Does Vitamin D Status Influence the Exertional Stress Induced Neutrophil Response to 2 Hours Running at 70% V O2max?

Objective: It is well established that endurance exercise depresses neutrophil function, a key immune response in post-exercise recovery. Considering vitamin D (25(OH)D) role in immune maintenance, the study aimed to determine whether circulating 25(OH)D concentration influences bacterially challenged neutrophil degranulation in responses to endurance running. Methods: Male runners (n = 8) completed two hydration interventions (euhydrated and dehydrated) in a randomised order during 2 h running at 70% ˙V O2max in thermoneutral conditions (0.4 and 3.1% body mass loss, 290 and 303 mOsmol·kg−1 plasma osmolality, and −0.9 and −5.8% plasma volume change, respectively). Immediately and 1 h post-exercise participants received water (equating to × 1.5 body mass loss). Venous blood samples were collected pre- and post-exercise, and during recovery. Whole blood was used to determine total circulatory neutrophil counts and their maturity status. Additionally, 1000 μl of whole blood was incubated with 1 μg·mL−1 of lipopolysaccharide for 1 h at 37ºC. Aliquots were assayed for plasma elastase by ELISA. Serum and plasma samples were also analysed for 25(OH)D and IL-8 concentrations, respectively. Results.—Sub-optimal 25(OH)D levels were observed in three runners (50.1 ± 14.7 nmol·L−1), while five runners presented optimal 25(OH)D levels (99.1 ± 18.6 nmol·L−1) (P = .008), with only one athlete having insufficient levels (<50 nmol·L−1). A time effect (pre- to post-exercise increase) was observed for neutrophil degranulation (P = .001) and IL-8 response (P = .003), with no differences between the groups.A significant negative correlation was observed between serum 25(OH)D concentration and IL-8 responses in the euhydrated trial during recovery (P = .031). Conclusions: The current study suggests that circulating 25(OH)D concentration has little impact on IL-8 or bacterially challenged neutrophil functional responses to endurance running. It is likely that the levels of 25(OH)D within the cohort were not low enough to induce substantial immune perturbations. However, the negative correlation between 25 (OH)D status and IL-8 responses in the euhydrated state warrants further investigation.