GLP-1 Medications Show No Harm to Fertility – and May Benefit Men With Obesity, New Analysis Finds
Key Takeaways:
- A new review of randomised controlled trials found that GLP-1 medications do not harm male hormones, sexual function or sperm quality after long-term use, and in some cases appear to improve them.
- In men with obesity and weight-related low testosterone, treating the underlying excess weight and poor metabolic health may naturally restore hormone levels while preserving fertility – potentially offering an alternative to testosterone replacement therapy.
- The findings are encouraging but preliminary; with only five eligible trials and varying results, the research team stresses that larger, better-designed studies are still required.
A reassuring picture for male reproductive health
Long-term use of GLP-1 medications does not harm male hormones or fertility, according to research being presented on Monday at ENDO 2026, the Endocrine Society’s annual meeting in Chicago, Illinois. The findings go further still: the research team reported that GLP-1 medications may actually raise testosterone levels and improve sperm quality in men who have obesity-related low testosterone, while simultaneously tackling the metabolic problems that sit beneath those hormonal changes.
How the study was carried out
The work was led by scientists at University Hospitals Coventry and Warwickshire and Warwick Medical School in Coventry, United Kingdom. The team searched medical databases for published randomised controlled trials, narrowing their focus to studies that compared GLP-1 medications against other treatments or a placebo in men aged 18 to 65.
Their primary interest lay in changes to testosterone and the other hormones that govern testicular function. Alongside this, they also assessed sperm quality, body weight, blood sugar, cholesterol and broader markers of metabolic health. To reduce the risk of bias, two independent reviewers checked each study, and five clinical trials ultimately met the eligibility criteria.
What the trials revealed
Across these reports, GLP-1 medications showed no negative effect on hormones, sexual function or sperm quality.
In one 24-week semaglutide study, men saw improvements in sperm shape and cholesterol levels, while their testosterone and other hormone levels remained stable. A separate 16-week liraglutide study, conducted in men with obesity and low testosterone driven by excess weight, found that participants experienced increases in testosterone and related hormones. Notably, their overall health outcomes were better than those achieved with testosterone replacement alone.
Treating the cause rather than the symptom
For the research team, the most significant implication concerns how clinicians approach low testosterone in men with obesity.
“This work supports a shift away from prescribing testosterone replacement in men with obesity and low testosterone and toward treating the underlying cause – excess weight and poor metabolic health – which can naturally restore hormone levels and preserve fertility,” said endocrinologist and team lead Pratibha Natesh, M.B.B.S., M.R.C.P., M.Res., at Warwick Medical School.
Important caveats
While the outcomes are positive, Dr Natesh was careful to temper expectations. The body of evidence remains small and the results vary between studies, so larger and better-designed trials are needed before the effects on male fertility can be fully understood.
She also cautioned that most of the reproductive benefits observed are likely to be indirect, flowing from improved metabolic health rather than from any direct action on the reproductive system. Importantly, GLP-1 medications have not been evaluated as treatments for male infertility or hypogonadism, and should not be regarded as such.
The wider takeaway
By providing clear, evidence-based information about GLP-1 weight-loss and diabetes medications, Dr Natesh hopes the research will help people make better-informed decisions about these treatments.
“Improving metabolic health can have positive effects far beyond weight alone,” she said.
Source: Endocrine Society
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Gut Microbiome May Shape Responses to GLP-1 Therapies in Obesity and Type 2 Diabetes
Key Takeaways:
- The gut microbiome may contribute to why people respond differently to GLP-1 receptor agonists, although causality remains unproven
- GLP-1 therapies and associated weight loss can alter gut microbial composition, but findings are inconsistent
- Dietary and behavioural changes during treatment are likely key drivers of microbiome shifts rather than direct drug effects
Emerging link between the gut microbiome and GLP-1 treatment response
A recent review published in the Canadian Journal of Physiology and Pharmacology highlights growing interest in the role of the gut microbiota in shaping responses to glucagon-like peptide-1 receptor agonists, commonly referred to as GLP-1 RAs. These medications are widely used in the management of people living with type 2 diabetes and those living with overweight or obesity.
The analysis suggests that gut microbial communities and their metabolites may contribute to the variability observed in treatment responses. At the same time, GLP-1 receptor agonists may themselves influence the composition of the gut microbiota. This bidirectional relationship positions the microbiome as both a potential contributor to treatment variability and a possible future target for more personalised metabolic therapies.
The gut microbiome as a regulator of metabolic health
There is increasing evidence that gut microbes play an active role in metabolic regulation. Preclinical studies using faecal microbiota transplantation have demonstrated that microbial communities can transfer traits such as glucose regulation and body mass. However, findings in human studies have been less consistent.
Despite the widespread use of GLP-1 receptor agonists, research exploring their interaction with the human gut microbiome remains limited. This is particularly notable given the considerable variation in how individuals respond to these treatments. Differences in microbial composition may partly explain this variability, although a definitive causal relationship has not yet been established.
In the review, researchers examined potential interactions between GLP-1 therapies, diet, gastrointestinal symptoms, body weight, and gut microbial composition.
Interactions between GLP-1 signalling and gut microbes
GLP-1 is a naturally occurring hormone that regulates appetite and blood glucose levels by binding to the GLP-1 receptor. It is released by L-cells in the intestine following nutrient intake. Because this release occurs in the lower intestine, GLP-1 operates in close proximity to the gut microbiota, raising the possibility of direct interaction.
Microbial metabolites such as bile acid derivatives and short-chain fatty acids can influence endogenous GLP-1 secretion and activity. These findings suggest that gut microbes may play a role in modulating GLP-1 signalling pathways. However, direct evidence demonstrating that microbial composition determines treatment response to GLP-1 receptor agonists in humans remains limited.
GLP-1 therapies may also indirectly affect the microbiome through changes in appetite, gastrointestinal motility, and dietary intake.
Clinical effectiveness of GLP-1 receptor agonists and variability in response
GLP-1-based therapies including liraglutide, semaglutide, and tirzepatide are now well established in the treatment of type 2 diabetes and obesity. Clinical trials have demonstrated significant weight loss outcomes:
- Tirzepatide – approximately 11.9–17.8% greater weight loss than placebo over 72 weeks
- Semaglutide – approximately 12.4% greater weight loss over 68 weeks
- Liraglutide – approximately 8.0% weight reduction compared with placebo over 56 weeks
Despite these results, individual responses vary widely. Differences in the gut microbiome have been proposed as one potential contributing factor, although this remains an area of ongoing investigation.
Microbiome changes associated with GLP-1 therapies
GLP-1 receptor agonists may influence gut microbial composition in people living with type 2 diabetes and obesity. Historically, obesity has been associated with a higher Firmicutes-to-Bacteroidetes ratio. Some studies suggest that weight loss is linked to increased microbial diversity and a greater abundance of beneficial genera such as Akkermansia.
Clinical observations include:
- Increased Akkermansia levels after six weeks of liraglutide therapy
- Increased levels of Bacteroidota, Actinobacteriota, and Proteobacteria following 12 weeks of semaglutide
- A reduction in Firmicutes in some cohorts
However, these findings are based on a limited number of heterogeneous studies. Many have involved people living with type 2 diabetes who were also taking other medications such as metformin, which is known to independently influence the gut microbiome.
GLP-1 receptor agonists, when combined with lifestyle modification, can lead to approximately 8–20% body weight reduction over several months to one year. Nevertheless, reported microbiome changes remain inconsistent. Some studies show increased diversity, while others report minimal or no significant changes.
Dietary changes during treatment and their microbiome impact
Evidence suggests that observed microbiome shifts may largely reflect downstream effects of weight loss and metabolic improvement rather than direct drug–microbiome interactions.
GLP-1 therapies are associated with changes in eating behaviour, including reduced appetite, increased satiety, and altered taste perception. People using these medications often improve their dietary patterns, with reduced consumption of refined grains, processed foods, beef, and sugar-sweetened beverages. Caloric intake may decrease by approximately 16–39%.
In 2025, organisations including The Obesity Society, American College of Lifestyle Medicine, American Society for Nutrition, and Obesity Medicine Association issued clinical guidance emphasising the importance of nutritional assessment and management of gastrointestinal side effects during GLP-1 therapy.
This guidance recommends prioritising 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. Given the strong influence of diet on gut microbial composition, these dietary changes are likely to play a significant role in shaping the microbiome during treatment.
Future directions and research priorities
The review concludes that current evidence points towards behavioural and dietary changes, alongside weight loss, as the primary drivers of microbiome alterations observed during GLP-1 receptor agonist therapy.
However, the gut microbiome remains a promising area for future research. Microbiome profiling could eventually help predict treatment response more accurately in people living with obesity and type 2 diabetes.
Future studies should include:
- Larger and more diverse populations
- Longitudinal monitoring of microbiome changes
- Inclusion of people living with obesity who do not have diabetes
- Detailed tracking of dietary intake, gastrointestinal symptoms, and treatment adherence
Emerging therapies such as oral GLP-1 receptor agonists and new agents like orforglipron may offer further insight into direct interactions between medications and gut microbes, particularly as orally administered drugs come into direct contact with the gastrointestinal tract.
Understanding these relationships may ultimately support more personalised and effective approaches to obesity and diabetes care.
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