Casein peptides are derived from casein, the primary protein found in milk. Their use in bodybuilding is primarily justified by scientific evidence, especially in relation to muscle protein synthesis and recovery. Several studies have shown that casein protein, due to its slow digestion rate, provides a sustained release of amino acids into the bloodstream, which may help reduce muscle breakdown during prolonged periods without food, such as overnight. This property distinguishes it from faster-digesting proteins like whey.

Clinical research has demonstrated that casein supplementation can lead to increased muscle mass and improved body composition when combined with resistance training. For example, a study published in the American Journal of Physiology-Endocrinology and Metabolism found that casein ingestion before sleep increased overnight muscle protein synthesis in healthy young men. Another randomized controlled trial indicated that casein supplementation led to similar or slightly better lean mass gains compared to whey when taken in adequate amounts.

While casein peptides (hydrolyzed casein) may offer faster absorption than intact casein, most studies focus on casein as a whole. Nevertheless, the general consensus is that casein proteins, including peptides, are effective for supporting muscle growth and recovery in bodybuilders, especially when used as part of a high-protein diet and resistance training program.