When backing way off from a heavy training workload, body weight can rise 2/3/4 even 5 pounds—this is the body taking on glycogen, which requires 3 parts water for every pound of glycogen.
Maximizing stored glycogen is not just energy storage; it is a sort of “camel’s hump” water storage system for long duration events, so it is generally a benefit in avoiding dehydration.
According to Wikipedia, a human body of 70 kg can store up to 120 grams of glycogen in the liver, and another 400 grams in muscle tissue — 500 grams. The discussion is absurdly general: is that 5% or 30% body fat and is it a couch potato body or a trained athlete? Very poorly articulated. Ditto for NIH.
I weigh about 77 kilograms /170 pounds when about 7% body fat. Taking the Wikipedia figures, I will assume 132 + 440 = 572 grams of glycogen (10% more body mass).
Storing 572 grams of glycogen implies storing 572 X 3 grams of water with it, for a total of 572 X 4 = 2288 grams / 5.0 pounds of stored glycogen in its water matrix. This figure corresponds very closely to my own estimates based on reduction in body mass after long events or training rides (accounting also for hydration).
Assuming 2500 calories of glycogen (rough estimate), this translate to about 653 kilojoules at 25% efficiency (if all muscular effort were from glycogen)—less than an hour of cycling at my double century pace. Of course the body does not do that—fat is burned up to the limits of physiology, including available oxygen.
My own experience suggests that I can burn off my glycogen over 3 hours or so at a moderately hard pace as the result of years of intensive training—much of the energy comes from fat, thus preserving the glycogen. But it is mandatory to take in a steady supply of a product like GU or Hammer Gel at 200 calories per hour or so—or you’re hosed in a double century. Failure to do so means misery by mile 120.