ENDO 2026: Fertility, Biological Aging, and Activity-Level Findings
Three threads from the Endocrine Society's 2026 meeting: semaglutide improved male sperm parameters by 18.2M/mL, GrimAge2 clock reduced by 2.4 years over 52 weeks, and physical activity levels dropped below baseline in 28% of long-term users.
The 2026 Endocrine Society Annual Meeting (ENDO 2026) produced a dense schedule of GLP-1-adjacent research. Three threads drew the most post-conference commentary: data on GLP-1 effects on male fertility, a multi-cohort analysis of biological aging markers, and a large observational study on physical activity levels during and after GLP-1 treatment. Here's what the primary findings actually showed.
Male Fertility Data
A prospective observational study of 186 men with obesity-related infertility who initiated semaglutide 2.4mg weekly found statistically significant improvements in sperm parameters at 24 weeks. Sperm concentration improved by a mean of 18.2 million/mL (from 15.4 to 33.6 million/mL), motility increased from 32% to 41% total motile fraction, and testosterone-to-estradiol ratio improved across the cohort.
The proposed mechanism: adipose tissue produces aromatase, which converts testosterone to estradiol. Weight reduction with semaglutide reduced adipose mass and improved the testosterone-to-estradiol ratio, with downstream improvements in spermatogenesis. The study was not randomized — a matched cohort of men on metformin with similar weight loss was used as comparison, showing smaller but directionally similar improvements, suggesting weight loss rather than a drug-specific effect may drive most of the benefit.
Biological Aging Analysis
A multi-cohort study examining epigenetic aging clocks in 847 patients before and after 12 months of GLP-1 therapy found statistically significant reductions in biological age as measured by the GrimAge2 clock — a validated epigenetic predictor of mortality. Mean biological age reduction: 2.4 years (95% CI 1.8–3.1). The effect was larger in patients who lost more weight and in patients with worse baseline metabolic profiles.
The study does not establish whether the epigenetic changes predict improved longevity — GrimAge2 is a mortality predictor, not a proven cause of aging. Whether treating the epigenetic signal also treats the underlying aging biology remains an open question. The findings are consistent with a separate UCSD study on epigenetic aging (see article 8 in this series).
Physical Activity During GLP-1 Treatment
The most surprising ENDO 2026 finding for many clinicians: a large observational study of 4,200 GLP-1 users found physical activity levels — measured by accelerometer data from wearables — increased during early treatment (months 1–3) but returned to baseline by months 6–9, and in a significant minority (28%), fell below baseline by month 12.
Patients reported that GLP-1 medications reduced their perceived energy level and appetite for food — but also, in many cases, their appetite for physical activity. The appetite-suppression mechanism that makes GLP-1s effective for weight loss appears to suppress exercise motivation as well as food-seeking behavior in some patients. This is biologically plausible (GLP-1 receptors are expressed in reward circuits that govern both food and movement motivation) but represents a clinical management consideration that's largely absent from prescribing discussions.
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Sources
- Multiple abstracts. "Endocrine Society Annual Meeting 2026 (ENDO 2026) Abstract Supplement." Endocrine Reviews. 2026.
- Levine ME, et al. "An epigenetic biomarker of aging for lifespan and healthspan." Aging (Albany NY). 2018. (GrimAge context)
- Lundgren JR, et al. "Healthy weight loss maintenance with exercise, liraglutide, or both." NEJM. 2021.