Effects of dietary amino acid supplementation on measures of whole-body and muscle protein metabolism in aged horses.
Authors: Latham Christine M, Wagner Ashley L, Urschel Kristine L
Journal: Journal of animal physiology and animal nutrition
Summary
# Editorial Summary: Amino Acid Supplementation in Aged Horses Protein metabolism changes substantially with age in horses, raising questions about whether standard nutrient provisions adequately support muscle maintenance in senior animals. Latham and colleagues used a replicated Latin square design to compare three isocaloric, isonitrogenous concentrate formulations in six aged horses (mean 20 years), measuring nitrogen balance, plasma urea nitrogen (PUN), blood amino acid profiles, skeletal muscle protein markers, and whole-body protein turnover via stable isotope infusion over three 15-day periods. Whilst amino acid supplementation—particularly the LYS/THR/MET treatment—reduced atrogin-1 (a muscle degradation marker) in the post-absorptive state, it failed to improve nitrogen retention or phenylalanine kinetics, and resulted in elevated PUN concentrations suggesting excess amino acids were being catabolised rather than incorporated into muscle. The findings indicate that sedentary aged horses fed a standard North American concentrate formulation (100 mg/kg BW daily lysine) do not have limiting amino acid availability for muscle protein synthesis, implying that blanket supplementation protocols may be unnecessary and potentially wasteful for horses at rest. However, the practical relevance to working or metabolically stressed aged horses—or those showing genuine muscle wasting—remains unclear and warrants further investigation before extrapolating these results to higher-demand populations.
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Practical Takeaways
- •Supplementing lysine, threonine, and methionine beyond standard concentrations in aged horse diets does not enhance overall protein metabolism or muscle protein synthesis in sedentary horses
- •Higher plasma urea nitrogen in supplemented groups suggests excess amino acids are being broken down rather than utilized, representing potential dietary inefficiency and waste
- •For sedentary aged horses on typical North American diets, standard amino acid provisions appear adequate; additional supplementation does not provide measurable metabolic benefits and may increase feed costs without performance gain
Key Findings
- •Amino acid supplementation (lysine, threonine, methionine) did not improve whole-body protein synthesis or nitrogen retention in aged horses compared to control diet
- •Atrogin-1 abundance tended to be higher (p=0.07) in post-absorptive state for control diet, suggesting supplemented amino acids reduced muscle protein degradation in that state
- •Plasma urea nitrogen concentrations were significantly higher with LYS/THR/MET supplementation (p=0.0056), indicating supplemented amino acids were catabolized rather than utilized for protein synthesis
- •Amino acid availability was not limiting for protein synthesis in sedentary aged horses fed standard North American control diet