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

by Jason Fung

At baseline, eating normally, energy comes from a mix of carbohydrates, fat, and protein. As you start fasting, the body increases carbohydrate oxidation. This is just a fancy way of saying that it is burning sugar, in the form of glycogen, for the first twenty-four to forty-eight hours after you stop eating, until it runs out of glycogen. With no more sugar to burn, the body switches to burning fat. Fat oxidation increases as carbohydrate oxidation decreases toward zero. (See Figure 3.2.) At the same time, protein oxidation—that is, burning protein, such as muscle, for fuel—actually decreases. The normal protein breakdown of around seventy-five grams per day falls to fifteen to twenty grams per day during fasting. Rather than burning muscle during fasting, we start conserving muscle. Much of the amino acids that are broken down during regular turnover of cells are reabsorbed into new proteins.