The initial weight loss on any diet is mostly due to the loss of water, and you need to replace that water in order to prevent dehydration. Your body requires a lot of water every day; sometimes the body's signals for thirst are misinterpreted as hunger pangs, causing you to pick up something to eat instead of a glass of water. Many foods have high water content; approximately 40% of our daily water intake actually comes from food. Your body may signal that it is hungry in order to get more water through food. This would be OK, except for the fact that this excess food supplies excess calories in addition to water. If you're feeling hunger pangs, it's best to drink a large glass of water, wait 20 minutes, and then decide if you are truly hungry. Drinking water can help to fill you up and prevent you from overeating. You can exercise more effectively when you drink enough water. As you increase muscle mass through exercise, your body will burn more fat (muscles actually assist the body in burning fat). Muscle is made up of more water than fat is, so water becomes even more important as you become more active; dehydration slows down the fat-burning activity of muscles.
Water helps to maintain the muscle tone and lubricate the joints, helping to reduce muscle fatigue and soreness during exercise. Burning calories and fat creates toxins, and water is essential to the process of flushing them out of your body through the kidneys. Drinking an adequate amount of water will actually decrease your tendency to retain fluids.
Dehydration results in a decrease in your blood volume, leading to a reduced supply of blood and oxygen to the muscles and causes you to feel tired. Caffeine (coffee, tea, chocolate, some sodas) has a diuretic effect; for every cup of coffee, you need an additional cup of plain water to counteract this effect. The same goes for alcoholic beverages.
When you are eating a healthy, high-fiber diet you need additional water to dissolve the fiber in your body, helping it to move through the intestinal tract.