Discussions about physical pain tend to revolve around tissue damage and nerves, but the experience has proven to be far more complex. The sensation of pain can be influenced by a range of physiological processes that are not directly related to an injury, as well as several psychosocial factors like expectations and memories. Chronic pain last for more than six months after any apparent damage has healed, presenting a special challenge for those seeking and providing treatment. Accordingly, the impact of less obvious variables have become an important topic in chronic pain research.

A 2016 preliminary study, published in Psychophysiology, examined the experience of pain as it relates to the hydration of an individual. The motivation for this research came from evidence that hypohydration (not as severe as dehydration but still below average) can impair psychological characteristics that are also known to be related to pain sensations, like cognition, mood and energy levels.

Seventeen healthy males were recruited for the experiment, each of which visited the lab once when normally hydrated (control condition) and a second time after a prescribed 24-hour period of liquid restriction (hypohydration condition).

For both trials the subjects provided a urine sample so that hydration levels could be verified. To induce pain, participants were instructed to place their bare feet into water with a temperature near zero degrees Celsius, and to hold them there for as long as could be tolerated. They were also asked to report when they first became cold, first felt pain, and when they could no longer stand it. Three dependent variables were included in a multidimensional approach to account for the complexity of pain perception.

Pain threshold was defined by the time until pain was first felt, pain sensitivity (immediately after reaching threshold) was reported using a visual analogue scale and pain tolerance was measured as the total time with feet submerged. The tendency of subjects to exaggerate the magnitude or threat of pain was also recorded.

Several analyses, concluding with multiple regression, determined that pain sensitivity at threshold, hydration level and tendency to exaggerate were all correlated with changes in pain sensitivity between conditions (hydrated vs hypohydration). Most relevantly, the results confirmed the prediction that hypohydration would be associated with a higher sensitivity to pain.

Simple under-hydration, which is extremely common, can apparently intensify the experience of acute pain. Further research will be required to evaluate the processes that underlie this relationship, but the findings may aid in the understanding and treatment of more complicated conditions like chronic pain.