In one study, 14 healthy, normal-weight subjects (seven men and seven women), were used to assess the effects of drinking 500 milliliters (approximately 18 ounces) of water on energy expenditure, metabolism and fuel utilization . The scientists assessed energy expenditure and metabolic rate by using whole-room indirect calorimetry; a highly accurate method of assessing changes in the metabolism.
Results showed that drinking 500 milliliters of water increased metabolic rate by 30%!
The increase occurred within 10 minutes of consumption and reached a maximum after 30 to 40 minutes. Drinking water increases thermogenesis (calories expended via heat). About 40% of the thermogenic effect originates from the body warming the water from room temperature to body temperature.
In men, fat was the main fuel utilized during this process. In contrast, women tended to use carbohydrates to fuel thermogenesis. The total thermogenic response to water consumption in this study was only about 24 calories. However, Ddrinking 2 liters of water per day would augment energy expenditure by approximately 100 calories. If a person is careful with what they’re eating, this amount will definitely add up as weeks go by to ensure better fat loss results.
For more info' be sure to read Everything you want to know about hydration.
Source: Boschmann M, Steiniger J, Hille U, et al. Water-induced thermogenesis. J Clin Endocrinol Metab. 2003 Dec;88(12):6015-9.