Direct or Indirect Effects?
Most of the available evidence on GLP-RAs and sperm parameters comes from preclinical research. One of the few clinical trials, published last year in the Journal of Clinical Medicine, investigated the effects of liraglutide in men with metabolic hypogonadism, a body mass index between (BMI) 30 and 40, and severe erectile dysfunction.
Among the 110 men enrolled in the study, only the 35 participants who said that they were not seeking fatherhood received liraglutide. After 4 months of treatment, these men had significantly improved semen concentration, motility, and morphology than did those wanting to conceive who received conventional fertility treatment. Erectile dysfunction was also more improved in the liraglutide group, according to the researchers.
Though this study demonstrated the potential for liraglutide to treat metabolic hypogonadism, the men in that group also had greater weight loss and BMI reduction than the other participants. The review cited several other studies — albeit small ones — in which weight loss was associated with improvements in sperm parameters, including one randomized controlled trial in which one group lost weight with liraglutide and the other with lifestyle modifications; both groups showed increases in the concentration and number of sperm.
One of the key questions requiring further research, then, is whether GLP-1 agents have direct effects on male fertility independent of a reduction in obesity. The randomized controlled trials comparing liraglutide and lifestyle modifications failed to find additional effects on semen in the men taking liraglutide; however, the study had only 56 participants, and results from liraglutide cannot be generalized to potential effects of semaglutide or tirzepatide, Dr. Sadeghi-Nejad said.
“Determining the relative contributions of weight loss versus direct drug actions on fertility outcomes remains challenging without robust data,” Dr. Ramasamy said. “While acknowledged that diet and physical activity positively impact fertility, confirming the synergistic role of GLP-1 receptor agonists requires evidence from well-designed randomized clinical trials.”
Rodent studies suggest that GLP-1 RAs may independently affect testicular function because GLP-1 receptors exist in Sertoli and Leydig cells of the testes. In one study, for example, obese mice who received the GLP-1 agonist exenatide for 8 weeks had “improved sperm motility, DNA integrity, and decreased expression of pro-inflammatory cytokines,” the authors of the review reported. But the precise mechanisms aren’t well understood.
“We know that there are GLP-1 receptors in the reproductive tract, but the extent of the downstream effect of stimulating those receptors, I don’t think we know well,” said John P. Lindsey II, MD, MEng, assistant professor of urology at University of California San Francisco Health.
Other hormonal effects of GLP-1 agonists, such as stimulating insulin production and better regulating blood glucose levels, are better understood, said Raevti Bole, MD, a urologist at Cleveland Clinic, in Ohio, but still other effects of the drugs may not yet be identified.
“I think the really big unknown is whether these types of drugs have effects that are not hormonal on male fertility and what those effects are, and how those affect sperm,” Dr. Bole said. “For example, we know that these drugs slow gastric emptying. Is it possible that slow gastric emptying affects some of the nutrients that you absorb, and that could affect fertility?” Similarly, she said, it’s not clear whether GLP-1 agonists would have any effects on the thyroid that could then affect fertility.