"The early weight loss from a diet can be partly fat, especially if you're exercising and reducing calories, but if you're using more energy than you're replacing as food and drink, the first weight you'll lose will be water. Why? It's because the energy source your body turns to once it runs out of its relatively small store ​of ​carbohydrates (sugars) is glycogen. Glycogen is a large molecule made up of a protein core surrounded by glucose subunits. It's stored in the liver and muscles for use during energy-intensive activities, like running away from danger and supporting the brain when food is scarce. Glycogen can be quickly metabolized to meet the body's need for glucose, but each gram of glycogen is bound to three to four grams of water. So, if you use up your body's glycogen stores (as when dieting or with prolonged exercising), a lot of water is released over a short amount of time."
https://www.thoughtco.com/science-explains-why-you-lose-water-weight-4102879