The Complete Guide to Fasting: Heal Your Body Through Intermittent, Alternate-Day, and Extended Fasting

by Jason Fung

Does performance suffer? Not really. In one study, a three-and-a-half-day fast did not affect any measurements of athletic performance, including strength, anaerobic capacity, and aerobic endurance. However, during the period when you are adjusting to the change from burning sugar to burning fat, you may notice a decrease in your athletic performance. This lasts approximately two weeks. As you deplete the body of sugar, your muscles need time to adapt to using fat. Your energy, muscle strength, and overall exercise capacity will go down, but they will recover. This process is sometimes called keto-adaptation. Very low carb diets, ketogenic diets, and training in the fasted state may all have benefits in training your muscles to burn fat, but your muscles do need time to adapt. You can store far more energy in the form of fat than in glycogen, and for endurance athletes, the increase in available energy when they start burning fat is a significant advantage. If you are running ultra marathons, being able to utilize your almost unlimited fat stores instead of highly limited glycogen stores means that you won’t “hit the wall,” giving you a chance to win that race. Since your body is relying on your fat stores, there’s no shortage of energy during fasting, and you can and should do all your usual activities. There is no reason why exercise should stop during fasting. In fact, many elite athletes and endurance athletes train in the fasted state. The combination of low insulin a...