Use of mulberry pomace in dog foods as a functional component

The effects of dietary inclusion of mulberry pomace (MP) at 4% of the diet on nutrient digestibility, fecal quality, fermentative metabolites, serum biochemistry and serum immunoglobulins were investigated in dogs. MP is the skin-, pulp- and seed-rich residue remaining after mulberry fruit is pressed for molasses production; it is rich in insoluble dietary fiber and in anthocyanin-dominated polyphenols, and it is currently discarded as waste. Sixteen healthy Golden Retriever dogs (age 2–8 years, mean body weight 31 kg) were randomly allocated to a control group and an MP group ( n = 8 per group) and fed for 30 days. Dry matter (DM) digestibility, determined by the acid-insoluble ash (AIA) indicator method, decreased from 82.81 ± 0.53% in the control group to 79.27 ± 0.69% in the MP group ( p < 0.001). Crude protein (CP) digestibility decreased from 80.23 ± 0.60% to 76.89 ± 0.73% ( p < 0.001), whereas crude fiber (CF) digestibility did not differ between groups ( p = 0.613). Fecal score was numerically higher in the MP group, but the difference was not statistically significant ( p = 0.105), whereas fecal pH increased from 6.01 ± 0.12 to 6.62 ± 0.06 ( p < 0.001). Fecal ammonia rose from 6.02 ± 0.42 to 7.31 ± 0.27 µmol/g DM ( p = 0.030) and fecal valeric acid increased from 5.70 ± 1.23 to 15.26 ± 2.88 µmol/g DM ( p = 0.050), while butyric acid tended to decrease from 82.32 ± 7.70 to 59.95 ± 7.66 µmol/g DM ( p = 0.083). Acetic and propionic acid concentrations were unaffected. Hematological values remained within reference limits; serum cholesterol, glucose and blood urea nitrogen (BUN) increased significantly in both groups over the trial, and BUN was higher in the MP group at day 30 ( p = 0.034). No significant differences were observed in serum IgE and IgG. Inclusion of MP at 4% decreased nutrient digestibility, softened fecal consistency and shifted fecal fermentation from a saccharolytic towards a more proteolytic pattern, without producing clinically relevant changes in hematological or serum biochemical variables. The reduction in butyric acid is the finding that most warrants caution, and inclusion levels below 4%, longer feeding periods and molecular profiling of the colonic microbiota are required before MP can be recommended as a functional ingredient in dog foods.