The mediating effect of skeletal muscle index on the relationship between menarcheal age and bone mineral density in premenopausal women by race/ethnicity

Objective:To examine the contribution of skeletal mass index (SMI) as a mediator in the relationship between menarcheal age and hip/spine bone mineral density (BMD) in premenopausal women by race/ethnicity.Methods:The data of 4,329 participants (age ≥ 18; mean age=35.7 ± 9.5) of Whites (n = 2,543), African Americans (n = 1,236), and Asians (n = 550) enrolled from October 2011 to January 2019 from the Louisiana Osteoporosis Study were analyzed. After adjustment for physiological and behavioral factors, multivariable linear regression analyses were conducted to evaluate each component of the proposed mediation models, and mediation was verified by the bootstrapping resampling approach.Results:Premenopausal women with early menarcheal age tended to have higher SMI and BMD than women with normal menarcheal age among all races/ethnicities included. Women with late menarcheal age were, however, more likely to have a lower SMI than their counterparts with normal menarcheal age (r = -0.212, 95% CI = [-0.321 to-0.103] for White women; r = -0.181, 95% CI = [-0.410 to-0.008] for African-American women; r = -0.174, 95% CI = [-0.343 to-0.006] for Asian women). Similar results were found for both spine and hip BMD. SMI fully mediated the difference in BMD due to different menarcheal ages among Whites, African Americans, and Asian women with early menarcheal age; however, no mediating effects were observed for Asian women with late menarcheal age.Conclusions:SMI, as a full mediator, affected the relationship between menarcheal age and BMD among premenopausal women, and the mediating effects varied by race/ethnicity. To prevent or slow down the loss of hip/spine BMD and the development of osteoporosis, measures aiming at minimizing the risk for muscle mass loss should be recommended, especially for White and African-American women with late menarcheal age.