PSIX-13 Methionine requirements across the life cycle of working Labrador Retrievers using the indicator amino acid oxidation (IAAO) technique

Abstract Methionine is a limiting amino acid in canine diets, but few requirement studies have been conducted across life stages in large-breed dogs. The objective of this study was to determine the minimum requirement (MR) for methionine (Met) in Labrador retrievers at 4 life stages using the indicator amino acid (AA) oxidation (IAAO) technique. In total, 24 dogs were used: 6 puppies aged 10 to 14 wk, 6 puppies aged 14 wk to 9 mo, 6 adults aged 2 to 5 yr, and 6 seniors aged over 8 yr in feeding studies to evaluate the changing requirements of methionine (Met) as canines age. After 2-d adaptation to Met-adequate basal diet (Met = 0.56% dry matter), dogs underwent individual IAAO studies. In brief, all dogs were randomly fed one of six test diets with varying levels of Met ranging from deficient to sufficient (final Met content in experimental diets was 0.1982, 0.2386, 0.279, 0.49, 0.525, 0.56% dry matter, with indispensable amino acids formulated at 1.6x NRC values. Test diet was divided into 13 equal meals; at the 5th meal, a tracer amino acid was supplied (a bolus L-[1-13C]phenylalanine based on body weight was first given, followed by [1-13C]Phe doses every 30 min spanning a 4-h period), and breath samples were collected via respiration mask every 30 min. Total production of 13CO2 during isotopic steady state was determined by enrichment of 13CO2 in breath samples and total CO2 production measured via indirect calorimetry and isotope ratio mass spectrometry (IRMS). Results for IRMS were converted to atom percent excess (APE) and analyzed using segmented linear regression. The Four Rivers mean and population requirements for Met were estimated to be 0.84 ± 0.05 g/1000 kcal (mean ± 2SD) for puppies 10 to 14 wk, 0.78 ± 0.16 g/1,000kcal (mean ± 2SD) for puppies 14 wk to -9 mo, 0.68 ± 0.19 g/1,000kcal (mean ± 2SD) for adults 2 to 5 yr, and 0.62 ± 0.17 g/1,000kcal (mean ± 2SD) for seniors. This research provides important information to assist in accurate formulation of diets that contain appropriate methionine for large breed dogs at all life stages.