The benefits of fasting and fasting-mimicking diets

by Nils Osmar.  © 2017, 2019

  • This article should not be construed as offering either formal or informal medical advice. The content is intended solely for informational purposes. Any changes in your lifestyle or diet should be done in consultation with your doctor or health care professional. 
  • Fasting is a powerful tool, but can have dangers. So if you do it, make sure you’re doing it safely. Please see this article by Jason Fung.

Several interesting things happen when we go without food for an extended stretch. Two of the most interesting are autophagy and apoptosis.  

Imagine that you’re in the woods without a jacket, and it’s cold outside (the equivalent of your body having no food for a while).  So you gather up all the twigs and sticks lying around, and start a fire.

Gathering up the twigs to burn is similar to AUTOPHAGY, the period (when you’re fasting) when your body is going through its cells finding viruses, bacteria and random accumulated junk to “burn” for fuel.  The body needs at least 15 grams of protein just to keep the heart beating and carry on other vital processes.  So if no food is coming in, it finds the building blocks of protein by scavenging for them in your cells. (It breaks cellular debris down into amino acids, then reassembles them into proteins to do the job.)

Eventually (in a day or two) you run out of twigs and sticks, or run low on them. (You’ve burned the ones that are readily available.) (And your body has burned all the easy-to-find debris in your cells.) But it’s still cold outdoors.  So you decide to cut down a tree, and use the wood for fuel. You choose an old, half-dead tree to cut down, so as to do minimal damage to the forest.

This is similar to APOPTOSIS, when  your body kills its own cells for fuel.  It happens two or three days into the fast. The body starts by killing old, broken, malfunctioning senescent cells — which is great, because they’re the sorts of cells that leak toxins into nearby cells and tissues, soak up needed nutrients, and tend to trigger autoimmune problems and illnesses.

Then after your fast, when you start eating again, your body grows new stem cells to replace the cells that were used as fuel. Think of this as planting a new, young, healthy tree in the forest to replace the old dying tree you burned.

You’ve cleaned the forest of a bunch of twigs and sticks… cleared out an old, sickly dying tree… and planted some healthy young trees in its place. Congratulations… good fast 🙂

In a nutshell, autophagy “takes out the cellular trash.” During autophagy, the body starts scouring the interiors of our cells looking for items to use as fuel. The items found include things like viruses, bacteria, and various types of useless clutter which have accumulated in the cell over time — junk that is “overflowing the trash bins” of our cells.  Burning this readily available “kindling” keeps our bodies happy for a while, but eventually the “trash” will be emptied, and we’ll have run out of available items to use.

Some autophagy is always going on in the body; but fasting shifts it into high gear. The longer our fast, the deeper it goes.

Apoptosis occurs later in a fast, and involves the actual deaths of old, broken, poorly functioning senescent cells. Fasting for three days or more should move you deeply into apoptosis. The longer the past (within reason), the greater the benefit. After two or three days of fasting, the body is in “starvation” mode and has to make a triage-like decision.  So it goes looking for senescent cells, which have special markers to make them easy to find. It then cannibalizes them, breaking them down into amino acids it can use to fuel essential bodily functions.

During apoptosis, misfolded proteins get cleared out of our brains, and neurons that are broken and senescent can get cleared away too. The body also gets rid of damaged mitochondria during this stage of fasting.

Apoptosis may sound harsh, but it’s great for the body, because senescent cells are the ones that trigger allergies, autoimmune reactions, and even conditions like multiple sclerosis. And the cells that are “killed” weren’t really fully alive in a biological sense… they were broken, malfunctioning cells, lingering and accumulating in the body but carrying on no essential functions. Apoptosis doesn’t happen in a short fast, but can occur in a longer one, if you’re fasting for three days or more.

  • How long it takes to fully activate autophagy appears related to our glycogen stores. Mild autophagy can kick in after just 12 hours. Or it may take longer if we have more glycogen reserves.
  • If we typically eat a ketogenic diet (low in carbohydrates, high in fat, moderate in protein), burning ketones rather than glucose as our main source of fuel, autophagy can start very quickly, because there’s not much glycogen in our bodies to burn. If we typically eat a high-carb diet, with lots of fruit, grains, and other starches and sugars, it can take 24 hours or more before autophagy begins. 
  • People are sometimes concerned about fasting destroying muscle tissue when deprived of nutrients. Recent evidence suggests that muscle is actually the last thing to go. The body burns fat, then protein (not muscle). It takes inessential proteins like skin tags first. If you fast for months, of course, you’ll eventually metabolize your own body. But according to Dr. Jason Fung, fasts of several days should be safe for everyone except those with health problems that counter-indicate fasting.


People pay thousands, or hundreds of thousands, of dollars for stem cell therapy. But prolonged fasting not only kills off your senescent cells (and senescent mitochondria), it results in the creation of new stem cells, for free.

When the fast is over, if we then eat enough highly nutritious foods (including ample protein rich in taurine, a necessary building block for cells), the body will replace the cannibalized senescent cells with new, healthy stem cells. 

This is truly remarkable, and was only recently verified in laboratory studies. It was thought for a long time that only very young humans (babies) could produce new stem cells; adult bodies could not. But it turns out that they can, and will do so when we fast. Doing a prolonged or extended fast followed by a re-feed is the only known way for an adult to create new stem cells.

Being true stem cells, the ones created in the hours and days after a fast can become heart cells, liver cells, pancreas cells, or even brain cells, increasing our intelligence and our “processing power.”

This is why it’s important that periods of fasting be balanced by periods of feasting, during which we’re eating the most nutritious foods we can find. We can’t simply fast then eat a poor quality, or low calorie diet; we’ll be damaging ourselves if we do. We need to eat foods rich in high quality proteins, vitamins, minerals, fats, and even some carbohydrates. For those of us who are carnivores or omnivores, foods like fish and organ meats are a highly nutritious choice.

To summarize:

  • We reach autophagy by fasting.
  • We reach apoptosis (killing senescent cells), an even more beneficial process, by continuing to fast.
  • 3-5 day fasts should give most people a good dose of both.
  • The longer you fast, the more of both processes you’ll experience.
  • Of course, past a certain point, being deprived of nutrients starts doing harm. We can’t fast forever! And people with medical conditions that contraindicate fasting need to do so only in consultation with their doctors.
  • Animals who are fasted in a laboratory setting, and experience both apoptosis and autophagy, have been found to live 30 to 50 percent longer than ones which don’t fast. (No one has tested this in people, because it would take several generations of study to test. But due to similarities between humans and lab animals in other respects, many researchers, including Dr. Jason Fung and Dr. Valter Longo, believe that it may work the same way in us.)
  • Dr. Valter Longo, who did much of the original research that established the benefits of fasting, pointed out that the “refeed” after a fast is the period when the real benefits occur.  The fast sets you up for regeneration; but if you skip the refeed, it won’t happen.
  • He recommends doing a several-day fast four times a year. Some people do it more often.
  • It’s important to remember that fasting alone does not rejuvenate the body. It clears out the debris and kills the malfunctioning cells, setting the stage for the real miracle to begin. It’s the stretch right after a prolonged fast, when we meet the most highly nutritious foods available, that the body uses the nutrients in those foods to create new stem cells. So it’s essential to pay as much attention to what we eat, and eating enough nutrients, as we do to the fast itself. Fasting needs to alternate with feasting, eating the most nutritious foods we can, rich in protein, vitamins, minerals and amino acids, to be fully effective.

As a side note, there are foods that can deepen autophagy, or trigger it. They include drinking coffee and green tea, and eating coconut oil, MCT oil, and ginger.  Oh, and getting some exercise.  Here’s an excellent article about things that we can to support autophagy in addition to fasting.

Want to try it? 

If you’re new to fasting, you may want to start with intermittent fasting, such as just skipping breakfast, and seeing how you feel. If you have dinner at 7 pm, then don’t eat till noon the next day, you’ll have fasted (gone without food) for 17 hours.  This will give you about three hours of autophagy.

Then work your way up to a day long fast (spend a day a week not eating at all.) Remember that the safest way to do a prolonged fast is to do a fasting mimicking diet (FMD). Apoptosis kicks in during a fast of three to five days.

And remember to eat highly nutritious foods when you stop fasting, rich in high quality proteins and healthy fats, and particularly the amino acid taurine, so that your body will have the nutrients it needs to make new stem cells, including the ones that can make new neurons in your brain.

Myths about fasting: There are many, many very silly myths about fasting, such as the notion that if you skip dinner, you’ll keel over and die, or that your metabolism will slow down. It’s true that low calorie eating can slow down your metabolism. But lab test after lab test has shown that fasting actually speeds it up. It also appears to sharpen out wits, as we need more intelligence when we’re out hunting for food.  Another myth is that we lose muscle when we fast. We may lose a little but we primarily lose fat. And any lost muscle mass will be quickly regrown, because fasting makes our growth hormone levels shoot through the roof.

Hunger and fasting: If you’ve been eating a high carb diet, you may feel ravenous when fasting, because your body is used to running on glucose, which isn’t being provided. As you start burning your stored body fat, releasing ketones in your bloodstream, the hunger should go away. If you’ve already been eating a high fat, low carb diet, you should feel little if any hunger when fasting.

Electrolytes and fasting: Most researchers agree that it’s helpful to replenish your electrolytes when fasting for more then a few hours. You can do this by adding some sea salt to the water you’re drinking. I recommend using Real Salt, a type of sea salt which incidentally is free of plastic contamination.  (Some people make an electrolyte drink, made up of water, apple cider vinegar, sea salt, a splash of lime, and some cream of tartar, to replace the full range of electrolytes.) (See the video below for the recipe).

Don’t overdose on water; just drink when you’re thirsty. Drinking too much water, especially if you’re not adding electrolytes, can be as damaging to the body as drinking too little.

Types of fasts: Most people who think of fasting picture themselves drinking only water (with some sea salt added for electrolytes). This is the most common type of fast.  But technically, a fast can involve restricting any nutrient, or only eating certain foods.  Water fasting is usually a good place to start. (One fast I would personally never try is a “dry fast,” which involves avoiding both food and liquids, and can result in dehydration and even death if done for an extended period.)

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 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.)

© Nils Osmar, 2017