Divergent early trajectories in young transgender individuals: sexual dimorphic shifts in physical performance and body composition during the first six months of gender-affirming hormone therapy

Background Gender-affirming hormone therapy (GAHT) early effects on physical performance and body composition remain partially explored, particularly in non-athletic individuals.Aim This pilot study aimed to evaluate the short-term, within-individual changes in strength, anthropometry, and body composition in non-athletic transgender individuals during the first six months of GAHT.Methods This prospective longitudinal study enrolled 19 participants (8 assigned female at birth [AFAB]; 11 assigned male at birth [AMAB]). Participants were assessed at baseline (T0) and after six months of GAHT (T6). Assessments included bioelectrical impedance analysis (BIA) for body composition, isometric leg extension (maximal force [Fmax] and endurance), 1-repetition maximum (1RM) bench press, handgrip strength (HG), and submaximal cycle ergometry for VO2max estimation.Results In the AFAB group, GAHT led to a significant increase in total testosterone (TT), fat-free mass (+3.5 kg; p = 0.020), Fmax (p = 0.015), and 1RM bench press (+13.0 kg; p = 0.009). A strong correlation was found between TT increases and 1RM improvements (r = 0.718). Conversely, the AMAB group achieved significant suppression of TT and gonadotropins but exhibited remarkable stability in all anthropometric parameters and functional performance metrics, with no significant declines in strength or cardiorespiratory fitness (p > 0.05).Conclusions These findings describe for the first time a distinct sexual dimorphism in the early response to GAHT. While AFAB individuals experience rapid functional and anabolic gains, AMAB individuals maintain physical stability during the first six months of therapy. These preliminary findings highlight the need for future interventional studies to investigate whether implementing structured physical training early during GAHT can optimize body composition and potentially mitigate long-term metabolic risks in transgender cohorts.