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

by Jason Fung

glycerol is used for gluconeogenesis, so the amino acids formerly used can be reserved for protein synthesis. The fatty acids are used directly for energy by most tissues of the body, though not the brain. The body uses fatty acids to produce ketone bodies, which are capable of crossing the blood-brain barrier and are used by the brain for energy. After four days of fasting, approximately 75 percent of the energy used by the brain is provided by ketones. The two major types of ketones produced are beta-hydroxybutyrate and acetoacetate, which can increase over seventyfold during fasting. 5. The protein conservation phase (five days after beginning fasting): High levels of growth hormone maintain muscle mass and lean tissues. The energy for basic metabolism is almost entirely supplied by fatty acids and ketones. Blood glucose is maintained by gluconeogenesis using glycerol. Increased norepinephrine (adrenaline)