The Science of Intermittent Fasting

Navigating the balance between resting and replenishing nutrients for optimum health benefits.
While fasting, the body draws upon stored fats for energy. Photo: iStock by Getty Images

Intermittent fasting (IF) is not a diet as much as it is an eating style. Intermittent fasters wait anywhere from 12 to 48 hours between meals, with the idea of getting the body into a fasting state. Throughout history, we’ve always had periods of feast and famine. Our bodies are set up to do specific jobs in each state. In our current three-square-meals-a-day food culture, 24-hour grocery stores and food-delivery services, we spend all of our time in a state of nutritional cellular abundance; our bodies never get a chance to reset.

Cellularly speaking, there are two roads a cell can take: fed or fasted. On the fed road, cells are in a period of acquisition and accumulation. They are taking in glucose and nutrients, using what they need and storing the rest. Insulin and glucose are high. Excess is stored for later: The body stockpiles all of the extra glucose in the form of glycogen. A stored form of carbohydrates, glycogen is tucked away in tissues and the liver.

On the fasted road, the body draws upon stored fats and carbohydrates for energy. It uses up glycogen in a process called glycolysis, and then consumes stored fat in a process called ketosis. Though glucose is the body’s preferred fuel source, the body adapts pretty quickly to the transition from using stored glucose to using ketone bodies (or fats) for energy.

In ketosis, the body is able to go into a cellular recycling process called autophagy. This process is unique to a fasted state. Autophagy, a Greek word meaning “self-eating,” is a way for the body to clean up and eliminate damaged cells. This can initiate molecular changes in the body that improve cellular repair and gene expression. According to registered dietitian Felicia Stoler, autophagy is a critical cellular process for proper immune system function and, possibly, longevity. In 2016, the Nobel Prize in Physiology or Medicine was awarded to cellular biologist Yoshinori Ohsumi for his work in identifying and observing this extraordinary cellular phenomenon.

It should be noted here that IF, though very good for your body, should not be taken to extremes. Although ketosis can trigger cellular cleanup, it can also cause cellular death when incurred over long periods of time. The key with proper IF is to follow periods of fasting with consumption of nutrient-rich whole foods.

There are four styles of IF:

16-8: The baseline for fasting is 12 hours of fasting (including sleep) followed by an eating window of 12 hours. This does not mean eating for 12 hours straight!
It refers to maintaining a normal diet in the 12 hours where eating is permitted. Working up to a 16-hour fasting period with an eight-hour eating window is recommended, as it allows for a longer amount of time in ketosis.

5 and 2: This type of intermittent fasting entails eating a normal diet with no designated fasting periods for five days, and then spending two days a week eating only 500 calories per day.

The Warrior: Fasting with nothing but water for 24 hours, one day a week.

5-Day Fasting Mimicking: This diet limits carbohydrates and proteins, both of which can be turned into glucose, and focuses on a low level of fat-based foods for a period of five days. Having your body in ketosis for a five-day period allows your body to do the maximum amount of cellular repair. It is recommended by Dr. Valter Longo, creator of the ProLon system, that this style of diet be done once per month for three months every year.

Tapp Francke is Purist ’s Contributing Health Editor and the founder of