This is what I read recently:
Staying hydrated will help you lose weight. Nutritionists and trainers always notice that their clients seem to lose weight more easily if they drink enough water, and now researchers from the University Medicine Berlin in Germany may have discovered why. Their studies show that drinking two cups of water raises metabolic rate (the rate at which your body burns calories) by about 30 percent. This effect peaks in about thirty minutes and disappears completely in about an hour. Researchers estimate that if you drink about six cups of water daily, you could, theoretically, burn an extra 48 calories a day, which could translate into a 5-pound weight loss for the year.
Other ways water may lower the number on your scale: it might help you eat a little less. We often mistake thirst for hunger; instead of filling up a glass with water, we turn to food. So stave off thirst, and you may shave off some calories. And going into your workout well-hydrated gives you more energy and endurance. You'll get the most out of your hour at the gym or your walk, burning more calories and increasing fitness.