Post-Exercise Muscle Protein Synthesis in Rats after Ingestion of Acidified Bovine Milk Compared with Skim Milk

Bovine milkproteins have a low absorption rate due to gastric acid-induced coagulation. Acidified milkremains liquid under acidic conditions; therefore, the absorption rate of its protein may differ from that of untreated milk. To investigate how this would affect muscle protein synthesis (MPS), we compared MPS after ingestionof acidifiedversus skim milkin rats. Male Sprague-Dawley rats swam for 2 h and were immediately administered acidifiedor skim milk, then euthanized at 30, 60, 90, and 120 min afterwards. Triceps musclesamples were excised for assessing fractional synthetic rate(FSR), plasma components, intramuscular free amino acids and mTOR signaling. The FSR in the acidified milkgroup was significantly higher than in the skim milkgroup throughout the post- ingestiveperiod. Plasma essential amino acids, leucine, and insulin levels were significantly increased in the acidified milkgroup at 30 min after administration compared to the skim milkgroup. In addition, acidified milk ingestionwas associated with greater phosphorylation of protein kinase B(Akt) and ribosomal protein S6kinase (S6K1), and sustained phosphorylation of 4E-binding protein 1 (4E-BP1). These results indicate that compared with untreated milk, acidified milk ingestionis associated with greater stimulation of post-exercise MPS.