Controlled trial of whole body protein synthesis and plasma amino acid concentrations in yearling horses fed graded amounts of lysine.

The research article explores the impact of varying lysine intake levels on whole body protein synthesis and the concentration of amino acids in plasma and muscle in yearling horses. The study finds a correlation between increasing lysine intakes and plasma lysine concentrations, but it does not establish a distinct lysine requirement, suggesting that another amino acid may be the limiting factor.

Aim and Methodology of the Study

• The main goal of this research was to measure the impact of different lysine intake levels on whole body protein synthesis and the presence of amino acids in muscles and plasma, in yearling horses. The study endeavored to expand upon prior knowledge that recognizes lysine as a limiting amino acid in standard equine diets.
• For the purpose of this study, six Thoroughbred colts were monitored in different conditions with varying lysine intakes, ranging from 76 to 136 mg/kg of body weight per day.
• Measurement of amino acids was done via blood sampling and muscle biopsies taken after the morning concentrate meal, and phenylalanine kinetics—an aspect of whole body protein synthesis—were determined using infused tracers.

Results of the Study

• Findings from this study showed that plasma lysine concentrations in Thoroughbred colts increased linearly with incrementing lysine intakes. This demonstrates a direct correlation between the quantity of lysine eaten and its presence in the bloodstream, marking significant benefits of lysine intake.
• The concentrations of free muscle asparagine, aspartate, arginine, glutamine, lysine, taurine and tryptophan were found to respond in a quadratic manner to the levels of lysine intake, indicating that these amino acid concentrations likely have a curvilinear relationship with the lysine diet of the horses.
• Despite the changes observed in plasma lysine and muscle amino acid concentrations, phenylalanine kinetics (reflecting protein synthesis) did not differ noticeably in response to different lysine intakes, indicating that lysine levels may not be a primary regulatory factor for protein synthesis in horses.

Limitations and Implications

• The study failed to establish a clear “breakpoint,” or a point at which additional lysine intake does not lead to further benefits, suggesting another amino acid may be the limiting factor in protein synthesis.
• This result emphasizes the existing data gap related to amino acid requirements and bioavailability in horses, indicating a need for further research to guide the formulation of equine diets.