Carbohydrate source modulates protein and amino acid digestibility patterns in domestic cats

Domestic cats are obligate carnivores with high dietary protein requirements, yet carbohydrates remain functionally necessary in extruded dry foods. Because carbohydrate matrices may be associated with nutrient accessibility during digestion, this study evaluated whether defined protein- and carbohydrate-source formulations are associated with nutrient and amino acid apparent total tract digestibility (ATTD) in cats. A total of 117 Korean short-haired domestic cats were enrolled and underwent a two-week adaptation period before digestibility evaluation. Twelve diets were formulated to compare six protein sources (chicken, duck, salmon, pollock, beef, and cricket) using rice as a reference carbohydrate source and six carbohydrate sources (rice, corn, potato, tapioca, sweet potato, and wheat) using chicken as a reference protein source. ATTD of nutrients and amino acids was measured, and an efficiency ratio (ER) was calculated as an exploratory descriptive ratio reflecting the relationship between crude protein ATTD and the carbohydrate digestibility denominator. Among protein sources, duck, chicken, and salmon showed the highest protein digestibility (86.36, 85.72, and 83.05%, respectively), whereas beef and cricket showed lower values (74.77 and 74.97%). Among carbohydrate sources, rice and corn showed higher protein digestibility (85.72 and 76.82%, respectively) than wheat, tapioca, potato, and sweet potato (75.23, 74.49, 72.52, and 72.04%, respectively) within the tested chicken-based formulations. Although rice showed the lowest carbohydrate digestibility (7.71%), it yielded the highest ER (11.12), reflecting high crude protein ATTD relative to the carbohydrate digestibility denominator rather than direct evidence of protein utilization efficiency. Seven candidate indicator amino acids (Ala, Ser, Val, Thr, His, Phe, and Ile) were identified based on their strong linear agreement with crude protein ATTD across both source categories. Exploratory group-level regressions showed positive associations between ER and selected candidate amino acid ATTD values, with the strongest relationships observed for Phe (R ² = 0.89), Ala (R ² = 0.87), and Ile (R ² = 0.85). These findings suggest that carbohydrate source may be associated with protein and amino acid ATTD within fixed-matrix feline diet formulations. However, ER should be interpreted as a preliminary, matrix-sensitive descriptive index requiring further validation rather than as a direct measure of protein bioavailability or post-absorptive amino acid utilization.