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

Lysine has been reported as the first limiting amino acid in typical equine diets. Indicator amino acid oxidation (IAAO) has become the standard method for determining amino acid requirements in other species, but prior to this study, it has not been used to determine equine requirements. The aim of this study was to evaluate whole body protein synthesis and plasma and muscle amino acid concentrations in response to graded levels of lysine intake in yearling horses. Six Thoroughbred colts (358 ± 5 kg) were fed each of six treatment lysine intakes ranging from 76 to 136 mg/kg body weight/day. Blood samples were taken before and 90 min after the morning concentrate meal. Gluteal muscle biopsies were taken ~100 min after the morning concentrate meal. The next day, whole body phenylalanine kinetics were determined using a 2 h primed, constant infusion of [¹³C] sodium bicarbonate followed by a 6 h primed, constant infusion of [1-¹³C] phenylalanine. Plasma lysine concentrations increased linearly (P < 0.05) at both the 0 and 90 min time points with increasing lysine intakes. Free muscle asparagine, aspartate, arginine, glutamine, lysine, taurine and tryptophan concentrations responded quadratically to lysine intake (P < 0.05). Phenylalanine kinetics did not differ between treatment intakes (P > 0.10). A broken line analysis of lysine intake and phenylalanine oxidation failed to yield a breakpoint from which to determine a lysine requirement. These diets may have been limiting in an amino acid other than lysine, underscoring the lack of data concerning amino acid requirements and bioavailability data in the horse.