ENDO 2026, the Endocrine Society's annual meeting, was held June 13–16 in Chicago. Two presentations are worth examining in detail: a systematic review addressing whether GLP-1 therapy harms male reproductive function, and a real-world study using wearable-device data to measure what actually happens to physical activity levels after starting treatment.
Male fertility: a systematic review of five RCTs
A research team from University Hospitals Coventry and Warwickshire and Warwick Medical School conducted a systematic search of PubMed and the Cochrane Library for randomized controlled trials involving men aged 18–65 receiving a GLP-1 receptor agonist with an appropriate comparator.[1] Five trials met the inclusion criteria; three provided data suitable for quantitative analysis. Primary outcomes assessed were total testosterone, luteinizing hormone, follicle-stimulating hormone, and sex hormone-binding globulin; secondary outcomes included semen analysis, weight, lipids, and HbA1c.
What the individual trials showed
- Two acute infusion studies of native GLP-1 in healthy men showed no meaningful change in mean LH, FSH, or total testosterone, though one reported reduced testosterone pulse frequency without a change in LH.
- A four-week dulaglutide crossover trial in healthy men reduced A1c with no significant effect on reproductive hormones, sexual function scores, or semen parameters.
- A 16-week liraglutide trial in men with obesity and functional hypogonadism found significant increases in total testosterone, LH, and FSH — and the researchers reported this approach outperformed testosterone replacement therapy on overall health outcomes in that trial.
The findings support biological activity without hypothalamic-pituitary-gonadal axis suppression.
What "no harm, possible benefit" actually rests on
The positive testosterone findings come specifically from a population with obesity-related functional hypogonadism — a group where weight loss itself is expected to improve reproductive hormones, since excess adipose tissue converts testosterone to estradiol via aromatase. It's not yet established whether the same benefit would appear in men without obesity-related hormonal dysfunction. The authors themselves called for larger, adequately powered trials specifically assessing reproductive outcomes, given the limited and heterogeneous evidence base — five trials, only three analyzable quantitatively.
Physical activity decline after GLP-1 initiation
A separate study, presented by Sajana Maharjan, MD, of HSHS Saint John's Hospital, used a retrospective pre-post cohort design drawing on the NIH's All of Us Research Program, which integrates electronic health records with objectively measured wearable device data.[2] Of 1,950 adults with obesity newly prescribed a GLP-1 receptor agonist, 753 (38.6%) had sufficient Fitbit activity data before and after initiation to include in the analysis.
Daily step counts declined by an average of 560 steps, and moderate-to-vigorous physical activity declined by 5.7 minutes per day — both statistically significant changes. The decline in daily steps was numerically larger in men than in women. No significant difference in activity change was observed across age group, morbid obesity status, stroke history, or heart failure status, suggesting the pattern was broadly consistent across the studied subgroups rather than concentrated in one demographic.[3]
This is a retrospective, observational design without a non-GLP-1 comparison group tracked over the same period — it cannot fully rule out that activity would have declined for other reasons unrelated to the medication.
The 38.6% data-availability rate means the analyzed subgroup — people who reliably wore a Fitbit before and after starting treatment — may not be representative of the full prescribed population.
Why this finding matters clinically
The activity decline finding connects directly to a separate, related concern raised in a May 2026 JMIR analysis of the clinical support gap in telehealth GLP-1 care: exercise is a primary tool for preserving lean muscle mass during GLP-1-driven weight loss, and this ENDO 2026 data suggests that, contrary to a common assumption, successful weight loss does not naturally lead to increased activity — it may do the opposite.[3] The study's authors specifically framed this as an argument for treating exercise as a mandatory, structured component of GLP-1 treatment programs rather than an assumed side effect of weight loss.
Compare GLP-1 telehealth providers
Source GLP-1 tracks pricing, licensing, and provider structure across the market.