I find this quite interesting:
In the absence of dietary carbs, the body converts glycogen (stored primarily in the liver and muscles) to glucose. Each gram of stored glycogen binds with 3 to 4 grams of water. So when a gram of glycogen is converted to glucose, 3 to 4 grams of water are also released. BTW, this accounts for the "supernaturally" fast loss of scale weight many people experience in the first week of a low-carb diet. It's mostly water weight associated with glycogen storage. Plus a gram of glycogen yields only about 4 kcal of energy, as opposed to about 9 kcal from a gram of fat. So, e.g., satisfy a 9 kcal deficit via burning fat, and you lose only 1 gram of weight, but satisfy it via burning glycogen, and you lose about 9/4 = 2.25 grams of glycogen plus 3 to 4 times that much again in associated water weight.
One of insulin's lesser-known effects is to increase sodium retention by the kidneys, and the more sodium stored, the more water the body retains. Insulin levels fall when fasting, which leads to sodium excretion, which leads to increased water excretion.
Nicola
