Effects of threonine supplementation on whole-body protein synthesis and plasma metabolites in growing and mature horses.
Authors: Mastellar S L, Moffet A, Harris P A, Urschel K L
Journal: Veterinary journal (London, England : 1997)
Summary
# Editorial Summary: Threonine Supplementation in Growing and Mature Horses Current NRC guidelines may underestimate equine threonine requirements by not accounting for its role in maintaining intestinal integrity and mucin production, prompting researchers to investigate whether supplementing this amino acid would enhance protein synthesis across different life stages. Using a crossover design, six weanling colts and six mature mares each received both basal and threonine-enriched diets (weanlings: 79 vs 162 mg/kg bodyweight/day; mares: 58 vs 119 mg/kg bodyweight/day) with stable nitrogen levels, whilst whole-body protein synthesis was measured via phenylalanine kinetics using isotopic tracer methodology. Supplemental threonine elevated plasma methionine, threonine and glycine concentrations post-feeding and paradoxically reduced lysine and valine availability in both groups; however, overall phenylalanine flux—the key marker of whole-body protein synthesis—remained unchanged between treatments. These findings challenge the assumption that threonine is limiting in typical equine diets and suggest that single amino acid supplementation creates complex metabolic interactions that may mask benefits to gut health; practitioners should consider that improving threonine status may support mucosal function without necessarily driving systemic protein accretion, and that balancing the entire amino acid profile may be more important than targeting individual amino acids in isolation.
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Practical Takeaways
- •Threonine supplementation beyond current NRC recommendations does not enhance protein synthesis in growing or mature horses, suggesting current requirement estimates are appropriate
- •Single amino acid supplementation creates metabolic interactions affecting other amino acids—consider whole dietary amino acid balance rather than isolated nutrient supplementation
- •Resources spent on threonine supplementation above requirement levels are unlikely to improve protein metabolism or performance in well-managed horses
Key Findings
- •Threonine supplementation increased plasma threonine, methionine, and glycine concentrations (P<0.10) but did not increase whole-body protein synthesis in either weanling colts or adult mares
- •Phenylalanine flux, intake, oxidation and non-oxidative disposal remained similar between basal and supplemented diets (P>0.05)
- •Supplementation of a single amino acid affected metabolism of multiple amino acids including lysine and valine concentrations
- •Current NRC threonine requirements may be adequate despite theoretical increased demands for gut health and mucin synthesis