Body mass index affects EEG microstate dynamics through blood viscosity in high-altitude environments.

High-altitude hypoxia imposes substantial challenges on cerebral oxygen delivery and brain functional regulation. Body mass index (BMI) may influence neurophysiological adaptation to such environments through metabolic and hemorheological pathways, yet its relationship with resting-state Electroencephalography (EEG) microstate dynamics remains unclear. This study examined whether blood viscosity (BV) mediates the association between BMI and EEG microstate characteristics in a high-altitude population. A total of 123 permanent residents from Lhasa, Tibet, were initially included. Because only five participants met the obesity criterion, the main statistical analyses were conducted in 118 participants after excluding the obese subgroup. Resting-state eyes-closed EEG data were recorded using a 64-channel system, and standardized 180-s artifact-free segments were used for microstate analysis. Four canonical microstates were identified, and temporal parameters, including coverage, duration, global explained variance, global field power, occurrence, and transition probabilities, were extracted. BMI was calculated from measured height and weight, and BV was estimated using hematological indicators based on a validated high-altitude prediction model. The four identified microstate maps showed topographic patterns broadly consistent with previous EEG microstate studies. Microstate C showed the highest values across several temporal parameters, suggesting a potentially dominant resting-state pattern under high-altitude conditions. BMI was positively correlated with BV, while BV was negatively correlated with the occurrence of microstate B. The mediation model suggested an indirect statistical pathway from BMI to microstate B occurrence through estimated BV: higher BMI was associated with higher estimated BV, which in turn was associated with reduced microstate B occurrence. These findings suggest that BMI-related alterations in resting-state brain dynamics at high altitude may be linked to hemorheological changes. Overall, this study provides preliminary evidence for a BMI-blood viscosity-brain microstate pathway in high-altitude residents. The results highlight BV as a potential physiological bridge between body composition and spontaneous brain activity under chronic hypoxic exposure.