Evaluation of In Vitro Method for Estimating In Vivo Digestibility in Mixed-Breed Dogs across Each Life Stage

This study aimed to evaluate the agreement between a two-step in vitro digestion procedure and in vivo apparent total tract digestibility (ATTD) across three life stages in mixed-breed dogs. Eighteen dogs were divided into three groups: puppies (n = 6; < 1 year; initial body weight (iBW): 10.68 ± 1.55 kg), adults (n = 6; 2–7 years; iBW: 8.34 ± 0.38 kg), and seniors (n = 6; > 8 years; iBW: 8.87 ± 0.98 kg). An extruded diet based on oat, turkey, and chicken breast meal was used for both in vitro and in vivo digestibility determinations of dry matter (DM), organic matter (OM), crude protein (CP), gross energy (GE), crude fiber (CF), and ether extract (EE). In vitro digestibility values were consistently higher than in vivo values across all nutrients and life stages (p < 0.05). Linear regression revealed strong in vitro–in vivo agreement for gross energy across all life stages (r² = 0.87–0.98), for organic matter in adult dogs (r² = 0.96), and for ether extract and dry matter in senior dogs (r² = 0.91–0.93), as well as for crude fiber in puppies and adults (r² = 0.85–0.87) and for ether extract in puppies (r² = 0.91), whereas agreement was weaker for crude protein across life stages (r² = 0.55–0.66) and for organic matter in puppies (r² = 0.40). These findings indicate that the two-step in vitro digestion method can serve as a practical screening tool for energy and macronutrient digestibility in mixed-breed dogs of the body-size range studied here, with predictive accuracy that depends on both the nutrient and the life stage.