Sex influence on the resting physiological responses to different levels of normobaric hypoxia in young, trained population

BACKGROUND: This study investigated the influence of sex on the peripheral oxygen saturation (SpO<inf>2</inf>) and heart rate (HR) after being exposed to different levels of normobaric hypoxia (NH) at rest. METHODS: Sixty-five trained volunteers (44 males and 21 females) were randomly exposed twice to a low (FiO<inf>2</inf>=17.4%; 1500 m asl.), moderate (FiO<inf>2</inf>=15.4%; 2500 m a.s.l.), and severe (FiO<inf>2</inf>=13.4%; 3500 m a.s.l.) NH with at least 48 hours between them. SpO<inf>2</inf> and HR were monitored at baseline and during 25 min of hypoxic exposure at rest. RESULTS: Findings revealed that the female group presented higher SpO<inf>2</inf> values at normoxia (ES 0.146; P<0.01) and at low (ES 0.153; P<0.001), moderate (ES 0.067; P<0.05) and severe NH (ES 0.133; P<0.01). Additionally, the female group was able to stabilize their response after 5 min at every altitude, while the male group required 10 min to acclimatize at severe NH and 5 min at low and moderate intensity. The HR values did not differ at any moment of any altitude, but when exposed to severe NH males needed 5 min to acclimatize (P<0.001), while females values remained the same as baseline. CONCLUSIONS: In conclusion, females stabilized their SpO<inf>2</inf> values faster than men at severe altitudes, particularly at 3500 m, despite no HR differences between sexes. These findings suggest that physiological adjustments during NH exposure are different between sexes and should be considered when performing interventions or training sessions in hypoxic conditions.