Fasting for Health: What Are the Benefits?

Is fasting for health sustainable? And more importantly, does it work? Let me start by telling you first why I began 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 did 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.

A Novel Idea: Fasting for Health

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 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 I’d never come across this research before! (Maybe because no one wants to do a three-day fast?)

Learning More About Fasting for Health

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 to lower insulin levels and lose weight, among other health benefits. You can fast for 12 hours per day—for example, from 7:00 p.m. to 7:00 a.m.—or 14, 16, 18, or 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!

Eating Begets More Eating

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.

Our Ancestors Employed Fasting for Health

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.

Fasting for Health as a Spiritual Practice

After the development of agriculture made fasting avoidable, many cultures and religions continued to use fasting for health 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 to 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 eat continuously.

What Happens in the 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 to 36 hours, after which the body transitions to burning our stored fat for energy.

The Keto Diet and Fat Burning

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 and new neuron growth from stem cells, and slowing the progression of neurodegenerative diseases, including Alzheimer’s, Parkinson’s, and Huntington’s.

What is Autophagy?

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 your 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.

Eating Too Often Turns Off Autophagy

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