Effects of Vitamin D supplementation on iron metabolism in endurance athletes: A meta-analysis and randomised controlled trial

Vitamin D deficiency has been linked to anaemia, suggesting a protective role of sufficient 25(OH)D concentrations. This thesis investigated the relationship between vitamin D supplementation and iron status in endurance athletes. Chapter 3 assessed training volume and intensity, supplement use, and vitamin and/or mineral deficiencies among UK endurance runners using a cross-sectional study of 576 athletes aged ≥ 18 years. Most runners trained five to nine times per week, following a pyramidal intensity distribution (~70% in Zone 1) and included one to two resistance sessions and rest days. More males (47.8%) than females (24.4%) ran > 50 km/week, whereas more females (23.3%) than males (16.3%) ran 31- 40 km/week. Females most frequently consumed vitamin D, while multivitamins were most common in males. Iron and magnesium were most commonly consumed by females and males, respectively. Deficiency in iron was most frequently reported for both sexes, followed by vitamin D. These findings prompted investigation of whether vitamin D supplementation impacts iron status. Chapter 4 presents a systematic review and meta-analysis of 14 double-blind RCTs (n = 1,131), showing vitamin D supplementation increased haemoglobin concentrations (T = 2.58, p = 0.02) when compared to the placebo, including a subgroup analysis of different patient populations (T = 2.26, p = 0.04) and dose administered in the intervention (T = 2.45, p = 0.03). Plasma/serum 25(OH)D concentrations significantly increased across all studies (T = 5.82, p < 0.0001), regardless of population or dose. A dose-dependent effect on 25(OH)D concentrations was observed with larger increases at higher doses and smaller increases at lower doses. No effect was observed for plasma/serum iron, ferritin, hepcidin, or transferrin saturation concentrations. Given the lack of RCTs in athletes, Chapter 5 presents a double-blind RCT in 18 male endurance runners supplemented with 5,000 IU/day or placebo for 4 weeks prior to completing a 21 km race. Bloods were collected at baseline, post-intervention, and pre-, post- and 24 h post-race. Plasma 25(OH)D concentrations increased significantly in the vitamin D group (37.4 ± 7.9 vs 60.9 ± 9.5 nmol/L). Haemoglobin, post-exercise ferritin, hepcidin, and inflammatory markers (IL-6, CRP) were unaffected. RBC counts were higher (p = 0.017) in the vitamin D group at all time-points when compared to the placebo group (pre-race: 4.8 ± 0.8 vs 4.4 ± 0.7; post-race: 4.9 ± 0.3 vs 4.5 ± 0.2; 24 h post-race: 4.9 ± 0.3 vs 4.6 ± 0.3, p = 0.017). This thesis provides novel insights for coaches and endurance athletes, demonstrating that vitamin D supplementation effects depend on baseline vitamin D status, iron-related markers, and exercise demands, and highlighting the importance of regular blood screening to support adequate 25(OH)D concentrations, particularly in athletes at risk of iron deficiency. Future research should focus on longer-term RCTs in athletic populations to clarify the effects of vitamin D on iron depletion following strenuous endurance exercise.

Citation

Galea, R. (2026). Effects of Vitamin D supplementation on iron metabolism in endurance athletes: A meta-analysis and randomised controlled trial [Unpublished doctoral thesis]. University of Chester.

Publisher

University of Chester

URI

https://chesterrep.openrepository.com/handle/10034/630032

Type

Thesis or dissertation

Language

en