Let me add to what I wrote in “Fasting Tips” a little insight into a minor side effect of fasting that could puzzle you if you don’t know about glycogen. Near the beginning of a several-day fast, there is a day or two when I am much less thirsty than usual; then after I go back to eating normally I there is a day or two when I am much thirstier than usual. What is going on is that glycogen, the body’s short-term sugar storage doubles as water storage as well. Evolutionarily, this seems like a good trick.
The body taps into these glycogen sugar + water stores at the beginning of a fast before dipping into the long-term storage of energy in body fat. After going back to eating normally, the body needs extra water along with carbs to reconstitute the glycogen sugar + water stores.
You probably have 6 cups to half a gallon of water bound up with your glycogen. The glycogen proper amounts to about half a kilogram. Quoting from the current version of the Wikipedia article “Glycogen”:
… the liver of an adult, weighing 1.5 kg, can store roughly 100–120 grams of glycogen. In skeletal muscle, glycogen is found in a low concentration (1–2% of the muscle mass) and the skeletal muscle of an adult weighing 70 kg stores roughly 400 grams of glycogen.
Then there is 3 to 4 times as much water bound up with the glycogen:
Glycogen in muscle, liver, and fat cells is stored in a hydrated form, composed of three or four parts of water per part of glycogen associated with 0.45 millimoles (18 mg) of potassium per gram of glycogen.
3 times 500 grams is 6.34 cups of water. 4 times 500 grams is 8.45 cups of water. Your glycogen probably won’t go below 10% of its normal level, so to get available stored water these need to be multiplied by .9, which is 5.7 to 7.6 cups of water. That is a lot!
In terms of calories, there are about 4 calories per gram of glycogen, so if 450 grams are available to be run down, that is 1800 calories—a large share of the calories needed for a day. So you might not burn much fat with a fast of just one day if your body leans strongly toward running down the glycogen first. I talk about that in “Increasing Returns to Duration in Fasting.”
Despite knowing about glycogen, I have had many moments after I begin eating again after a several-day fast when I have been alarmed at how thirsty I am, until I remember what is going on. Then I am reassured.
Being less thirsty at the beginning of a fast hasn’t ever gotten me needlessly worried. Note that this is something you could use if you are ever in a situation in which you have food available but not much water. If you don’t eat the food, then you will get access to your body’s stored water and for a while are likely to suffer less from thirst.
Let me share one more bit of experience. I just came off of three weeks in which I did a modified fast very low on the insulin index as I describe in “Forget Calorie Counting; It’s the Insulin Index, Stupid” for 4 days each week and then did an almost total fast for 3 days each week. It was only when I began eating additional things higher on the insulin index after the three weeks were over that I experienced the extra thirst. Thus, eating low enough on the insulin index seemed to also be lowcarb enough that my glycogen couldn’t reconstitute.