Oddly enough, drinking water increases the metabolic rate substantially: Water-induced thermogenesis.
Drinking lots of water is commonly espoused in weight loss regimens and is regarded as healthy; however, few systematic studies address this notion. In 14 healthy, normal-weight subjects (seven men and seven women), we assessed the effect of drinking 500 ml of water on energy expenditure and substrate oxidation rates by using whole-room indirect calorimetry. The effect of water drinking on adipose tissue metabolism was assessed with the microdialysis technique. Drinking 500 ml of water increased metabolic rate by 30%. The increase occurred within 10 min and reached a maximum after 30-40 min. The total thermogenic response was about 100 kJ. About 40% of the thermogenic effect originated from warming the water from 22 to 37 C. In men, lipids mainly fueled the increase in metabolic rate. In contrast, in women carbohydrates were mainly used as the energy source. The increase in energy expenditure with water was diminished with systemic beta-adrenoreceptor blockade. Thus, drinking 2 liters of water per day would augment energy expenditure by approximately 400 kJ. Therefore, the thermogenic effect of water should be considered when estimating energy expenditure, particularly during weight loss programs.
The mechanism by which water does this is somewhat mysterious. A later paper by the same group, Water Drinking Induces Thermogenesis through Osmosensitive Mechanisms, found that saline solution had no effect.
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.
I am a little skeptical of the authors’ claim that this might help weight loss, since other homeostatic mechanisms would come into play, but sure enough, a paper looked at that, and found that water drinking is associated with weight loss: Drinking Water Is Associated With Weight Loss in Overweight Dieting Women Independent of Diet and Activity.
Learn something new every day.
On the other hand, another study, Water-Induced Thermogenesis Reconsidered: The Effects of Osmolality and Water Temperature on Energy Expenditure after Drinking, could find no effect of water drinking on metabolism. Oh, well.