I've seen that same "5 lb" photo over and over for years and it doesn't add up. That picture makes it appear that the fat is at least two, if not three or more, times the volume of the muscle. But does that pass a simple sanity check? Have you ever tried to dive under water taking even a small inner tube with you? It's basically impossible. Yet if fat displaced even as much more water than muscle, it wouldn't take very many pounds of fat at all to displace as much additional water as that inner tube. Yet people that are even two hundred pounds over weight can dive under water, though it is admittedly more difficult.

On average muscle has a density of about 1.1 g/mL while fat has a density of about 0.9 g/mL (water has a density of 1.0 g/mL -- this is how the gram was originally defined). This means that fat occupies about 22% more volume than the same weight of muscle.

Turning back to the original question, and this is primarily my personal take on it based on my experience and discussions with numerous other people over the years, as you lose fat your body is essentially assuming that the lose is temporary and that you will soon be able to consume enough food to get back to your prior weight. So it wants to preserve the size and shape of the fat cells from which the fat is being withdrawn (as best it can, like most things it can only do this to a certain degree) and it does this by replacing it with water. As a result, you see no weight loss and might even see a weight gain. I've had this happen countless times. But at some point your body recognizes that it can't keep this up and so it dumps the water out of the fat cells over a short period of time. This is what we call a Whoosh.

So, as frustrating as it is when you step on that scale for days, weeks, or perhaps even a month or more and see no decline (or even an increase), the key is to recognize it for what it probably is -- provided you have been staying on the LC path -- you ARE losing fat and your body is clinging to water to keep the volume of the fat cells close to their prior shape. It is a GOOD sign. Embrace it for the sign of future success that it is. If you are not able to get into that mindset, then quite using the damn scale as it is only going to lead you into a phase of discouragement that might derail you altogether.

Now, knowing that a portion of the water you are drinking is being diverted to these emptying fat cells, it becomes all the more important to keep you water intake up so as to provide your body with the water it needs to function, particularly to keep your blood volume where it should be and maintain your electrolyte balance.