When someone begins a weight-reducing diet, they often lose a lot of weight at first. This is commonly said to be a result of water loss. Why is it?
Well it is the converse of where the discussion went here … glycogen in particular is stored with a good amount of water, roughly three grams of water stored for every gram of glycogen stored. Glycogen tends to get mobilized first (especially in low carb plans or highly calorie restricted ones), hence that water gets released in the circulation and peed off. Fat has less water stored with it and is more energy dense per gram stored.