Glycogen, Water Weight, and Why the Scale Drops Fast in 72 Hours

If you have ever started a water fast and watched the scale fall 3–5 pounds in two days, you have seen glycogen physiology in action. Understanding the difference between glycogen-linked water and true fat loss is essential for interpreting any fasting calculator—and for setting realistic expectations when you refeed.

What glycogen is and where it lives

Glycogen is your body's stored form of carbohydrate. The liver holds roughly 80–100 grams; skeletal muscle stores several hundred grams more, though muscle glycogen is less readily exported as blood glucose. Each gram of glycogen is stored with approximately 3–4 grams of water in a hydrated gel structure.

When you eat regularly, glycogen stores are continuously topped up. When you stop eating, falling insulin signals the body to break glycogen back into glucose for fuel. As those stores shrink, the bound water is released and excreted—often showing up as rapid scale loss that has little to do with adipose tissue.

The 72-hour window

Most people deplete a large fraction of liver glycogen within 24 hours and make substantial progress through muscle glycogen over the following days. By roughly 72 hours of water-only fasting, much of the glycogen-linked water shift has already occurred. That is why day 1–3 scale changes can look dramatic while days 4–7 often appear slower—even when fat oxidation continues.

Water Fasting Calc models this pattern explicitly: up to about 3.5 lbs of glycogen water across the first three days, then a taper on longer fasts. Your starting diet matters—a person who ate very low carbohydrate before fasting starts with smaller glycogen stores and may show a smaller early water drop.

What returns after you eat again

When you refeed—especially with carbohydrates—glycogen stores rebuild and water returns to bind with them. It is normal to regain several pounds quickly; that does not mean you "failed" or that the fast had no effect. Fat loss from your calorie deficit is a separate line item from temporary water movement.

This is also why comparing scale weight mid-fast vs. one week after refeed tells different stories. For educational planning, track fat-loss estimates from a personalized calculator, consider waist measurements or body composition tools if available, and focus on how you feel and whether refeeding was safe—not on the peak low number on day 3.