Sex-hormone induced short-term changes in body composition are paralleled by an increase in ad libitum energy intake

Background. Body weight is governed by balance between energy intake and energy expenditure. Transgender individuals undergoing gender-affirming hormone therapy (GAHT) are at increased risk of overweight and obesity. A comprehensive assessment of energy expenditure and intake during GAHT is lacking. Objective. To investigate adaptations in body composition, energy expenditure, and energy intake during GAHT. Methods. In a prospective observational cohort study, body weight, body composition, ad libitum energy intake during buffet, snack food intake, energy expenditure (REE), and physical activity were assessed at baseline, after three, six, 12 months of GAHT in 17 trans men (female-to-male transgender; median±interquartile range; body mass index [BMI] 25.7 ± 7 kg/m²; age 22.9 ± 3.9 years) and 16 trans women (male-to-female transgender; BMI 22.5 ± 7.3 kg/m²; age 24.9 ± 5.8 years). Cis men (n = 7) and women (n = 10) without hormone treatment served as controls. Results . In trans men, body weight (χ²=16.27, p = 0.001; Δ median = 2.4 kg) and fat-free mass (χ²=24.95, p < 0.001; Δ median = 0.8 kg) increased over 12 months of GAHT, accompanied by a gain in fat mass over 6 months (χ²=9.14, p = 0.028; Δ median = 1.1 kg). In trans women, body weight (χ²=16.54, p < 0.001; Δ median = 3.6 kg) and fat mass (χ²=18.06, p < 0.001; Δ median = 4.7 kg) increased, whereas fat-free mass remained stable (p > 0.05). In both groups, energy intake was higher (trans women χ²=13.73, p = 0.003, Δ median = 132 kcal; trans men χ²=18.39, p < 0.001, Δ median = 222 kcal). Snack food intake, REE, physical activity, and controls remained unchanged (all p > 0.05) Conclusion. During GAHT, sex hormone–induced changes in body composition are paralleled by an increase in energy intake.