Many diets focus on what to eat, but intermittent fasting is all about when you eat. With intermittent fasting, you only eat during a specific time. Research shows fasting for a certain number of hours each day or eating just one meal a couple days a week may have health benefits.

Our bodies have evolved to be able to go without food for many hours, or even several days or longer. In prehistoric times, before humans learned to farm, they were hunters and gatherers who evolved to survive — and thrive — for long periods without eating. They had to. It took a lot of time and energy to hunt game and gather nuts and berries.

Experts note that even 50 years ago, it was easier to maintain a healthy weight in the United States. There were no computers, and TV shows turned off at 11 p.m., people stopped eating because they went to bed. Portions were much smaller. More people worked and played outside and, in general, got more exercise.

With internet, TV and other entertainment available 24/7, many adults and children stay awake for longer hours to watch TV, scroll through social media, play games and chat online. That can mean sitting and snacking all day — and most of the night.

Extra calories and less activity can mean a higher risk of obesity, type 2 diabetes, heart disease and other illnesses. Scientific studies are showing that intermittent fasting may help reverse these trends.

How did it become so popular?

It has become obvious that conventional medical treatments are failing patients. Many of today’s chronic medical issues are related to diet and obesity, yet treatments are focused on medications and surgeries. If you don’t deal with the root cause, the problem never improves. A dietary problem requires a dietary solution.

Dr. Jason Fung, MD, is a Toronto-based nephrologist and a world-leading expert in intermittent fasting and low-carb diets. After graduating from the University of Toronto and completing his residency at the University of California, Los Angeles, Dr. Jason returned to Toronto to work as a kidney specialist. He began to work with patients to prevent and treat Type 2 diabetes and obesity — both precursors to kidney disease.

He started by prescribing more conventional solutions — low-fat, low-calorie diet, and exercise — but he found that such conventional treatment plans weren’t truly helping his patients lose weight and get healthier. That’s when he decided to think a little outside the box.

Dr. Jason, along with his friend and professional partner, Megan Ramos, co-founded the Intensive Dietary Management program and The Fasting Method to help their patients truly lose weight, prevent obesity and Type 2 diabetes, and completely avoid kidney disease. They started seeing real results in their patients, and now, they’re beginning to see that fasting can also lower our chances of getting cancer. Dr. Jason fundamentally believes in the power of intermittent fasting to help people lose weight, lower their blood glucose levels, reduce their medication dependency, and improve their overall health.

According to the International Food Information Council (IFIC) ‘s most recent annual survey, intermittent fasting has just surpassed the ketogenic/high-fat diet as the “most popular” diet. The IFIC showed that about 10% of Americans between the ages of 18 and 80 reported adhering to some sort of intermittent fasting diet. The specifics around the timing and caloric regime were not included in the survey; however, nonetheless, this 10% barely edged out “clean eating,” which accounted for 9% of diets. Although there are many large consumer/customer surveys out there showing mixed rankings, and intermittent fasting is far from a break-away winner here, it does show a paradigm shift in Americans’ relationship with food.

How does it actually work?

There are several different ways to do intermittent fasting, but they are all based on choosing regular periods to eat and fast. For instance, you might try eating only for eight hours each day and fast for the remainder. Or you might choose to eat only one meal a day two days a week. There are many different intermittent fasting schedules. Intermittent fasting works by prolonging the period when your body has burned through the calories consumed during your last meal and begins burning fat.

During the times when you’re not eating, water and zero-calorie beverages such as black coffee and tea are permitted. During your eating periods, “eating normally” does not mean going crazy. Research shows that you’re not likely to lose weight or get healthier if you pack your feeding times with high-calorie junk food, super-sized fried items and treats. But what some experts like about intermittent fasting is that it allows for a range of different foods to be eaten — and enjoyed. Sharing good, nutritious food with others and savoring the mealtime experience adds satisfaction and supports good health.

Intermittent fasting benefits research has revealed so far:

• Thinking and memory. Studies discovered that intermittent fasting boosts working memory in animals and verbal memory in adult humans.
• Heart health. Intermittent fasting improved blood pressure and resting heart rates as well as other heart-related measurements.
• Physical performance. Young men who fasted for 16 hours showed fat loss while maintaining muscle mass. Mice who were fed on alternate days showed better endurance in running.
• Type 2 diabetes and obesity. In animal studies, intermittent fasting prevented obesity. And, in six brief studies, obese adult humans lost weight through intermittent fasting. People with type 2 diabetes may benefit: Most of the available research shows that intermittent fasting can help people lose body weight and lower their levels of fasting glucose, fasting insulin and leptin while reducing insulin resistance, decreasing levels of leptin and increasing levels of adiponectin. Certain studies found that some patients practicing intermittent fasting with supervision by their doctors were able to reverse their need for insulin therapy.
• Tissue health. In animals, intermittent fasting reduced tissue damage in surgery and improved results.

Some people try intermitting fasting for weight management, and others use the method to address chronic conditions such as irritable bowel syndrome, high cholesterol or arthritis. But intermittent fasting isn’t for everyone.

• Children and teens under age 18.
• Women who are pregnant or breastfeeding.
• People with type 1 diabetes who take insulin. While an increasing number of clinical trials have shown that intermittent fasting is safe in people with type 2 diabetes, there have been no studies in people with type I diabetes.
• Those with a history of eating disorders.

Can intermittent fasting have side effects?

Over the last several years, intermittent fasting has become increasingly popular for its promises of improved health and weight control. The idea is that it’s easier to sharply restrict calo­ries a few days a week or to limit eating to a shortened “eating window” each day than it is to moderately cut calories at every meal, every day.

Propo­nents claim that extended fasting periods (beyond the normal time between meals) promote cellular repair, improve insulin sensitivity, increase levels of human growth hormone, and alter gene expression in a way that promotes longevity and disease protection. But are there any risks?

Depending on the length of the fasting period, people may experience headaches, lethargy, crankiness, and constipation. To decrease some of these unwanted side effects, you may want to switch from adf fasting to periodic fasting or a time restricted eating plan that allows you to eat everyday within a certain time period.

There’s a strong biological push to overeat following fasting periods because your appetite hormones and hunger center in your brain go into overdrive when you are deprived of food.

It’s human nature for people to want to reward themselves after doing very hard work, such as exercise or fasting for a long period of time, so there is a danger of indulging in unhealthy dietary habits on non-fasting days.

Two common effects of calorie-restricted diets—a slowed metabolism and increased appetite—are just as likely when people practice intermittent fasting as when they cut calories every day. And in studies of time-restricted eating, evidence is accumulating that eating that misaligns with a per­son’s circadian rhythm (your body’s natural daily pattern) may lead to metabolic trouble.

While intermittent fasting shows promise, there is even less evidence about the benefits or how fasting might affect older adults. Human studies have looked mostly at small groups of young or middle-aged adults, for only short periods of time.

But we do know intermittent fasting could be risky in some cases. If you’re already marginal as far as body weight goes, you could lose too much weight, which can affect your bones, overall immune system, and energy level.

If you want to give intermittent fasting a try, make sure to discuss it with your doctor first. Skipping meals and severely limiting calories can be dangerous for people with certain conditions, such as diabetes. Some people who take medications for blood pressure or heart disease also may be more prone to imbalances of sodium, potassium, and other minerals during longer-than-normal periods of fasting.

People who need to take their medications with food — to avoid nausea or stomach irritation — may not do well with fasting.

What is autophagy?

Autophagy is your body’s process of reusing old and damaged cell parts. Cells are the basic building blocks of every tissue and organ in your body. Each cell contains multiple parts that keep it functioning. Over time, these parts can become defective or stop working. They become litter, or junk, inside an otherwise healthy cell.

Autophagy is your body’s cellular recycling system. It allows a cell to disassemble its junk parts and repurpose the salvageable bits and pieces into new, usable cell parts. A cell can discard the parts it doesn’t need.

Autophagy is also quality control for your cells. Too many junk components in a cell take up space and can slow or prevent a cell from functioning correctly. Autophagy remakes the clutter into the selected cell components you need, optimizing your cells’ performance.

Autophagy is essential for a cell to survive and function. Autophagy:

• Recycles damaged cell parts into fully functioning cell parts.
• Gets rid of nonfunctional cell parts that take up space and slow performance.
• Destroys pathogens in a cell that can damage it, like viruses and bacteria.

Autophagy plays an important role when it comes to aging and longevity, too. As a person ages, autophagy decreases, which can lead to a build-up of cellular junk parts and, in turn result cells that aren’t functioning at their best.

You can induce autophagy by stressing your cells to send them into survival mode. You can induce autophagy through:

• Fasting: Fasting means that you stop eating for a certain amount of time. Fasting deprives your body of nutrients, forcing it to repurpose cell components to function.
• Calorie restriction: Restricting your calories means decreasing the number of energy units, or calories, your body consumes. Instead of depriving your body of calories completely (as with fasting), you limit them. This forces your cells into autophagy to compensate for the lost nutrients.
• Switching to a high-fat, low-carb diet: This type of diet, commonly referred to as a keto diet, changes the way your body burns energy, so that instead of burning carbs or sugar for energy, it burns fat instead. This switch can trigger autophagy.
• Exercise: Exercise stimulates processes that increase the activity of ATGs, such as stressing your skeletal muscles. Exercise can induce autophagy, depending on the type of exercise you’re doing and its intensity.

Is intermittent fasting the same as autophagy?

Researchers found that fasting for 16 hours is beneficial to your body as long as you do not have comorbidities. If planned well, a 16-hour intermittent fasting diet provides you with many long-lasting health benefits. It is one of the easy diets to follow. But initially, it will be hard on the body.

Initially, with fasting, the body physiology undergoes changes that include:

• Hunger and fatigue as the body feel low on the energy from glucose.
• Gluconeogenesis, a process where glucose is generated from fat stores in the body to provide energy, is initiated.
• This process of using stored glucose and fat leads to weight loss.
• During the initial stages of fasting, you feel hungry at the usual times of your meals, as your brain is conditioned in such a manner.
• Gradually, your body adapts to fasting, and the hunger levels decrease.
• Having a healthy meal after fasting increases insulin sensitivity.
• Insulin sensitivity helps in fat loss and protects the body from diseases such as diabetes, high blood pressure, and heart diseases.
• During the 16-hour fast, your body undergoes a process known as autophagy, a process where the body destroys old or damaged cells in the body.

Autophagy is not an on/off switch — instead, it is much more similar to a dimmer that gradually illuminates as your fast duration increases past 16 hours of fasting. According to clinical research, your fasting autophagy dimmer switch is not at peak “brightness” until at least 36–72 hours into a fast. For those who want to try to maximize the deep benefits of autophagy, the majority of studies present evidence that suggests you may need to target longer fasts lasting 2 to 4 days. It’s important to remember these longer fasts can cause rapid changes that may require professional support in managing fluids and electrolytes as well as even reducing or adjusting medications.

The length of time needed to trigger autophagy through fasting can vary depending on a number of factors, including your individual metabolism and dietary habits. Generally, it is recommended to fast for at least 16 hours to stimulate autophagy, but this varies by individual. Autophagy fasting can be something you invest in through small, consistent fasts lasting 17 or more hours, or you can opt to do longer fasts less frequently; perhaps a few times each year, you do a 48–72 hour fast to maximize your fasting autophagy.

While the specific length of time needed to induce autophagy may vary from person to person, intermittent fasting is an effective way to stimulate autophagy. Based upon the research, you may want to work up to a fast of at least 17 hours, and you may opt to fast longer so that you can slide that autophagy dimmer switch further into the “on” position.

After completing an autophagy fast, especially one that is greater than 24 hours, it is important to ease back into eating and support your nutrition needs. Start with small, high-protein meals that are easy to digest, such as bone broth, cooked vegetables or scrambled eggs. Avoid processed and high-sugar foods, as they can cause blood sugar spikes and interfere with the body’s natural processes. Focus on whole foods, which are rich in vitamins, minerals, and antioxidants to help support cellular health and function.

Some examples of foods to eat after an autophagy fast include:

• Bone broth, which is easy to digest and rich in protein (from collagen) and other nutrients. Like bone broth, miso soup is a great plant-based option to break your autophagy fast. 
• Lean proteins, such as chicken, fish, and tofu, are rich in amino acids that play several important roles in rebuilding proteins and cellular repair after autophagy.
• Healthy fats, such as eggs, avocado, nuts, and seeds. Healthy fats help to rejuvenate your cell membranes. 
• Fermented foods, such as kimchi and sauerkraut, are rich in probiotics that can help to support gut health and immunity. Autophagy can shift your gut microbiome, so it’s always a great idea to rebalance your microbiome species with a probiotic, especially if your autophagy fast is greater than 24 hours.