Casein protein is widely used in bodybuilding due to its unique slow-digesting properties, which make it distinct from other protein sources such as whey. Scientific evidence supports the use of casein for muscle recovery and growth, particularly when consumed before periods of fasting, such as overnight. Several studies have shown that casein ingestion leads to a sustained increase in plasma amino acid levels and muscle protein synthesis over several hours. For example, a study published in the American Journal of Physiology-Endocrinology and Metabolism (2012) demonstrated that consuming casein protein before sleep increased overnight muscle protein synthesis in healthy young men. Additionally, a meta-analysis in Sports Medicine (2014) concluded that both whey and casein protein supplementation can significantly enhance lean mass gains and strength in resistance-trained individuals, with casein being particularly effective for promoting an anti-catabolic environment during periods without food intake.

While the majority of research supports the efficacy of total protein intake for muscle hypertrophy, casein's slow absorption profile makes it a preferred choice for bodybuilders seeking to minimize muscle breakdown during prolonged fasting, such as overnight. The evidence does not indicate that casein is superior to other high-quality proteins for all aspects of muscle building, but it does support its specific use for sustained amino acid delivery. Thus, the use of casein protein in bodybuilding is well-justified by scientific research.