Obesity is a global health crisis linked to numerous metabolic disorders, including type 2 diabetes and cardiovascular disease. The gut microbiota plays a crucial role in metabolic homeostasis, and dysbiosis is associated with the development of obesity and related conditions. Several Lactobacillus species have shown promise in ameliorating obesity in animal models and humans, but the underlying mechanisms remain unclear. This study aimed to investigate the protective effects of L. fermentum LM1016 against diet-induced obesity in mice and elucidate the molecular mechanisms involved. The study's significance lies in potentially identifying novel therapeutic strategies for obesity management by targeting the gut microbiota and its interaction with host metabolism. The high prevalence of obesity and its associated comorbidities necessitates the exploration of novel therapeutic avenues, and manipulating the gut microbiota presents a promising approach. This study directly addresses the gap in knowledge regarding the specific mechanisms through which L. fermentum LM1016 exerts its anti-obesity effects.

Previous research has demonstrated the potential of various Lactobacillus strains in mitigating diet-induced obesity. Studies have shown reductions in abdominal adiposity, body weight, and body mass index in both animal models and human subjects. However, a comprehensive understanding of the underlying molecular mechanisms involved in the anti-obesity effects of Lactobacillus strains has been lacking, particularly at the genetic and metabolomic levels. Existing literature highlights the intricate interplay between the gut microbiome, host metabolism, and immune function in obesity pathogenesis. The role of bile acids, brown adipose tissue (BAT) function, and adipose tissue inflammation in obesity has been well-established. The relationship between these factors and the effects of probiotic administration warrants further investigation.

Male C57BL/6N mice (6 weeks old) were divided into three groups: normal diet (ND) with PBS, high-fat diet (HFD) with PBS, and HFD with probiotics (L. fermentum LM1016 or other Lactobacillus strains). Mice received daily oral administration of 1 × 10⁹ CFU of bacteria for 8 weeks. Body weight was monitored weekly. After 8 weeks, mice were sacrificed, and blood, liver, and adipose tissues were collected. Quantitative real-time PCR (qPCR) was used to assess L. fermentum LM1016 colonization. Histological analysis (H&E staining) was performed on liver, BAT, and gWAT. Serum metabolic parameters (glucose, insulin, leptin, cholesterol, bile acids, CRP) were measured. Whole-transcriptome sequencing (WTS) was performed on mouse intestines and gWAT. Metabolomic profiling using ultra-performance liquid chromatography/quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF/MS) was conducted on mouse plasma, bacterial culture supernatants, and MRS media. In vitro studies using 3T3-L1 preadipocytes and RAW 264.7 macrophages were conducted to evaluate the effects of L. fermentum LM1016 bacterial extracts and linoleic acid on adipogenesis and inflammation. Data were analyzed using two-way ANOVA, Student's t-test, or one-way ANOVA, as appropriate.

Administration of L. fermentum LM1016 significantly reduced diet-induced body weight gain compared to other Lactobacillus strains tested. LM1016 treatment led to a reduction in lipid droplet size in BAT and increased expression of thermogenesis genes (Ucp1, Dio2, Acadm, Esrr, Aox). Serum bile acid levels were significantly elevated in LM1016-treated mice, along with upregulation of bile acid synthesis genes (CYP7A1, CYP27A1) in the liver and downregulation of FGF15 expression in the ileum and colon. LM1016 improved serum metabolic parameters, including lower fasting blood glucose, insulin, leptin, and cholesterol levels. Metabolomic analysis revealed changes in several serum metabolites, including increased avenoleic acid and reduced 20:3 cholesteryl ester. LM1016 reduced hepatic steatosis by decreasing the expression of hepatic genes associated with gluconeogenesis, lipogenesis, and lipid sequestration. LM1016 reduced the weight of iWAT and gWAT and decreased adipocyte size. The expression of inflammatory cytokine genes was significantly reduced in gWAT of LM1016-treated mice. GSEA showed enrichment of oxidative phosphorylation genes and downregulation of genes involved in inflammation in gWAT of LM1016-treated mice. In vitro studies demonstrated that L. fermentum LM1016 bacterial extracts reduced adipogenesis in 3T3-L1 cells. Linoleic acid, identified as a metabolite produced by LM1016, also inhibited adipogenesis and inflammation in vitro. Both L. fermentum LM1016 extracts and linoleic acid suppressed inflammatory responses in RAW 264.7 macrophages and 3T3-L1 cells.

The findings of this study provide strong evidence for the beneficial effects of L. fermentum LM1016 in combating diet-induced obesity. The observed improvements in glucose tolerance, insulin sensitivity, and lipid profiles suggest a significant impact on metabolic homeostasis. The mechanism appears to involve a complex interplay between bile acid signaling, BAT activation, and reduction of adipose tissue inflammation. The identification of linoleic acid as a key metabolite produced by LM1016 and its anti-adipogenic and anti-inflammatory effects further elucidate the mechanisms. These results support the potential of L. fermentum LM1016 as a therapeutic agent for the management of obesity and related metabolic disorders, highlighting the importance of the gut microbiota in regulating host metabolism and inflammation.

This study demonstrates that L. fermentum LM1016 effectively ameliorates diet-induced obesity in mice through a multi-faceted mechanism involving enhanced bile acid signaling, increased BAT activity, and reduced adipose tissue inflammation. The identification of linoleic acid as a key anti-obesity metabolite produced by LM1016 adds to the understanding of its beneficial effects. Further research is needed to confirm these findings in larger cohorts and explore the clinical applications of L. fermentum LM1016 in humans.

This study used a murine model, which may not fully recapitulate the complexity of human metabolic responses. The transient colonization of L. fermentum LM1016 in the gut requires daily administration, which may be a practical limitation in clinical settings. Further research is needed to examine the long-term effects and optimal dosage of L. fermentum LM1016 for human applications.