The effect of body mass index on prefrontal cortex hemodynamic and cognitive functions in exercise-induced hypohydration in male athletes: preliminary study

This preliminary study aimed to examine whether exercise-induced hypohydration is associated with different prefrontal hemodynamic and working-memory responses in male athletes with high and low body mass index (BMI). We hypothesized that athletes with high BMI would show less favorable prefrontal oxygenation patterns and reduced cognitive task performance following exercise-induced hypohydration. Twelve trained male athletes were divided into a high-BMI group (HBMIG; BMI = 33.91 ± 5.73 kg/m²) and a low-BMI group (LBMIG; BMI = 24.68 ± 2.54 kg/m²). Participants completed a familiarization session and an experimental session. During the experimental session, body mass and hematocrit were measured before and after exercise. Prefrontal hemodynamic responses were recorded using functional near-infrared spectroscopy during a 2-back working-memory task. The exercise protocol consisted of 30 min of constant-load cycling at 100 W and 60 rpm, followed by 4 × 20-s sprint intervals. Hematocrit levels increased in both groups compared to pre-exercise levels ( p < 0.05), with a higher increase in HBMIG ( p = 0.045). On the other hand, pre-post exercise responses in prefrontal hemodynamic activity during the 2-back test showed different patterns for each group: an increase in oxyhemoglobin for the LBMIG ( p = 0.002), an increase in deoxyhemoglobin for the HBMIG ( p = 0.033), and an increase in total hemoglobin for the HBMIG ( p = 0.050). Furthermore, the number of missing answers in the post-exercise 2-back test results was significantly higher in the HBMIG compared to the LBMIG. These Preliminary findings suggest that male athletes with high BMI may exhibit different prefrontal hemodynamic responses and reduced task engagement or working-memory performance under exercise-induced hypohydration. However, given the small sample size and absence of a euhydrated control condition, these findings should be interpreted with caution and confirmed in larger controlled studies.