Mild hypohydration in healthy older adults increases pain-related brain activity without affecting pain perception: a single-blind study

Understanding how hydration status influences pain perception is particularly important in older adults, as both dehydration and pain are prevalent in this population. Ten individuals (70 ± 4 yr) completed two randomized and counterbalanced trials. They were exposed to passive heat until they lost 1% body mass through sweat and urine (∼100 min), with the loss either unreplaced (sham infusion, HYPO) or fully replaced via 0.45% saline infusion (EUH). Nociceptive electrical stimulation was applied to the sural nerve <i>1</i>) before heat exposure (baseline), <i>2</i>) 60 min following hydration manipulation (R60, ∼160 min after baseline), <i>3</i>) after mouth rinsing with water (MR, ∼170 min after baseline), and <i>4</i>) following water ingestion (ING, ∼185 min after baseline). Pain-related event-related potentials were assessed using electroencephalography (EEG) at R60, MR, and ING. After hydration manipulation, body mass loss and plasma osmolality were greater, and plasma volumes were lower in HYPO than in EUH, although thirst did not differ between the conditions. There were no differences between the two conditions regarding pain intensity and unpleasantness. Still, EEG analyses revealed that the peak-to-peak amplitude of the pain-related N200-P300 potential (∼136-310 ms) was significantly greater in HYPO than in EUH (<i>P</i> = 0.036) and significantly greater in R60 compared with both MR (<i>P</i> = 0.01) and ING (<i>P</i> = 0.03), either with HYPO and EUH. These results suggest that mild hypohydration in healthy older adults may influence some neurophysiological processes related to nociception without significantly affecting pain perception.<b>NEW & NOTEWORTHY</b> This study reveals, for the first time, that mild hypohydration equivalent to ∼1% of body mass does not alter pain perception in healthy older adults when they are blinded to their hydration status, despite electroencephalography signals showing modulation of pain-related brain responses.