What is fasting?

Fasting is a practice of eating only during certain specific times throughout the day in order to support your health and wellness. Someone who fasts might do all their eating between the hours of 8 AM and 6 PM, for example, or might refrain from eating at all during some days of the week. Unlike following a specific diet, fasting is not so much about what you put in your body but when you put it in.

Fasting is an ancient practice. Our prehistoric ancestors likely fasted, as a natural part of their hunter-gatherer way of living. Historically, people often fasted for religious or spiritual reasons, as well as to promote wellness. The Bible, classic Buddhist and Hindu texts, and the Quran all mention or promote fasting as a way of purifying the body and mind. Traditional Chinese medicine has long advocated fasting, while the Greek physician Hippocrates, sometimes described as the father of medicine, often urged his patients to fast. He is believed to have said "to eat when you are sick is to feed your sickness".

Today, many people use fasting primarily as a wellness tool.

What are the benefits of fasting?

The benefits of fasting can be considerable: many people who fast find that they lose weight, feel more energetic, and experience better health later in life. Research indicates that fasting may help repair your gut, improve your memory and help you think more clearly. Fasting can also encourage your body to produce more human growth hormone (HGH), a hormone that helps you burn fat and build muscle.

More controversially, fasting may help you live longer. The evidence for this is based on studies of mice, not human beings, and is therefore not conclusive. However, fasting has been proven to help you stay healthier while you age.

Fasting has also been shown to boost your metabolism, decrease inflammation in your body, and help you become more resistant to sickness. It can also support your heart health and lower your blood pressure. In other words, properly done, fasting can have a powerful effect on your overall wellness.

The spiritual and emotional aspects of fasting are not just relics of a distant past. There is evidence fasting can help lift depression, and improve your overall emotional health by benefiting your brain and gut. In addition, many people who fast today continue to do so for spiritual reasons, whether during religious events like Ramadan or Lent, or as a way to practice renunciation and develop greater mindfulness and mental clarity year-round.

What about Hippocrates' recommendation that we use fasting to treat illness and disease? There is indeed evidence that fasting may benefit people with health conditions ranging from obesity to diabetes to cancer. However, there are also risks. If you have a serious health condition, fasting is best practiced as a collaboration with your doctor.

What is the difference between dieting and fasting?

Dieting is another traditional approach to losing weight and boosting wellness. Typically, someone who follows a diet eats only certain foods. The Mediterranean Diet, for example, is based on the foods traditionally eaten in cultures surrounding the Mediterranean Sea, including lots of olive oil, fresh vegetables, and fresh seafood. People following a diet may also choose to limit the amount of food they eat each day.

Fasting has a different emphasis. While eating lots of natural, nutrient-rich unprocessed foods is always a foundation for wellness, someone who fasts will also set clear boundaries on when they will eat. No matter what diet they may or may not be following, they eat on a certain schedule that has long gaps between periods of eating. This may give them a little more flexibility in what they eat, the benefits of fasting won't be erased by a rogue chocolate bar, for example.

Why does fasting work?

While it isn't a cure-all, fasting can help many people to feel better. Why? Scientists don't have a complete picture for how fasting works to change your body, but they have suggested several theories.

One of the most well-known involves what is called the metabolic switch. Normally, our bodies burn glucose "sugar" for fuel. Our digestive system breaks down glucose from the carbohydrates we eat, and uses the glucose to power everything we do.

However, after a certain amount of time without eating - between 12 and 36 hours - our bodies run out of glucose. This is when the metabolic switch gets flipped; with no glucose left to burn, we start using fat for fuel instead. Specifically, our bodies release stored fatty acids, which are then converted into chemicals called ketones, and used for energy.

This process is thought to be responsible for many of the health benefits of fasting. Ketones are believed to be a better source of energy by many researchers and wellness experts, they are more stable and longer lasting than glucose. People whose bodies use ketones for fuel don't experience the same spikes in blood sugar that most people do, and often report feeling consistently high levels of energy.

Ketones may also be especially good for your brain. Sugar is a well-documented adversary of good brain health, and can cause depression and brain fog. Meanwhile, fat helps your brain to thrive.

In addition, some scientists have suggested that fasting may stress our cells in a beneficial way. Most of us think of stress as a bad thing, and it often is. However, a little stress can actually have a positive effect on our health.

For example, we stress our muscles when we exercise, which actually helps them to grow. Similarly, some researchers believe that fasting stresses our cells in a way that strengthens them against damage and aging, and helps them to eliminate waste more efficiently. Fasting has also been shown to positively influence how our bodies respond to insulin.

Fasting may also help support our health by giving your digestive system a break. A significant period of time without eating allows your gut to put energy towards releasing toxins instead of breaking down food. It can also act as a check on the growth of unhealthy gut flora.

How do I start fasting?

There are many different specific kinds of fasting. One of the most well-studied is what is called intermittent fasting. Intermittent fasting is an umbrella term that just means you stop all eating for at least 12 hours a day. This is long enough to potentially trigger your metabolic switch, and help your body start burning fat for fuel.

If you want to try fasting, intermittent fasting is a good place to start. There is no rigid protocol you have to follow. Instead, it's more of a journey. Try fasting for 12 hours at a time for a few weeks, and see how you feel.

For example, if you normally eat breakfast at 7 AM, you can start by fasting during the hours between 7 at night and breakfast. During the fasting period, you won't eat anything, or drink anything except for water, black coffee or tea. This is a common traditional approach - the word breakfast literally means - to break your fast.

Many people who experience good results with fasting end up increasing the length of their fasts. Fasting for periods of 14-18 hours each day is not unusual. Some people also choose to eat very little on some days of the week, and eat regularly on other days, a practice known as alternate day fasting. You can experiment, and see what works for you.

However you choose to fast, it is important to understand that if you really want to support your health and wellness, you need to eat healthy too. This means lots of greens, good fats like olive or coconut oil and healthy proteins, and as little sugar, junk food or processed foods as you can manage. Fasting is not the whole toolkit, but is a great tool.

Safety and side effects

It is important to note that intermittent fasting is not for everyone. Fasting is a stressor, although a potentially beneficial one. However, if you are experiencing mental, physical or emotional stress already, this may be compounded; so be aware of additional feelings of stress in your life. If you suffer from a chronic illness, it is especially important to speak to your doctor before beginning a fast.

As your body adjusts to fasting, you may experience transient symptoms, such as headaches, brain fog, lightheadedness, lower energy, hunger, irritability, a runny nose, bloating, or heartburn. These often taper off in a matter of days. Staying well-hydrated can help. Subsequently, people who do well with fasting often report exceptionally clear thinking, improved memory and positive energy gains.

A cautionary note for women: Intermittent fasting may cause women to experience hormonal imbalances, decreased menstrual cycles and fertility issues. Modifying fasting to suit you may, on the other hand, prove very effective. See Learn More below for an article on "Crescendo Fasting" as an alternative for women, and seek the advice of your holistic practitioner.


How Intermittent Fasting Might Help You Live a Longer and Healthier Life
David Stipp
ScientificAmerican, 2013

The Secret to Intermittent Fasting for Women
Amy Shah, MD

Paleo, keto, fasting, Whole 30: Why food tribes are on the rise. Washington Post
Sophie Egan
The Washington Post, 2018

Selected Studies:

Flipping the Metabolic Switch: Understanding and Applying Health Benefits of Fasting
Stephen D. Anton, et al
Obesity (Silver Spring), 2018