Body composition changes in transgender individuals undergoing gender-affirming hormone therapy are accompanied by an increase in ad libitum energy intake.

Background: Obesity is a major risk factor for the development of type 2 diabetes. In transgender individuals, the risk of developing obesity and related disorders may be underestimated owing to limited understanding of how gender-affirming hormone therapy (GAHT) affects body weight, body composition and underlying energy balance.

Aim: To assess the longitudinal effects of GAHT on body weight, body composition and measures of energy balance in transgender individuals.

Methods: Body weight, fat-free mass (FFM), fat mass (FM), ad libitum total energy intake and macronutrient intake during a buffet, as well as resting energy expenditure were measured in 19 trans men (mean body mass index, BMI,±standard deviation 25.7±8.3 kg/m2, age 22.9±4.2 years) and 18 trans women (BMI 21.6±7.7 kg/m2, age 24.5±5.1 years) before and during GAHT at 3, 6, and 12 months. A control group of 11 cis women (BMI 23.7±4.0 kg/m2, age 24.8±3.9 years) and 10 cis men (BMI 23.2±3.3 kg/m2, age 24.5±5.0 years) were included. Changes in hormone levels, body weight, body composition and energy balance were analyzed using the Friedman test. Statistical significance was set at p<0.05.

Results: In trans men, testosterone concentrations increased over 12 months of GAHT relative to baseline (p<0.0001, median=16.9 pmol/L, inter-quartile range, IQR, 13.5–21.4 pmol/L). During the same period, body weight (p=0.002, median=73.5 kg, IQR 62.8–85.0 kg) and FFM (p<0.0001, median=49.5 kg, IQR 47.3–55.2 kg) increased, while FM remained stable (p>0.05). Changes in FFM did not result in altered REE over time (p>0.05). The increases in body weight and FFM were accompanied by a higher total energy intake during the buffet (p=0.0008, median=904.7 kcal, IQR 778.6–1044.0 kcal) driven by greater intake of carbohydrates (p=0.004, median=373.7 kcal, IQR 321.4–458.4 kcal) and proteins (p=0.04, median=113.4 kcal, IQR 85.3–156.3 kcal). In trans women, estradiol concentrations increased over 12 months of GAHT and relative to baseline (p=0.0004, median=155.8 pmol/L, IQR 155.8–419.5 pmol/L). Body weight (p=0.002, median=73.4 kg, IQR 66.8–83.7 kg) and FM (p=0.0006, median=19.8 kg, IQR 11.7–23.3 kg) were higher after 12 months of GAHT, while FFM remained unchanged (p>0.05). Trans women reveiving GAHT over 12 months consumed more total calories during the buffet relative to baseline (p=0.009, median=1161.0 kcal, IQR 933.8–1704.0 kcal), with a pronounced increase in protein intake (p=0.03, median=196.4 kcal, IQR 173.0–244.2 kcal). REE did not change over time (p>0.05). In cis individuals, none of these parameters changed over time (all p>0.05).

Conclusion: GAHT may increase the risk of obesity and related disorders in transgender individuals by promoting greater ad libitum energy intake. The absence of changes in resting energy expenditure after 12 months of GAHT may exacerbate energy imbalance, leading to body weight gain and fat accumulation in trans women