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

by Jason Fung

The underlying cause of obesity turns out to be a hormonal, rather than a caloric, imbalance. Insulin is a fat-storage hormone. When we eat, insulin increases, signaling our body to store some of this food energy as fat for later use. It’s a natural and essential process that has helped humans survive famine for thousands of years, but excessively and persistently high insulin levels result inexorably in obesity. Understanding this leads naturally to a solution: if excessive insulin is causing obesity, then clearly the answer lies in reducing insulin. Both the ketogenic diet (a low-carb, moderate-protein, high-fat diet) and intermittent fasting are excellent methods of reducing high insulin levels.

Excessive insulin causes obesity, and excessive insulin causes insulin resistance, which is the disease known as type 2 diabetes.

How can people fast for days without being hungry? It comes down to the fact that hunger is not determined by not eating for a certain period of time. Rather, it is a hormonal signal. It does not come about simply because the stomach is empty. When you avoid natural stimuli to hunger, such as the sight and smell of food, as well as the conditioned stimuli to hunger—specific mealtimes, movies, ball games, any event where you normally eat and have learned to expect food—you help avoid that hormonal signal. Fasting helps to break all the conditioned stimuli and thus helps to reduce, not enhance, hunger. Hunger is a state of mind, not a state of stomach.

It’s useful to categorize fasts by two features: what is allowed on the fast and how long or frequent the fast is.

Most definitions of fasting allow noncaloric drinks only. This means that water, tea, and black coffee are all allowed during fasting,

So the optimal strategy seems to be eating the largest meal in the midday, sometime between noon and 3:00 p.m., and only a small amount in the evening hours. Interestingly, this is the traditional Mediterranean eating pattern. They eat a large lunch, followed by a siesta in the afternoon, and then have a small, almost snack-sized dinner. While we often think of the Mediterranean diet as healthy due to the type of foods in it, the timing of meals in the Mediterranean may also play a role.

Since all foods increase insulin to some degree, the most efficient method of lowering insulin is eating nothing at all. Even shorter fasts of less than twenty-four hours can prevent the development of insulin resistance and reverse relatively minor levels of resistance, and they certainly help with weight loss. But because breaking insulin resistance requires not just low insulin levels but persistently low levels, we need longer fasting periods.