Effects of a mildly cooked human-grade dog diet on gene expression, skin and coat health measures, and fecal microbiota of healthy adult dogs

Purported benefits of human-grade pet foods include reduced inflammation, enhanced coat quality, and improved gut health, but research is scarce. Therefore, we compared gene expression, skin and coat health measures, and the fecal microbiome of dogs consuming a mildly cooked human-grade or extruded kibble diet. Twenty beagles (BW = 10.25 +/- 0.82 kg; age = 3.85 +/- 1.84 yr) were used in a completely randomized design. Test diets included: 1) chicken and brown rice recipe [feed-grade; extruded; blue buffalo (BB)]; and 2) chicken and white rice [human-grade; mildly cooked; Just Food for Dogs (JFFD)]. The study consisted of a 4-week baseline when all dogs ate BB, and a 12-week treatment phase when dogs were randomized to either diet (n = 10/group). After the baseline and treatment phases, fresh fecal samples were scored and collected for pH, dry matter (DM), and microbiome analysis; blood samples were collected for gene expression analysis; hair samples were microscopically imaged; and skin was analyzed for delayed-type hypersensitivity (DTH), sebum concentration, hydration status, and transepidermal water loss (TEWL). Data were analyzed as a change from baseline (CFB) using the Mixed Models procedure of SAS (version 9.4). At baseline, fecal pH was higher (P < 0.05) and hair surface score, superoxide dismutase (SOD) expression, and tumor necrosis factor-alpha (TNF-alpha) expression was lower (P < 0.05) in dogs allotted to JFFD. The decrease in CFB fecal pH and DM was greater (P < 0.05) in dogs fed JFFD, but fecal scores were not different. The increase in CFB hair surface score was higher (P < 0.05) in dogs fed JFFD. The decrease in CFB TEWL (back region) was greater (P < 0.05) in dogs fed JFFD, but TEWL (inguinal and ear regions), hydration status, and sebum concentrations in all regions were not different. Hair cortex scores and DTH responses were not affected by diet. The increase in CFB gene expression of SOD, COX-2, and TNF-alpha was greater (P < 0.05) in dogs fed JFFD. PCoA plots based on Bray-Curtis distances of bacterial genera and species showed small shifts over time in dogs fed BB, but dramatic shifts in those fed JFFD. JFFD increased (adj. P < 0.05) relative abundances of 4 bacterial genera, 11 bacterial species, 68 KEGG pathways, and 167 MetaCyc pathways, and decreased (adj. P < 0.05) 16 genera, 25 species, 98 KEGG pathways, and 87 MetaCyc pathways. In conclusion, the JFFD diet dramatically shifted the fecal microbiome but had minor effects on skin and coat measures and gene expression. Lay Summary This study tested the effects of a mildly cooked human-grade diet and a feed-grade extruded kibble diet on the fecal microbiome, skin and coat health measures, and expression of genes related to inflammation and oxidative stress in healthy adult dogs. During a 4-week baseline, 20 beagles consumed the kibble diet. After baseline, 10 dogs continued to consume that diet, while 10 dogs consumed the mildly cooked diet for 12 weeks. After baseline and treatment phases, fresh fecal, blood, and hair samples were collected and skin was analyzed. The mildly cooked diet led to lower fecal pH and dry matter percentage, but fecal scores were not affected. The mildly cooked diet dramatically altered the fecal microbiome, shifting the relative abundances of over 30 bacterial species and 165 bacterial metabolic pathways. Measures of skin sebum content and hydration status were not different between groups, but skin water loss was lower in dogs consuming the mildly cooked diet. Baseline and post-treatment gene expression and hair surface scores were noted, but hair cortex and delayed-type hypersensitivity testing were not altered by diet. Our results demonstrate that mildly cooked diets dramatically change the fecal microbiome, but may not impact skin and coat in healthy adult dogs over a short time period. The current study expanded the research on mildly cooked human-grade dog diets, demonstrating that they dramatically alter the fecal microbiome without changing the fecal scores of healthy adult dogs. While minor differences in blood gene expression and skin and coat health measures were observed after 12 weeks of feeding, our results suggest that improvements would require a longer period of study or testing in animals with lower health status.