This article is intended solely for informational purposes. It is not meant to be taken as, and should not be construed, as medical advice. Any changes to your lifestyle or diet should be done in consultation with your doctor or health care professional.

Is prolonged fasting safe? 

Opinions vary.  It depends on the person, their state of health before fasting, their medical issues (if any), and what’s meant by the word “prolonged.” Some people report having fasted for days, weeks, or even months, and believe that it has been beneficial to their health.  Others feel that too much fasting, or too long of a fast without medical supervision, may be harmful.

One influential proponent of fasting, Dr. Jason Fung, feels that a three to five day fast is likely safe for most people. But he recommends that if you fast for longer than five days, you should do so under the care of a physician,  particularly if you’re on medications that may be affected by the changes in your blood sugar. 

Fasting is really just postponing eating, which our ancestors did many times, and which animals in the wild are used from time to time too. Our bodies are designed to store fuel as fat; when we’re hungry, we use up our fat reserves. Then we go look for food again. So there’s nothing unnatural about it. But obviously, we do need to be taking in nutrients at some point. We can’t fast forever.

Dr. Fung does state that fasting can be dangerous to some groups, including children, teenagers, pregnant or nursing women, and those with health conditions that contraindicate fasting.  In my own case, I found it helpful to read Dr. Fung’s writings and watch several interviews with him before trying a prolonged fast. More information: Fasting and Autophagy by Dr. Jason Fung

Below: An excellent video about Dr. Valter Longo’s work. Longo is describing the results of what he calls a “fasting mimicking diet.” (What’s true of an FMD, which simulates a fast, would of course also be true of an actual fast.)