Glycerol Monolaurate Supplementation in Extruded Diets Enhances Immune Function and Antioxidant Capacity and Alters the Gut Microbiota and Fecal Metabolome in Cats

This study evaluated the effects of dietary glycerol monolaurate (GML) supplementation on feline growth performance and intestinal health. Twenty-four adult British Shorthair cats (3.03 ± 0.07 kg) were initially enrolled. Following a 56-day pre-feeding period, 18 eligible cats were allocated to two dietary groups, of which 12 cats were prespecified for final sample collection and statistical analysis (n = 6): a control group (CON, basal diet) and an experimental group (EXP, basal diet + 2000 mg/kg GML product) for a 28-day trial. Dietary GML significantly increased the average daily feed intake. GML supplementation significantly increased the changes in fecal score and fecal pH, and a significant GML × day interaction was observed for the change in fecal pH. GML improved the apparent digestibility of dry matter, gross energy, crude fat, and crude protein. Furthermore, serum albumin increased within the normal range. Cats in the EXP group showed elevated antioxidant enzyme activities, alongside lower malondialdehyde concentrations. GML also elevated serum immunoglobulin A and reduced pro-inflammatory cytokines. Fecal analyses showed reduced acetic acid and total volatile fatty acids, but elevated propionic acid. Microbiome and metabolomic profiling revealed that GML decreased the relative abundance of Streptococcus and metabolites such as Linoleic Acid and 12,13-Epome, while increasing the relative abundances of Collinsella, Peptoclostridium, Clostridium, and (-)-Jasmonic Acid. These findings provide a theoretical foundation for applying GML in pet foods to optimize companion animal extruded diets.