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.[4][6] 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.[4]

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.[5]

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.

For annotated links to other posts on diet and health, see:

• Miles Kimball on Diet and Health: A Reader's Guide