Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance

by Alex Hutchinson

Subsequent biopsy studies confirmed that the amount of glycogen you can stuff into your muscles is a pretty good predictor of how long you’ll last on a treadmill or stationary bike test to exhaustion. There are other sources of carbohydrate in the body; your liver, for example, can store 400 or 500 calories of glycogen for use throughout the body, compared to about 2,000 for fully loaded leg muscles. (That’s why it’s useful to eat a small breakfast a few hours before a morning marathon: while your muscles remain fully stocked, your liver glycogen gets depleted because it fuels your energy-hungry brain while you sleep.)12 Your muscles can also dip into the glucose circulating in your blood, though the total amount of glucose in circulation at any given moment is very small. Overall, the picture that emerged from these studies is relatively simple, harking back to A. V. Hill’s “human body as a machine” view: you can store a finite amount of carbohydrate fuel in your body, and when you use it up, you bonk.