Grain proteins ameliorate glucose metabolism disorders by activating intestinal AhR and the hepatic NLK/FOXO1 pathway via gut microbiota-derived indole metabolites

INTRODUCTION: Consumption of gran proteins has been associated with lower risk of type 2 diabetes (T2D), but the underlying mechanisms remain unclear. Gut microbiota plays a key role in glucose metabolism, and dietary proteins can modulate microbial composition and function. OBJECTIVE: This study aims to evaluate the effects of grain proteins on glucose metabolism, identify key gut microbiota-derived metabolites, and elucidate the molecular mechanisms underlying grain protein-mediated glucose metabolism regulation. METHODS: Glucose homeostasis and gut microbiota composition were assessed in mice fed a high-fat diet (HFD) supplemented with proteins isolated from rice, soybean, highland barley (HB), oats, and quinoa. The alterations in gut microbiota and their causal roles in glucose regulation were determined by in vitro fermentation and fecal microbiota transplantation (FMT). Key tryptophan (Trp) metabolites in fecal and serum samples were identified, and their glucose-regulating effects were validated in mice and HepG2 cells. Liver transcriptomics and molecular analyses were subsequently performed to elucidate the underlying regulatory pathways. RESULTS: HB, oat, and quinoa proteins significantly improved glucose metabolism, increased Lactobacillus and Bifidobacterium abundance, and enriched Trp-derived metabolites. FMT reproduced the metabolic improvements in recipient mice, supporting a microbiota-mediated mechanism. Among Trp metabolites, indole-3-ethanol (IEt), indole-3-acrylic acid (IArA), and indole-3-aldehyde (IAld) promoted glucose homeostasis via aryl hydrocarbon receptor (AhR) activation and intestinal homeostasis maintenance. Cell-based and liver transcriptomic analyses demonstrated that these metabolites suppressed hepatic gluconeogenesis by modulating the Nemo-like kinase (NLK)/forkhead box protein O1 (FOXO1)/phosphoenolpyruvate carboxykinase 1 (PCK1)/glucose-6-phosphatase (G6PC) pathway. CONCLUSIONS: Grain proteins alleviate glucose metabolic disorders by enriching gut microbiota-derived indole metabolites, which improve intestinal homeostasis and inhibit hepatic gluconeogenesis through AhR and NLK/FOXO1 signaling. These findings highlight the therapeutic potential of grain proteins for T2D prevention and management.