Sex differences in iron status during military training: A prospective cohort study of longitudinal changes and associations with endurance performance and musculoskeletal outcomes

This study investigated sex differences in iron status, and associations between iron status and endurance and musculoskeletal outcomes, in military training. 2,277 British Army trainees (581 women) participated. Iron markers and endurance performance (2.4 km run) were measured at the start (week 1) and end (week 13) of training. Whole-body areal body mineral density (aBMD) and markers of bone metabolism were measured at week 1. Injuries during training were recorded. Training decreased haemoglobin in men and women (mean change [95% CI], −0.1 [−0.2, −0.0] and −0.7 [−0.9, −0.6] g∙dL-1, both p < 0.001), but more so in women (p < 0.001). Ferritin decreased in men and women (−27 [−28, −23] and −5 [−8, −1] ug∙L, both p ≤ 0.001), but more so in men (p < 0.001). sTfR increased in men and women (2.9 [2.3, 3.6] and 3.8 [2.7, 4.9] nmol∙L, both p < 0.001), with no difference between sexes (p = 0.872). RDW increased in men (0.3 [0.2, 0.4]%, p < 0.001), but not women (0.1 [−0.1, 0.2]%, p = 0.956). MCV decreased in men (−1.5 [−1.8, −1.1] fL, p < 0.001), but not women (0.4 [−0.4, 1.3] fL, p = 0.087). Lower ferritin was associated with slower 2.4 km run time (p = 0.018), sustaining a lower limb overuse injury (p = 0.048), lower aBMD (p = 0.021), and higher βCTX and P1NP (both p < 0.001) controlling for sex. Improving iron stores before training may protect haemoglobin in women and improve endurance and protect against injury.