You just started your low carb diet and after a couple days you jump on the scale to see a 5 pound drop in weight. You click your heals in excitement and proclaim that low carb dieting is awesome and are optimistic that at this rate you will have all your extra weight off in no time. Then to your disappointment over the next few weeks you are losing weight but nowhere near the same rate. A week later you take care of the rest of the birthday cake in your fridge and get on the scale and holy crap, you just gained 4 pounds overnight. What happened? This is the effect of water weight within your body.
You body carries a substance call glycogen and for each gram of this stuff it also requires 3 grams of water. This is stored in your body in your muscles and your liver and with a high carb diet we always have plenty of this to spare in case of an emergency or you decide to go on a quick sprint. When you start your low carb diet your body starts to use up these stores at a rate of about 70 grams per day. So with your body fully charged up with glycogen depending on your muscle mass and in some cases even the size of your liver you can carry and excess of 4 pound of water in glycogen in your body. So this is where your rapid weight loss is coming from.
Fortunately/unfortunately this works both ways, so if you move off your diet for even a couple of meals (anything over 70 grams of carbohydrates for an average male) will result in these glycogen stores getting recharged along with the needed 3 molecules of water resulting in a depressing weight gain literally overnight. The good news is this is easily reversed by going back on your diet and you should get the benefits of the “rapid weight loss” again after a few days.