Apparent total tract nutrient digestibility of frozen raw, freeze-dried raw, mildly cooked, and retorted dog foods and their effects on fecal characteristics, metabolites, and microbiota of healthy adult dogs.

The objectives of this study were to determine how diets distinctly processed from the same raw batch to mimic common pet food formats may affect apparent total tract macronutrient digestibility (ATTD), gastrointestinal transit time (GTT), and fecal characteristics, metabolites, and the microbiota of healthy dogs. Ten spayed female adult beagles (age: 6.10±1.20 yr; body weight: 8.63±0.77 kg) were used in a replicated 5x5 Latin square design to test the following diet formats: retorted (RT), mildly cooked [sous vide (SV), steamed (ST)], and raw [high-pressure processed (HPP), freeze-dried (FD)]. The study comprised five 28-d periods, including a 7-d diet transition, 14-d treatment, and 7-d collection phase. All feces excreted during the collection phase were pooled and analyzed for dry matter (DM), organic matter (OM), crude protein (CP), lipids, and gross energy. On the first day of fecal collections, one fresh fecal sample was collected to measure pH, DM, fermentative metabolite concentrations, and microbiota composition. Data were analyzed by Mixed Models using SAS 9.4, with P<0.05 being statistically significant. Dogs consuming the RT and FD diets had a higher (P<0.05) as-is fecal output to DM intake ratio than dogs consuming the ST and HPP diets. The HPP and ST diets had higher (P<0.05) DM and OM ATTD than the RT, FD, and SV (ST not different) diets. All diets had greater (P<0.05) CP ATTD than the RT diet and higher (P<0.05) fat ATTD than the FD diet. Energy ATTD was higher (P<0.05) in the ST, HPP, and SV diets than in the FD and RT (SV not different) diets. Fecal acetate and butyrate concentrations were higher (P<0.05) in dogs consuming the RT diet than the ST and SV (butyrate only) diets. Fecal isobutyrate, isovalerate, total branched-chain fatty acid, indole, and ammonia concentrations were higher (P<0.05) in dogs consuming the RT diet than all other diets (ST not different for indole and ammonia). Alpha and beta diversity measures were not impacted, but diet altered the relative abundances of 3 bacterial phyla and 19 genera (P<0.05). In summary, processing method influenced nutrient digestibility as well as the fecal metabolite concentrations and microbiota populations of dogs, but did not impact GTT, fecal score, pH, DM, and alpha and beta diversity measures. More research is needed to evaluate additional factors such as packaging size, ingredient inclusion levels, macronutrient concentrations, different processing parameters, and other formats.