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

Abstract

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.

Seasonal Variation in Vitamin D Status Does Not Interfere with Improvements in Aerobic and Muscular Endurance in Conscripts during Basic Military Training

“…In our conscripts, hemoglobin levels did not change during BMT in the S-C, but there was a small (3.7%) statistically significant increase in the A-C. Contrary to our data, other research groups [63][64][65] observed a decrease in hemoglobin levels in male soldiers during BMT, lasting from 9 weeks to 4 months. The discrepancy between our findings and those of the previous studies may be due to many factors, such as the duration of training, the proportions of training loads with different goals, nutrient intake, and the physical performance of soldiers before the start of military training.…”

“…On the other hand, a significant decrease in ferritin levels observed during BMT in both S-C (14.6%) and A-C (39.4%) is in good agreement with previous data [63][64][65][66]. With a background of relatively stable hemoglobin concentrations, a decrease in ferritin levels did not prevent the improvement in aerobic endurance during BMT in our conscripts.…”

“…On the contrary, larger decreases in ferritin levels were associated with greater improvements in the 3200 m-run time during BMT. The same phenomenon, a positive association between a decrease in ferritin levels and improvement in aerobic endurance during military training, was recently noted by O'Leary et al [64]. Ferritin is considered a biomarker of iron stored in the bone marrow, liver, and spleen [67,68], while the portion of iron that transports and uses oxygen in the production of energy is known as functional iron [69].…”