Dehydration Enhances Pain-Evoked Activation in the Human Brain Compared with Rehydration

Ogino, Yuichi MD, PhD*; Kakeda, Takahiro RN, PHN, PhD; Nakamura, Koji MD; Saito, Shigeru MD, PhD*

doi: 10.1213/ANE.0b013e3182a9b028
Pain Medicine: Research Report

BACKGROUND: Negative effects of dehydration on the human brain and cognitive function have been reported. In this study, we examined the effects of dehydration on pain thresholds and cortical activations in response to pain, compared with rehydration with an oral rehydration solution (ORS) by functional magnetic resonance imaging.

METHODS: Five healthy adult men were subjected to dehydration and rehydration on 2 different days. The condition on the first day was randomly assigned to each subject. They completed a 40-minute exercise protocol using a walking machine after 12 hours of fasting under both conditions. For rehydration, the subjects consumed up to 3000 mL ORS starting from the night before the test day. After exercise, a painful stimulus (cold pressor test) was applied to the subjects’ medial forearm in a magnetic resonance imaging scanning gantry, and pain-evoked brain activation was analyzed.

RESULTS: On the rehydration day, each of the subjects consumed an average of 2040 mL (range; 1800–2500 mL) ORS. Physiological data revealed that subjects when dehydrated lost more weight from exercise than subjects when rehydrated had a larger heart rate increase, a higher tympanic temperature, and a higher urine osmolality. Subjective data revealed that the subjects reported significantly stronger thirst while dehydrated than while rehydrated with ORS, although the levels of hunger and anxiety and mood did not significantly differ between conditions. The cold pressor test robustly activated the pain-related neural network, notably the anterior cingulate cortex, insula, and thalamus. Such activations in the dehydrated subjects were greater than those in the rehydrated subjects in terms of peak and cluster, accompanied by a decrease in pain threshold (P = 0.001).

CONCLUSION: Our findings suggest that dehydration brings about increased brain activity related to painful stimuli together with enhanced thirst, whereas rehydration with ORS alleviates thirst and decreases brain activity related to painful stimuli.

Published ahead of print December 31, 2013

From the *Department of Anesthesiology, Gunma University Graduate School of Medicine, Maebashi; Department of Nursing, Faculty of Health and Welfare, Kawasaki University of Medical Welfare, Kurashiki; and Department of Anesthesiology, Saiseikai-Hita Hospital, Hita, Japan.

Accepted for publication August 14, 2013.

Published ahead of print December 31, 2013

Funding: This research was supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.

The authors declare no conflicts of interest.

Reprints will not be available from the authors.

Address correspondence to Yuichi Ogino, MD, PhD, Department of Anesthesiology, Gunma University Graduate School of Medicine, 3-39-22 Showa-machi, Maebashi 371-8511, Japan. Address e-mail to

© 2014 International Anesthesia Research Society