The Complete Guide To Fasting

by Jason Fung

During the initial stages of fasting, insulin and blood glucose levels fall but remain in the normal range, maintained by the breakdown of glycogen as well as gluconeogenesis. After glycogen is used up, the body begins to switch over to burning fat for energy. Longer-duration fasts reduce insulin more dramatically. Regularly lowering insulin levels leads to improved insulin sensitivity—your body becomes more responsive to insulin. The opposite of insulin sensitivity, high insulin resistance, is the root problem in type 2 diabetes and has also been linked to a number of diseases, including: Heart disease Stroke Alzheimer’s disease High cholesterol High blood pressure Abdominal obesity Nonalcoholic steatohepatitis (fatty liver disease) Polycystic ovary syndrome Gout Atherosclerosis Gastroesophageal reflux disease Obstructive sleep apnea Cancer Figure 1.3. Extended fasting over 4 days leads to decreased insulin and blood sugar levels. Source: Zauner et al., “Resting Energy Expenditure in Short-Term Starvation Is Increased as a Result of an Increase in Serum Norepinephrine.” Lowering insulin also rids the body of excess salt and water because insulin is well known to cause salt and water retention in the kidneys. This is why low-carbohydrate diets often cause diuresis, the loss of excess water—in fact, much of the initial weight lost on a low-carb diet is water. This diuresis is beneficial in reducing bloating and helping you feel lighter. Some may also note a lower blood pres...