Controlled trial of whole body protein synthesis and plasma amino acid concentrations in yearling horses fed graded amounts of lysine.
Authors: Mastellar S L, Coleman R J, Urschel K L
Journal: Veterinary journal (London, England : 1997)
Summary
# Editorial Summary Lysine is widely assumed to be the first limiting amino acid in equine diets, yet its precise requirement in growing horses has never been established using rigorous methodology. Researchers fed six Thoroughbred yearling colts six different lysine intakes (76–136 mg/kg body weight daily) over successive periods, measuring plasma amino acid concentrations post-feeding and conducting stable isotope tracer studies to assess whole body protein synthesis via phenylalanine kinetics. Plasma lysine concentrations increased predictably with dietary intake, and several free muscle amino acids (asparagine, aspartate, arginine, glutamine, taurine and tryptophan) showed dose-dependent responses; however, the tracer protocol failed to identify a distinct breakpoint in phenylalanine oxidation that would indicate a lysine requirement threshold. The most striking implication is that lysine may not actually be the limiting amino acid under the conditions tested—instead, deficiencies in other amino acids or gaps in our understanding of equine amino acid bioavailability may be masking lysine's true role in protein metabolism. For practitioners, this highlights the substantial uncertainty underpinning current lysine supplementation recommendations and reinforces the need for comprehensive amino acid profiling and requirement studies across different classes and ages of horses.
Read the full abstract on PubMed
Practical Takeaways
- •Current equine nutrition recommendations for lysine may be inadequate or oversimplified—horses fed typical diets may be deficient in multiple amino acids, not just lysine
- •Yearling horses show metabolic responses to lysine intake across the tested range (76-136 mg/kg BW/day), but optimal requirements remain undefined; work with nutritionists using balanced amino acid profiles rather than single amino acid supplementation
- •This research highlights significant gaps in equine amino acid science; standardized requirement data for horses lags behind other species, so consider consulting current research before adjusting concentrate formulations
Key Findings
- •Plasma lysine concentrations increased linearly with lysine intake from 76-136 mg/kg body weight/day in yearling horses
- •Free muscle amino acid concentrations (asparagine, aspartate, arginine, glutamine, lysine, taurine, tryptophan) showed quadratic responses to lysine intake
- •Phenylalanine kinetics and whole body protein synthesis did not differ significantly across treatment lysine intakes, suggesting diets may have been limiting in other amino acids beyond lysine
- •Indicator amino acid oxidation method could not establish a definitive lysine requirement breakpoint in horses, indicating need for additional amino acid requirement and bioavailability research in equines