I've heard that from the persperation, you could lose the water weight and that would affect the scale....once you gulp a glass, it's right back on (but it's water). I've heard it suggested that you only weigh yourself in the morning with nothing on, that way, you are weighing yourself, not the clothes, food.....but as long as you weigh yourself at the same time of day, every time, you should be able to tell if you are making progress.