Why do some people respond better to GLP-1 drugs? Gut microbiome may hold clues

New research explores whether gut microbes may help explain why patients respond differently to GLP-1 weight-loss medications, revealing how diet, metabolism, and microbiome shifts could shape future personalized obesity treatments.

Study: The Potential for Complex Interplay Between GLP-1 Receptor Agonists, Gut Microbiome, and Obesity Management. Image Credit: Vicente Fernandez Garcia / Shutterstock

A recent review published in the Canadian Journal of Physiology and Pharmacology highlights growing evidence that the gut microbiota may influence the therapeutic effectiveness of glucagon-like peptide-1 receptor agonists (GLP-1 RAs). These medications are commonly used to manage both type 2 diabetes (T2D) and weight-related disorders.

The analysis suggests that gut microbial communities and metabolites may contribute to variability in patient responses to these treatments. Conversely, GLP-1 RA drugs may also reshape the gut microbiota. This bidirectional relationship highlights the gut microbiota as a potential contributor to treatment variability and a promising target for personalized metabolic therapies.

Growing evidence indicates that gut microbes play an active role in metabolic health. Studies using fecal microbiota transplantation (FMT) show that microbial communities can transfer metabolic traits such as glucose regulation and body mass in preclinical models. However, findings in humans have been variable.

Although GLP-1 RAs are widely used to treat T2D and obesity, research examining their interactions with resident gut microbial communities in humans remains limited. This gap is notable given the considerable variation in patient responses to therapy, suggesting that microbiota differences may partly contribute to, but have not yet been proven to determine, treatment outcomes.

In this review, researchers examined hypothesized interactions between GLP-1 receptor agonists, diet, gastrointestinal symptoms, body weight, and the gut microbiota.

Interactions between GLP-1 signaling and the gut microbiota

GLP-1, a naturally occurring hormone, regulates appetite and blood glucose by binding to its receptor (GLP-1R). L-cells in the intestine release GLP-1 after nutrient intake. Because this release occurs in the lower intestine, the hormone operates close to the gut microbiota, suggesting potential interactions between the hormone and microbial communities.

Microbial metabolites, including bile acid derivatives and short-chain fatty acids (SCFAs), can influence endogenous GLP-1 secretion and activity. These observations suggest that gut microbes may influence GLP-1 signaling pathways. However, direct evidence showing that microbial composition determines responses to GLP-1 RA drugs in humans remains limited.

These medications may also alter the microbiota indirectly through changes in appetite, gastrointestinal motility, and diet.

GLP-1-based therapies such as liraglutide, semaglutide, and tirzepatide are widely used to treat T2D and obesity. Clinical trials report significant weight-loss benefits. Tirzepatide has demonstrated approximately 11.9–17.8% greater weight loss than placebo over 72 weeks, semaglutide about 12.4% greater weight loss over 68 weeks, and liraglutide around 8.0% weight reduction compared with placebo over 56 weeks. However, responses vary widely among patients, potentially due to differences in the intestinal ecosystem.

Microbiome shifts linked to GLP-1 therapies and weight loss

GLP-1 RAs may modulate gut microbial populations in individuals with T2D and obesity. Obesity has historically been associated with a higher Firmicutes-to-Bacteroidetes ratio. Some studies have linked weight loss with greater microbial diversity and increased abundance of beneficial genera such as Akkermansia.

In one clinical study, liraglutide increased Akkermansia levels after six weeks. Following 12 weeks of semaglutide therapy, levels of Bacteroidota, Actinobacteriota, and Proteobacteria increased, while Firmicutes declined.

However, the available studies remain limited in number and heterogeneous. Most have been conducted in individuals with type 2 diabetes who were also receiving medications such as metformin, which may independently affect the gut microbiome.

GLP-1 RA therapy combined with lifestyle modification can produce approximately 8–20% body weight reduction over several months to a year, depending on the drug type and dosage. Nevertheless, microbiome findings remain inconsistent across studies. Some research reports increased microbial diversity, whereas other studies show minimal changes.

Dietary changes during GLP-1 therapy and implications for the microbiome

Research suggests that microbial shifts may follow metabolic improvements linked to weight loss, indicating they are more likely secondary effects rather than primary drivers of GLP-1-induced weight reduction.

These metabolic improvements often coincide with changes in eating behavior, including reduced appetite, increased satiety, and shifts in taste perception. Individuals using these medications often show improved diet quality and reduced intake of refined grains, processed foods, beef, and sugary beverages. As a result, caloric intake may decline by approximately 16–39%.

In 2025, several professional organizations, including The Obesity Society (TOS), the American College of Lifestyle Medicine (ACLM), the American Society for Nutrition (ASN), and the Obesity Medicine Association (OMA), issued clinical guidance emphasizing nutritional assessment and management of gastrointestinal side effects during therapy.

The guidance recommends prioritizing nutrient-dense foods such as vegetables, fruits, whole grains, lean proteins, seeds, and nuts while limiting refined carbohydrates, sugar-sweetened beverages, red and processed meats, fast foods, and highly processed snacks. Because diet strongly influences gut microbial composition, these dietary changes may also affect the microbiome during GLP-1 RA treatment.

Future directions

The review suggests that shifts in diet, behavior, and weight loss, rather than direct drug–microbiome interactions, currently appear most likely to drive microbiome changes during GLP-1 RA treatment.

Nevertheless, microbiome profiling could eventually help predict individual treatment responses more accurately among people with obesity and T2D.

Future clinical trials should include larger sample sizes and longitudinal monitoring in individuals with obesity who do not have diabetes. Emerging therapies, including orforglipron and oral semaglutide, may also provide opportunities to study potential direct interactions with gut microbes because orally administered drugs interact directly with the gastrointestinal tract.

Researchers recommend incorporating detailed dietary monitoring, microbiome profiling, and symptom tracking in future trials. These measures may help clarify whether microbial signatures influence treatment success, post-treatment weight regain, gastrointestinal side effects, or long-term treatment adherence.

• Johnson, A. et al. (2026). The potential for complex interplay between GLP-1 receptor agonists, gut microbiome, and obesity management. Canadian Journal of Physiology and Pharmacology. DOI: 10.1139/cjpp-2025-0219, https://cdnsciencepub.com/doi/10.1139/cjpp-2025-0219