Blood and Vascular Endothelial Responses to Mild Hyperbaric Oxygen During EPA-Supplemented Aerobic Exercise: A Randomised Crossover Trial

Background: Exercise, nutrition and environmental oxygen conditions may influence vascular function and haematological parameters. Whether repeated aerobic exercise under mild hyperbaric oxygen (HBO) during eicosapentaenoic acid (EPA) supplementation elicits different responses from the same exercise under normobaric normoxia (NN) remains unclear. Methods: Sixteen healthy young men completed an exploratory, randomised two-period crossover trial. During each 4-week period, participants cycled for 60 min at 60–70% of peak oxygen uptake twice weekly while taking EPA (2170 mg·day−1), with a 1-week washout. HBO exercise was performed at 1.41 atmospheres absolute (ATA) with 29–30% O2 and NN exercise at 1.00 ATA with 20.9% O2. Eight non-randomised EPA-only participants were included in secondary and supplementary analyses. Results: Linear mixed models showed no significant Condition effect on haematological, biochemical, or flow-mediated dilation outcomes. In the Period 1 analysis of covariance, the HBO-first group had a lower red blood cell count (4.980 vs. 5.211 × 106/µL, p = 0.035) and higher mean corpuscular haemoglobin (29.928 vs. 29.422 pg, p = 0.009) and mean corpuscular haemoglobin concentration (33.303 vs. 32.922 g/dL, p = 0.031), with no differences in haematocrit or haemoglobin. Conclusions: No consistent additional HBO effects were detected; Period 1 erythrocyte-related findings require confirmation in larger studies.