids or enzymes and water.

A French study found that hydrolyzed casein (HC) was absorbed faster than intact casein (IC). The body absorbs HC better than IC, which results in higher blood levels of specific amino acids (e.g., leucine and phenylalanine) and a greater potential for muscle protein synthesis. The study showed that hydrolyzed casein protein supplements increased blood levels of amino acids and insulin (a highly-anabolic hormone) faster and greater than intact casein, but that IC was a longer-acting protein. (American Journal of Clinical Nutrition, 90: 1011-1022, 2009)