When you eat carbohydrates they retain water in the body. As I've started eating Keto (Mainly protein and fats) the body loses the water as you have no carbohydrates in the body. So whilst this week I'm hoping to lose lets say 5lbs, I know approx 4lbs will be water weight and 1lb will be actual fat. This is why you hear of people with big weight losses in the first week, but then they slow down. Initially its just the water weight..