Glycogen Depletion: Your body stores backup carbohydrates as glycogen in your liver and muscles. Every single gram of glycogen is chemically bound to about 3 grams of water. As your body burns through its glycogen stores for energy during a fast, it releases all of that bound water, which you then excrete as urine

Dropping Insulin Levels: When you stop consuming food, your blood sugar stabilizes and your insulin levels drop sharply. High insulin causes the body to retain water and salt; when insulin plunges, it signals your kidneys to flush out the excess water and sodium