J Clin Endocrinol Metab.2006: 5; [Epub ahead of print]
Brown CM, Dulloo AG, Montani JP
Certain foods and liquids are thought to increase the body's metabolism and produce heat. This is called thermogenesis. A recent study reported that drinking 500 ml water causes a 30% increase in metabolic rate. If verified, this previously unrecognized thermogenic property of water would have important implications for weight-loss programs. However, the concept of a thermogenic effect of water is controversial because other studies have found that water-drinking does not increase energy expenditure. An increase in energy expenditure is measured by a method called calorimetry in which the amount of oxygen and carbon-di-oxide in the breath allows calculation of how much 'fuel' is being converted to heat.
A research group at the University of Fribourg, Switzerland designed a controlled experiment to test whether :
1. Water-drinking has a thermogenic effect in humans
2. The response is influenced by concentration or by water temperature.
Healthy young volunteer subjects were given 7.5 ml/kg body weight ( approximately 518 ml) of distilled water, 0.9% saline (dilute salt water) or a 7% sucrose solution (dilute sugar solution) on different days. In a sub-group of subjects, responses to cold water (3 C) were tested. The resting energy expenditure was measured by calorimetry for 30 min before and 90 min after the drinks.
Results: Energy expenditure did not increase after drinking either distilled water or 0.9% saline. Drinking the 7% sugar solution significantly increased energy expenditure as expected with any carbohydrate intake. Drinking water that had been cooled to 3 C caused a small increase in energy expenditure of 4.5% over 60 min.
Drinking distilled water at room-temperature did not increase energy expenditure. Cooling the water before drinking only stimulated a small thermogenic response, well below the theoretical energy cost of warming the water to body temperature. The authors suggest that these results cast doubt on water as a thermogenic agent that could speed up fat burning.
(We recommend however, that patients drink in response to thirst during weight loss in order to prevent dehydration during reduced food intake).
