One-year changes in body composition and phase angle during feminizing gender-affirming hormone therapy: a longitudinal and comparative study

Background Gender dysphoria (GD) refers to the clinically significant distress or discomfort that may arise from a marked incongruence between an individual’s gender identity and the sex assigned at birth, involving primary and/or secondary sex characteristics and social roles. Gender-affirming hormone therapy (GAHT) aims to alleviate this distress by aligning secondary sex characteristics with gender identity and improving psychological well-being. This study examined the effects of GAHT on metabolic and body composition changes in assigned male at birth (AMAB) individuals. Methods This single-center longitudinal study (2019–2022) followed 20 GAHT-naïve AMAB individuals with gender dysphoria, reassessed after 12 months of GAHT, with 24 cisgender women serving as a reference group. GAHT included estradiol with cyproterone acetate or spironolactone. Outcomes included bioelectrical impedance analysis (BIA)-derived phase angle (PhA) and body cell mass, as well as handgrip strength and gait speed. Results After 12 months of GAHT, AMAB participants receiving hormone therapy showed significant reductions in fat-free mass (49.9 [39.4–54.6] kg, p = 0.01), fat-free mass index (FFMI) (18.4 [16.5–19.4] kg/m², p = 0.04), body cell mass (25.7 [20.6–29.3] kg, p = 0.04), and phase angle (6.9 [6.0–7.6], p = 0.04). Fat mass increased from 10.6 [8.9–13.8] to 12.9 [10.8–16.1] kg, although this change did not reach statistical significance (p = 0.07). Grip strength showed a non-significant decline (32.5 ± 6.6 kg, p = 0.20), while gait speed remained stable (1.52 [1.30–1.80] m/s, p = 0.60). Despite these changes, lean mass indices and phase angle remained higher than in cisgender women at follow-up (all p < 0.001). Hormonal and metabolic markers remained stable over the one-year follow-up. Conclusion During the first year of GAHT, AMAB participants exhibited notable physiological changes, including decreases in fat-free mass (FFM) and phase angle. Because phase angle is an indicator of cellular integrity and muscle quality, its decline during GAHT warrants careful monitoring in future long-term studies. These findings underscore the importance of comprehensive body composition assessments in AMAB individuals receiving feminizing GAHT. Further studies with longer follow-up periods and larger cohorts are needed to elucidate the long-term clinical significance of these changes.