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In order to turn on muscle protein synthesis, a series of signals must be induced inside the muscle. Simply contracting muscle activates this cellular signaling, but nutrition has an important regulatory role as well. In particular, the signaling pathway that activates mTOR has received a lot of interest in the scientific community because the muscle requires mTOR signaling to increase protein synthesis and subsequently grow. When stimulated, mTOR activates other targets such as 70-kDa ribosomal protein S6 kinase, also called S6K1. This cascade ultimately results in increased translation of mRNAs encoding proteins. Researchers from the Netherlands asked the question: what happens to these critical signaling proteins in muscle when carbohydrate alone or carbohydrate and protein together are provided around a bout of resistance exercise. Before, immediately after, and 1 h after a single bout of resistance exercise, subjects consumed 0.3 gram per kilogram of carbohydrate or the same amount of carbohydrate with an equal amount of protein. The source of protein was a casein hydrolysate. Muscle samples were obtained before and after exercise to assess the level of activation of S6K1 in muscle. These investigators found that during recovery S6K1 was significantly higher when protein was provided with carbohydrate. The study provides a better understanding of how adding protein — in this case a casein hydrolysate - augments the signaling in muscle that turns on protein synthesis and promotes anabolism.

Koopman R, Pennings B, Zorenc AH, van Loon LJ. Protein Ingestion Further Augments S6K1 Phosphorylation in Skeletal Muscle Following Resistance Type Exercise in Males. J. Nutr. 2007 Aug;137(8):1880-6.