Hypohydration but not menstrual phase influences pain perception in healthy women

Chronic pain is a pervasive health problem and is associated with tremendous socioeconomic costs. However, current pain treatments are often ineffective due, in part, to the multifactorial nature of pain. Mild hypohydration was shown to increase experimental pain sensitivity in men, but whether this also occurs in women has not been examined. Fluctuations in ovarian hormones (i.e., 17 beta-estradiol and progesterone) throughout the menstrual cycle may influence a woman's pain sensitivity, as well as hydration levels, suggesting possible interactions between hypohydration and menstrual phase on pain. We investigated the effects of mild hypohydration (HYPO, 24 h of fluid restriction) on ischemic pain sensitivity in 14 eumenorrheic women during the early follicular (EF) and mid-luteal (ML) phases of their menstrual cycle. We also examined whether acute water ingestion could reverse the negative effects of hypohydration. Elevated serum osmolality, plasma copeptin, and urine specific gravity indicated mild hypohydration. Compared with euhydration, HYPO reduced pain tolerance (by 34 +/- 46 s; P = 0.02, eta(2)(p) = 0.37) and increased ratings of pain intensity (by 0.7 +/- 0.7 cm; P = 0.004; eta(2)(p) = 0.55) and unpleasantness (by 0.7 +/- 0.9 cm; P = 0.02; eta(2)(p)= 0.40); these results were not influenced by menstrual phase. Water ingestion reduced thirst perception (visual analog scale, by 2.3 +/- 0.9 cm; P < 0.001, eta(2)(p) = 0.88) but did not reduce pain sensitivity. Therefore, hypohydration increases pain sensitivity in women with no influence of menstrual phase. NEW & NOTEWORTHY Whether hypohydration can increase pain in women remains untested. Menstrual phase may influence pain, but findings are equivocal and may be confounded by the lack of hydration measures. We found that pain sensitivity in women increased after 24 h of fluid restriction versus ad libitum fluid intake, but did not differ between menstrual phases. Water ingestion did not acutely attenuate the negative effects of hypohydration on pain, highlighting the importance of staying wellhydrated throughout the day.