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

by Jason Fung

What Is Intermittent Fasting? The term intermittent fasting simply means that periods of fasting occur regularly between periods of normal eating. How long each period of fasting lasts, and how long the period of normal eating lasts, can vary widely. There are many different fasting regimens, and there’s no “best” one. They all work to different degrees for different people. One regimen may work for one person but be utterly ineffective for the next. One person may prefer shorter fasts while another prefers longer ones. Neither is correct or incorrect. It is all personal preference. Fasts can range from twelve hours to three months or more. You can fast once a week or once a month or once a year. Shorter fasts are generally done more frequently, even daily, while longer fasts—twenty-four to thirty-six hours is the most common duration—are usually done two to three times per week. Prolonged fasting may range from one week to one month. I categorize fasting periods as short (less than twenty-four hours) or long (more than twenty-four hours), but this is somewhat arbitrary. In the IDM Program, shorter regimens are generally used by those mostly interested in weight loss rather than in treating type 2 diabetes, fatty liver disease, or other metabolic diseases. However, shorter, more frequent fasts still often work well for these conditions.