Physiological and Nitrogen Balance Responses of Adult Dogs Fed Diets Containing Different Crude Protein Levels

This nonrandomized sequential study evaluated nitrogen (N) balance, apparent nutrient digestibility, serum biochemistry, and indoxyl sulfate (IS)-related outcomes in ten healthy adult Beagles. Dogs received the crude protein (CP) 30% formulation for four weeks followed by the CP 18% formulation for four weeks; urine and feces were quantitatively collected during four consecutive days in each period. Analyzed dietary CP concentrations were 31.36% and 19.20% of dry matter. Daily urinary N excretion was lower in the second period (5.60 ± 0.35 vs. 3.34 ± 0.15 g/day; false discovery rate (FDR)-adjusted q < 0.001), as was calculated N balance (2.52 ± 0.39 vs. 1.04 ± 0.14 g N/day; paired p = 0.0022). The N retention percentages were 28.78 ± 4.45% and 20.66 ± 2.81%, respectively (p = 0.077; q = 0.132), and were not demonstrated to be equivalent. Apparent total tract digestibility of CP and most analyzed amino acids was lower in the second period. Exploratory daily urinary IS excretion was 3.08 ± 0.35 and 2.59 ± 0.36 mg/day, respectively (p = 0.394; q = 0.394). The analyzed indispensable amino acid concentrations of both diets met the dietary nutrient profile benchmarks evaluated in this study. However, the formulations differed in ingredients and predicted daily energy intake, and period was completely confounded with feeding order. These observations cannot establish an isolated effect of dietary protein, actual long-term body protein retention, or nutritional equivalence.