Water drinking induces thermogenesis through osmosensitive mechanisms.

Context: Recently, we showed that drinking 500 ml water induces thermogenesis in normal-weight men and women. Objective: We now repeated these studies in a randomized, controlled, crossover trial in overweight or obese otherwise healthy subjects (eight men and eight women), comparing also the effects of 500 ml isoosmotic saline or 50 ml water. Results: Only 500 ml water increased energy expenditure by 24% over the course of 60 min after ingestion, whereas isoosmotic saline and 50 ml water had no effect. Heart rate and blood pressure did not change in these young, healthy subjects. Conclusions: Our data exclude volume-related effects or gastric distension as the mediator of the thermogenic response to water drinking. Instead, we hypothesize the existence of a portal osmoreceptor, most likely an ion channel.