Dietary Creatine Supplementation Reduced the Proportion of Dietary Arginine Directed toward Guanidinoacetic Acid (GAA) Synthesis and Reduced the Demand for Methionine‐Derived Methyl Groups, but Did Not Enhance Whole Body Protein Synthesis in Neonatal Piglets

Our previous work demonstrated that dietary arginine (Arg) intake influenced tissue creatine concentrations in neonatal piglets fed creatine‐free diets. Both Arg and methionine (Met) are required for creatine biosynthesis, as L‐arginine:glycine amidinotransferase (AGAT) converts Arg to guanidinoacetic acid (GAA),which is subsequently methylated by guanidinoacetate methyltransferase (GAMT) to form creatine, with Met as the methyl donor. In this study, we manipulated dietary Arg, Met, GAA and creatine supply to quantify the partitioning of amino acids towards GAA and creatine synthesis. Piglets (9 – 11 d old, N =35) were fed one of five elemental diets: 1) low Arg and low Met (Low Arg&Met), 2) low Arg and Met plus GAA, 3) low Arg and Met plus creatine 4) high Arg and high Met (High Arg&Met) or 5) low Arg with high Met plus GAA.On d 6, piglets underwent a primed, constant infusion of stable isotope tracers of Arg, GAA and creatine to measure Arg partitioning, and tracers of phenylalanine and tyrosine to measure whole body protein synthesis. On d 7, a second infusion of 3 H‐methyl‐methionine was conducted to trace the partitioning of methyl groups to creatine synthesis. GAA synthesis was limited by Arg and/or Met since piglets fed the Low Arg&Met diet had less Arg conversion to GAA compared to the High Arg&Met group (P<0.001). Renal AGAT activity in the Low Arg&Met was significantly higher than all other groups (P<0.01), suggesting that the lower conversion rate was due to inadequate substrate availability and not due to lack of enzyme activity. The inclusion of GAA with low Arg and Met resulted in significantly lower Arg partitioning to GAA (P<0.01) and to creatine (P<0.001). Liver GAMT activity was similar across diet groups, except when GAA was supplied with high Met (P<0.01), suggesting GAMT was induced to accommodate creatine synthesis only when Met was abundant. The addition of creatine to the Low Arg&Met diet resulted in higher kidney, liver and plasma creatine concentrations (P < 0.001), but these values were not different from those concentrations in the piglets fed High Arg&Met. The addition of creatine also resulted in a smaller proportion of dietary Arg directed towards GAA synthesis (63 versus 91%, respectively, P<0.001), and a lower incorporation of methionine‐derived methyl groups into creatine (P<0.05) compared to all other diet groups. Although supplementing GAA or creatine appeared to spare Arg and Met, whole body protein synthesis or breakdown were not different. When Arg and Met availabilities were limited, creatine supplementation led to a reduction in the demand for these amino acids to satisfy GAA and creatine synthesis. Consideration of dietary creatine availability is essential when determining Arg and Met requirements of the neonate. Support or Funding Information (NSERC and Evonik)

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