The Health Benefits of Multi-Day and Intermittent Fasting

On a Wednesday morning in January, I woke up sick. I had a sore throat, I was unusually tired, and I could feel myself fighting something off. I hadn’t been sick in three years! I wondered, why this cold get me?

I took an at-home COVID test and thankfully, it was negative. But by day four, my cold symptoms weren’t any better. I’d had three bad nights of sleep, and I was feeling absolutely lousy. I gave in and begged my boyfriend to go buy me some cold medicine. He raised his eyebrows and said, “You know this happened because you were bragging about not being sick in three years.” I knew he was right.

Later that day as I was scrolling through Facebook, a post caught my attention: “Fasting For Three Days Can Regenerate Entire Immune System.” The headline caught me at a weak moment. One of those moments in which I’d do pretty much anything to feel better—even consider not eating for three days.

I had never fasted before, or even skipped a meal. I’m not one of those people who gets busy during the day and just forgets to eat. I have never understood these people.

But the article intrigued me, and it was based on sound research, which I’ll describe in this post. By the end of my day of Googling, I was surprised that in all of my obsessive reading about nutrition that I’d never come across this research before! (Maybe because no one wants to do a three-day fast?)

On day five I woke up feeling much better. That afternoon, a close contact of mine tested positive for COVID after having tested negative earlier that week. I took another at-home test and…positive. It had been COVID all along. I was extremely grateful that I had been vaccinated and boosted, because I knew that if I hadn’t, it would have been much worse.

I was still gung-ho to learn more about fasting, so I started by reading The Complete Guide to Fasting by Dr. Jason Fung. I had a hard time putting it down. I expected it to be a simple guide on how to make it through a multi-day fast, but the book was far more than that. The biggest takeaway for me was: When we eat is just as important as what we eat.

In this post, I’ll discuss:

  • Why fasting is as natural as eating
  • What happens in our body when we fast
  • The benefits of fasting for Type 2 Diabetes, Cancer, and Alzheimer’s disease
  • Benefits of fasting for athletes
  • Pros and cons of fasting for women of childbearing age
  • Proven health benefits of fasting
  • How to incorporate fasting into your life

Fasting is as natural as eating

I had read research on intermittent fasting before and understood the major health benefit for people who have type 2 diabetes or want to lose weight: intermittent fasting lowers insulin levels quickly and naturally. But I had never thought that I personally would benefit from practicing it, nor did I want to. I hate being hungry, and I always tend to feel a little lightheaded and anxious when I get too hungry.

For those who aren’t familiar with intermittent fasting, it’s an approach to timing your meals with the intention of lowering insulin levels and losing weight, among other health benefits. You can fast for 12 hours per day—for example, from 7:00pm to 7:00am—or 14, 16, 18, even 20 hours per day. You consume all of your calories for the day during your “feeding window.” Doing a 12-hour fast might sound easy, but skipping that evening snack and not putting any sweetener in your morning coffee can be surprisingly challenging!

As I read Dr. Fung’s book, I quickly learned that the reason I felt the need to eat often was because I was eating often. I had trained my body to rely on frequent input of calories, so when I went for a longer period without eating, my body didn’t really know what to do. It didn’t shift efficiently into fat-burning mode as it should; I wasn’t “metabolically flexible.”

As I’ll explain in the next section, when you go longer periods of time without eating between meals, your body gets better at turning your stored fat into energy. For most people, the transition doesn’t happen immediately—it takes a few weeks for the body to become efficient at burning fat. And there are a lot of variables, like the timing of your intermittent fasting and what types of food you eat during your feeding window.

Many people who are interested in health and wellness want to eat a diet as similar as possible to the diet humans ate as we evolved. The theory is, of course, that many of our current health problems are the result of our modern day diet, and if we were to eat a “paleolithic” or primal diet, our health problems would resolve.

As it turns out, eating a paleolithic diet means incorporating intermittent fasting into our daily routine. Fasting for hours and sometimes days at a time has always been a part of our existence. Our bodies rely on periods of fasting to detoxify, recycle damaged cells, lower insulin levels, and use stored fat. Eating all the time as many of us do, myself included, is not normal for humans.

After the development of agriculture made fasting avoidable, many cultures and religions continued to use fasting for its health benefits and as a spiritual practice. Today, practitioners of religions around the world including Greek Orthodox Christians, Buddhists, Muslims, and Hindus practice fasting on a regular basis. Ancient doctors and academics including Hippocrates, Plutarch, Plato, and Aristotle encouraged fasting for health, as did Paracelcus, Benjamin Franklin, and Mark Twain.

Dr. Fung points out that up until the 1970s, most people fasted for 12-14 hours per day (overnight) and didn’t snack between meals. But today, the average American eats five or six times per day, with the misconception that it’s best for our health to continuously eat.

What happens in our body when we fast

When we eat, our pancreas releases the hormone insulin. Insulin allows us to turn the food we’re eating into glucose (sugar) and use it immediately as energy, and to store the rest as glycogen (stored sugar) or fat.

When we eat often, our insulin levels stay high, and our blood sugar levels stay high too. When insulin levels stay high, we’re constantly using glucose as energy and storing excess energy as glycogen or fat—we’re not burning any stored fat.

In order to burn stored fat, we must allow our insulin levels to get low. This happens when we don’t eat for a while. The glycogen we store in our body lasts for about 24-36 hours, after which the body transitions to burning our stored fat for energy.

This is how the trendy “Keto diet” works. After one to two days of fasting, our body fully enters the state of ketosis. In ketosis, low insulin levels stimulate the breakdown of fat for energy. Our stored fat is broken down into fatty acids, which are used by most tissues of the body as energy. The body also uses these fatty acids to produce ketone bodies, which are used by the brain for energy.

Other beneficial things happen in our body when we fast as well. Adrenaline increases, speeding up our metabolism and increasing our energy. Production of human growth hormone (HGH) increases, preserving muscle mass and slowing the aging process. And the production of brain-derived neurotrophic factor (BDNF) increases, stimulating new neuronal connections, new neuron growth from stem cells, and slowing the progression of neurodegenerative diseases including Alzheimer’s, Parkinson’s, and Huntington’s.

Autophagy also occurs when we fast—this was the topic of the article that originally sparked my interest in fasting. In 2016, Japanese cell biologist Yoshinori Ohsumi was awarded the Nobel Prize for his discovery of autophagy.

Autophagy is a way in which the body recycles damaged or diseased cells. When we fast and there isn’t enough incoming energy to sustain these damaged or diseased cells, the body breaks them down and recycles the components of the cells. Then, new cells are built to replace the ones that were recycled.

This includes immune system cells: old, damaged white blood cells are recycled when you fast, so white blood cell count goes down while you’re fasting. Then when you start eating again, stem cells are triggered to regenerate new white blood cells that can function properly and protect you more effectively against disease. This is how the immune system is renewed during the process of fasting.

Increased levels of glucose, insulin, and proteins turn off the process of autophagy. So, autophagy does not occur when we’re eating constantly. Animal studies show that autophagy begins after about 24 hours of fasting, and starts to peak around 48 hours of fasting.

If you want to learn more about what happens in your body over the course of a 72-hour fast, I highly recommend watching this video by Dr. Eric Berg.

The benefits of fasting for Type 2 Diabetes, Cancer, Alzheimer’s Disease, and Autoimmune Diseases

Type 2 diabetes occurs as a result of high blood sugar. When blood sugar levels are too high for too long, the body produces excess insulin, which causes insulin resistance. Insulin resistance means that the body stops responding to insulin as a result of prolonged exposure to excess insulin.

It’s clear why type 2 diabetes is the fastest growing chronic disease in the world—because we eat too much and too often, keeping our blood sugar and insulin levels high. For people with type 2 diabetes, dietary changes are the most successful treatment.

Fasting lowers insulin levels naturally, and very quickly reverses type 2 diabetes. If you have type 2 diabetes or pre-diabetes, I recommend reading Dr. Fung’s book, as he is an expert on using fasting to treat these conditions. If you have type 2 diabetes, fasting should be done under the supervision of a medical professional.

People with cancer can benefit from fasting in several ways. First, autophagy helps to prevent cancer by naturally removing potentially cancerous cells from the body. In this video, Dr. Eric Berg explains why the most important benefit of autophagy is cancer prevention.

Second, fasting can limit the growth of glucose-dependent tumors. Some leading scientists, such as Dr. Thomas Seyfried and Dr. Valter Longo recommend that everyone should fast on a regular basis as a cancer prevention strategy.

Third, fasting has been shown to prevent the toxic effects of cancer treatment. Both fasting and “fasting mimicking diets” increase resistance to chemotherapy in normal cells but not in cancer cells. After fasting, normal healthy cells are stronger and less affected by the toxic cancer treatment, while at the same time the cancer cells—which do not have the ability to adapt to fasting—have become weaker, and are more likely to be destroyed by the chemotherapy.

It’s been shown that autophagy is impaired in Alzheimer’s disease. Normally, the process of autophagy would remove the amyloid beta proteins that accumulate in the brain and cause Alzheimer’s. When autophagy is impaired, these proteins eventually destroy neural connections in the parts of the brain responsible for memory and cognition. Research shows that intermittent fasting protects against Alzheimer’s disease in animal models.

Fasting can also help protect against Alzheimer’s disease by lowering insulin levels. Alzheimer’s is now referred to by some doctors and scientists as type 3 diabetes because it involves insulin resistance that occurs in the brain. Research shows that having type 2 diabetes is one of the top risk factors for developing Alzheimer’s. You can learn more about the connection between Alzheimer’s disease and type 2 diabetes in this article.

Since fasting strengthens the immune system, it seems logical that it could help prevent and resolve autoimmune conditions. Research on this topic is still in its infancy, and there are over 100 autoimmune conditions that will need to be explored. Fasting has been shown to have multiple positive effects on the gut microbiome, an essential part of our immune system that is often affected in autoimmune disease. So far, fasting has been shown to have potentially positive outcomes for multiple sclerosis and rheumatoid arthritis. Some people use fasting to treat autoimmune conditions such as Hashimoto’s thyroiditis, Crohn’s disease, ulcerative colitis, and others, but little to no research has been done on the effects of fasting on these conditions.

Benefits of fasting for athletic training

Athletes may wonder if fasting is something they can benefit from, or if it is even safe. In fact, fasting periodically trains the body to become more efficient at burning fat, which is a huge advantage for athletes. When athletes feel as though they’ve “hit the wall” it means that they’ve used up all of their stored glycogen as energy and their body isn’t capable of using their stored fat quickly enough.

“Exercising in the fasted state trains your muscles to burn fat. Instead of relying on limited glycogen stores, you can use almost unlimited energy from your fat stores. Muscles adapt to use whatever energy source is available. When we deplete our glycogen through fasting, our muscles learn to become much more efficient at burning fat. The number of specialized fat-burning proteins is increased, and the breakdown of fat for energy is enhanced. After training in the fasted state, muscle fibers show increased available fat. All these are signs that the muscles are training to burn fat, not sugar.”
-Dr. Jason Fung, The Complete Guide to Fasting, Page 244

Upon starting a fasting regimen, athletes might notice a decrease in athletic performance while their body adapts to burning fat. This is referred to as fat-adaptation. Dr. Fung suggests that this period of decreased performance lasts about two weeks, though it varies from person to person depending on their current diet, what type of fasting regimen they adopt, and what type of workouts they do.

In addition to burning stored fat, there are two other significant advantages of fasting for athletes. First, fasting increases adrenaline, so people who train in a fasted state find that they can train harder. Second, fasting stimulates human growth hormone (HGH), which leads to quicker recovery and increased muscle building. For example, one study found that fasting for just two days increased HGH production by five times, while another study found that growth hormone levels were elevated 2000% above baseline after just 24 hours of fasting.

The pros and cons of fasting for women of childbearing age

Virtually all of the clinical studies on intermittent fasting have been carried out with test subjects who are men or post-menopausal women. Studies generally do not include pre-menopausal women due to concerns that fasting of any kind may negatively affect hormonal balance and the monthly cycle.

Some pre-menopausal women use intermittent fasting for weight loss, and with great success—you can search the internet to see countless before and after pictures. But there are an equal number of women out there who report that intermittent fasting had significant negative effects on their hormonal balance and monthly cycle. Since controlled studies with pre-menopausal women haven’t been done, we can only guess at the factors that could affect why some women can safely fast while others can’t. They likely include: overall health, how much excess weight they have to lose, how long they fast each day, and how much they restrict their overall caloric intake.

My guess is that overall caloric intake is a big factor. Intermittent fasting and caloric reduction are, in fact, two different things. You can intermittent fast every day while still eating your normal amount of calories during your feeding window. This allows you to get some benefits of intermittent fasting, like insulin reduction and training your body to burn fat, without dramatic weight loss. When we restrict our caloric intake for too long, our body can go into “starvation mode,” which can potentially upset hormonal balance and monthly cycle.

On the flip side, there some potential benefits of fasting for pre-menopausal women when it comes to fertility. First, intermittent fasting is becoming popular among women with polycystic ovary syndrome (PCOS) as a way to lower their insulin levels, lose weight, reduce inflammation, reduce stress, and alleviate depression and anxiety—all symptoms of PCOS. Some of these positive effects have now been proven in studies.

Second, intermittent fasting can lead to weight loss. Being overweight or obese negatively impacts fertility in a number of ways: by raising insulin and blood sugar levels, by increasing the risk of PCOS, by decreasing the sex hormones necessary for conception, and by increasing the risk of anovulation (when no egg is released by the ovaries).

Third, fasting may improve fertility in older women. A 2008 study carried out at Harvard Medical School found that caloric restriction significantly extended fertility in adult female mice. When the calorie-restricted mice reached 12 months of age—advanced age when it comes to mouse fertility—their eggs had far fewer abnormal chromosomes than the eggs of mice whose eating had been unrestricted. The calorie-restricted mice also produced more eggs than normal mice when their ovaries were artificially stimulated, their eggs were more likely to develop into embryos upon fertilization, and their offspring survival rates were dramatically higher than the control mice.

Another study showed that when worms are starved, they put reproduction on hold—not surprising. But in worms, their bodies destroy existing eggs during starvation and regenerate a new crop of healthy eggs from stem cells once they begin eating again. In the study, starvation allowed the worms to reproduce when they were 15 times older than normal reproductive age. The starved worms also lived three times longer than worms that were not starved.

This brings us to a topic that is now hotly debated: Do ovarian stem cells exist in humans?

Since 1951, we’ve believed that women are born with all the eggs they’ll ever have. However, a growing body of research suggests that mammals, including humans, do possess ovarian stem cells that are capable of generating new eggs as we age. If this is true, it could explain why women who have been through radiation treatment or bone marrow transplantation, which damage or destroy mature and immature eggs, can regain their fertility.

In the same way that fasting renews the immune system, it may renew women’s egg supply. Fasting induces autophagy, potentially triggering the body to recycle old, damaged eggs. Subsequent refeeding may then stimulate ovarian stem cells to regenerate new, healthy eggs. That’s what happened in the worm study—could a similar process happen in humans? It remains to be seen.

Experts recommend, and common sense dictates, that if you want to use fasting to improve your fertility you should do it before trying to conceive. Fasting while trying to conceive is counterproductive, because the body is much less likely to support a pregnancy when calories are scarce.

Proven health benefits of fasting

In summary, the proven health benefits of fasting include:

  • Induces body fat loss
  • Lowers blood sugar levels
  • Resolves type 2 diabetes and insulin resistance
  • Lowers blood cholesterol
  • Decreases inflammation
  • Triggers autophagy
  • Prevents neurodegenerative disease
  • Slows and prevents tumor growth
  • Lessens the toxic effects of chemotherapy
  • Slows the aging process
  • Increases energy, mental clarity, and concentration
  • Reduces food cravings
  • Improves digestive function by balancing the microbiome and stimulating regeneration of intestinal stem cells to heal the gut

How to incorporate fasting into your life

If you’re interested in fasting, there are many ways to do it, and they all have health benefits. You can experiment and find a fasting strategy that works for you. I recommend reading Dr. Fung’s book before starting.

And, be aware that certain people should not fast:

  • Women who are pregnant or breastfeeding
  • Children under the age of 18
  • Anyone who is malnourished or underweight (BMI under 18.5)
  • Anyone who has a history of disordered eating

Dr. Fung recommends fasting under the supervision of a healthcare provider if you have type 1 or type 2 diabetes, gout, gastroesophageal reflux disease, or are taking any medications.

If you’re a woman of childbearing age and you’re interested in intermittent fasting, experts recommend starting slowly and not fasting every day. Pay attention to your hormonal balance and monthly cycle, and back off from fasting if it isn’t working for you.

Types of fasting include:

Intermittent fasting: Fasting for anywhere between 12 and 20 hours per day, and consuming all of your calories during your “feeding window.”

24-hour fasting: Also known as OMAD (One Meal A Day), this is just as it sounds—eating just one meal per day, and consuming all of your calories for the day during that meal. Some experts advocate doing a 24-hour fast twice per week.

Alternate-day fasting: This approach involves fasting every other day, but you are allowed to eat 500 to 600 calories per day on fasting days.

The 5:2 Diet: This approach involves eating normally 5 days per week, and eating just 500 to 600 calories on the two fasting days.

Note: 500 to 600 calories are allowed on fasting days for these two types of fasting simply to increase compliance of people who are doing these fasts for weight loss.

36-hour fast: Dr. Fung uses this type of fast with his type 2 diabetes patients. He recommends doing a 36-hour fast three times per week until desired results are achieved, and then adopting a fasting strategy that works for each individual person in order to maintain results. A 36-hour fast can easily be stretched into a 42-hour fast by skipping breakfast on the final day.

Extended fasting: This describes fasts lasting at least 2 full days, or 48 hours. If you’re interested in extended fasting, Dr. Fung generally does not recommend doing just a 48-hour fast, because the first 48 hours are the hardest in terms of hunger. After the first two days, hunger dissipates and many people report feeling euphoric.

Another reason to not bother with a 48-hour fast is that it can take up to two days for your body to burn through its glycogen stores. Once glycogen is used up, you enter ketosis, and autophagy reaches peak levels. This is why a three-day fast is recommended to reset the immune system.

While this may be hard to believe, Dr. Fung reports that fasting for seven to 14 days is only marginally more difficult than fasting for two days, because the first two days are the hardest. He often uses seven- to 14-day fasts with patients who have severe type 2 diabetes in order to rapidly improve their blood glucose levels and prevent further damage to their organs.

My personal experience with fasting

I’ve now been intermittent fasting for six months, and I can’t imagine going back to how I used to eat. Like many people, I used to fast for only nine or so hours per night. I’d have a small snack around 8:30 or 9:00pm, then put a big spoonful of honey in my Rasa at 6:00am. Simply skipping that evening snack and leaving the sweetener out of my morning drink extended my fast to 12 hours. After a few of months of experimentation, I settled into fasting for 14-16 hours per day, which feels right for me.

The benefits of intermittent fasting that I’ve experienced are a dramatic reduction in food cravings, a general reduction of hunger, improved digestion, and loss of several pounds of fat. The reduction in food cravings is the biggest change for me. For the first time in my life, I’m not picking certain foods because of emotions, the feeling of low blood sugar, or an imaginary feeling that I need to eat. I’m able to make completely rational decisions about what to eat, and it’s the healthiest relationship I’ve ever had with food.

When you start fasting, it’s important to give yourself time to adapt, pay attention to what your body needs, and be prepared for ups and downs. And if you feel like you really need to eat, then eat!

It’s also important to know that if you want to maximize the benefits of intermittent fasting, you can’t just eat whatever you want during your feeding window—you do need to eat a healthy diet. You should reduce your intake of added sugar as much as possible, eat whole, unprocessed foods, and incorporate healthy fats (like avocados, nuts, seeds, and coconut and extra-virgin olive oil) into your diet.

If you’re looking for an easy way to keep track of your fasting schedule, I use the LIFE app and find it very helpful and easy to use.

And did I ever do the three-day fast? Yes! After four months of intermittent fasting, I felt ready and re-motivated to do the extended fast. The first 36 hours were easy, but Days 2 and 3 were tough. I am extremely I did it, and I felt so good afterward that I’m actually looking forward to doing it again!

If you want to do an extended fast, I strongly recommend that you:

  • Get very comfortable with intermittent fasting first
  • Consume high-quality electrolytes and sea salt during the fast
  • Do your research: Read Dr. Fung’s book or a similar book before starting
  • Consult with your doctor if you have any diagnosed medical conditions or any health concerns
  • Don’t put too much pressure on yourself—if you need to eat, then eat!