Endocrine Abstracts

Background: Bone marrow fat is a unique fat depot that is regulated independently of other fat depots. Estradiol is an important regulator of bone marrow fat. This is illustrated by an increase in marrow fat fraction after menopause and a decrease following estradiol replacement. We hypothesize that estradiol is the major sex steroid that regulates bone marrow fat not only in women, but also in men.

Methods: This is an open-label partly randomized intervention study in trans women (assigned male at birth, identify as female) and trans men (assigned female at birth, identify as male) receiving hormone treatment. Trans women were treated with triptorelin for 6 weeks, followed by triptorelin and estradiol for 52 weeks. Measurements were performed at baseline, 6, 8, 18 and 58 weeks. Trans men were randomized to receive triptorelin and testosterone with or without anastrozole for 12 weeks, followed by only testosterone until week 52. Measurements were performed at baseline, 6, 12 and 52 weeks. The marrow fat fraction was quantified by magnetic resonance imaging (MRI). The bone mineral density (BMD) was estimated by dual-energy X-ray absorptiometry (DXA) at baseline and after 12 months.

Results: In trans women, the marrow fat fraction increased by 0.09 (95% CI 0.05 to 0.12) after 6 weeks, compared to baseline. Compared to week 6, the marrow fat fraction decreased by 0.06 (0.01 to 0.11), 0.09 (0.04 to 0.14), 0.08 (0.03 to 0.13) at 8, 18, and 58 weeks, respectively. In trans men without anastrozole, the marrow fat fraction did not change at week 6 and 52, but was slightly higher at week 12 (0.05 (0.01 to 0.08)), compared to baseline. In trans men with anastrozole, compared to baseline, the marrow fat fraction increased by 0.06 (0.03 to 0.09) and 0.06 (0.03 to 0.09) at week 6 and 12, respectively. BMD did not change in response to hormone treatment in any of the groups.

Conclusion: The results of this study indicate that estradiol and not testosterone is the major sex steroid regulating bone marrow fat in both men and women.