Baseline differences in cardiorespiratory fitness by gonadotropin releasing hormone agonist treatment converge after testosterone in transgender adolescents

There are known sex differences in cardiorespiratory fitness (CRF). Little is known about the impact of pubertal blockade with a gonadotropin releasing hormone agonist (GnRHa) followed by hormone therapy on CRF for transgender adolescents. We aimed to (1) determine the effect of GnRHa monotherapy on CRF and mitochondrial function and associations with metabolomic profiles, and (2) evaluate for changes after 1 and 12 months of testosterone therapy among transgender adolescents . Participants assigned female at birth (n=19, baseline age 15.0+1.0 years) from two groups: GnRHa+ (n=8) and GnRHa- (n=11) were examined at baseline and 1- and 12-months post-testosterone therapy in a longitudinal observational study to assess cardiorespiratory fitness, mitochondrial respiration and metabolic profile. Fasted morning labs including assessment of metabolomics and peripheral blood mononuclear cell mitochondrial respiration and degree of mitochondrial coupling (respiratory control ratio, RCR). A graded cycle ergometer test was performed. Baseline differences were evaluated between groups. Changes were compared with mixed linear regression models evaluating time (baseline, 1 and 12 months), group (GnRHa treatment yes/no), and their interaction. At baseline GnRHa+ individuals had higher relative VO₂peak (30.1+4.83 vs. 25.24+4.47 ml/kg/min, p=0.042) than GnRHa- individuals. In regression models, GnRHa+ individuals had a significant increase in peak watts (p=0.011) and total exercise time (p=0.005)after 12 months of testosterone (p=0.012), but not GnRHa- individuals. GnRHa+ individuals have significantly higher RCR under carbohydrate (p=0.0007) and lipid (p=0.0002) conditions than GnRHa+ individuals. Pretreatment with GnRHa positively influences peak CRF and mitochondrial respiration in adolescent transgender males undergoing testosterone therapy.