Fasting: Definition, Types, & Benefits

By Charlie Huntington, M.A., Ph. D. Candidate
​Reviewed by Tchiki Davis, M.A., Ph.D.
Humans have been fasting for thousands of years, but modern research is just starting to unveil its power. Read on to learn the definition and benefits of fasting! 
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Our lives revolve around food. Our ancestors thought about little else, and it’s still one of the first things we think about in the mornings. We organize our social connections and our daily schedule around eating. We relish cooking shows, farmer’s markets, and the thriving gardens in our neighborhoods – they all exemplify the bounty and beauty of food in our lives.​
If you’re reading this article, you probably live in a place where food is abundant, affordable, and varied. Food is so easy to access that we scarcely give it a second thought. After all, eating is a fundamental human right and pleasure. But in our culture of abundance, we can easily lose sight of the benefits of going without – of intentionally choosing not to consume. As this article will demonstrate, the benefits of fasting are plentiful and powerful – enough to make it potentially a key component of your self-care routine.​
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What Is Fasting? (A Definition)

Fasting means not eating for a certain period of time. Our ancestors probably endured many periods of involuntary fasting, as their food supply was far less stable. For the vast majority of us today, fasting is an activity only undertaken voluntarily.

While fasting has become popular in recent years as a health-promoting lifestyle choice, it is a practice with millennia of history, deeply rooted in several religious traditions (Kerndt et al., 1982). Ancient Greeks practiced fasting to prepare for rituals and celebrations. The Old Testament suggests that fasting is a powerful way to connect with the divine, and monks have practiced fasting for centuries. Perhaps most familiarly, Muslims around the world fast throughout the holy month of Ramadan, forgoing all food and water until the evening meal (which some may now refer to as intermittent fasting). Fasting has also been utilized as a form of protest, with political prisoners often fasting during their incarceration.

In the last couple centuries, doctors and scientists began to observe and track the health benefits of fasting, especially for weight loss. In the last several decades, benefits far beyond reducing our waistlines have begun to be discovered (de Cabo & Mattson, 2019).

Fasting Diets

All fasting diets have in common the restriction of how much food one eats. Typically, a fasting period features no consumption of calories whatsoever, but some fasting plans include days of reduced overall caloric intake, rather than periods of complete fasting.

Water Fasting
​Water fasting is fasting while continuing to drink water. Although it is possible to fast without drinking water, it is not generally recommended. While the body can rely on energy stores for fuel in the absence of new food intake, essential bodily processes break down much more quickly without water (Popkin et al., 2010). For this reason, while water fasts of relatively short lengths (e.g., a week or less) are possibly safe, it is rarely advised that one abstain from fluid intake entirely while fasting (Finnell et al., 2018). In fact, it is sometimes recommended that people engaged in longer fasts drink water that has added (non-caloric) nutrients and salts in order to maintain bodily functioning (Finnell et al., 2018).

Intermittent Fasting

Since fasting indefinitely would ultimately lead to very bad consequences for your health, most people have found it’s best to fast on a schedule. Intermittent fasting means having regular periods of fasting built into your routine, while continuing to eat all the calories and nutrients you need during the periods you are eating.

Longer periods of fasting are often undertaken to address a specific health concern, such as obesity (Kerndt et al., 1982). In fact, many studies have shown that people can – under rigorous and consistent medical supervision – go without food for months at a time, shedding many pounds of excess weight in the process (Kerndt et al., 1982).

Most of us have neither the time nor the need for such extreme measures, and thankfully we don’t have to go to such lengths! Intermittent fasting delivers many of the health benefits of fasting in a less effortful and more sustainable way.

On this note, I speak from experience – I have practiced intermittent fasting for several years. Although I have not had the medical testing done to establish that I’m getting all the benefits I will describe later in the article, I do know my mind is clearer, my waistline a little trimmer, and my immune system stronger when I am intermittent fasting.

Intermittent Fasting Schedules
The most common pattern of intermittent fasting is to designate a short period during the day in which you eat all the food you need; for the rest of the time, you fast. A typical place to start is with a 16/8 schedule, in which you refrain from eating for 16 out of 24 hours and eat all your food for the day within a single eight-hour window. To get added benefits, many fasters gradually shrink their eating window, perhaps to the point of fasting for 20 hours and eating in a four-hour window. At the extreme are individuals who limit their eating to a single meal each day.
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There are two other, less commonly utilized approaches. On a 5/2 fasting schedule, one eats a limited number of calories two days out of the week (usually around 500-600 calories), while eating regularly on the other five days. Some people will take this further and go without food for an entire 24 hours, once or twice a week, while continuing to eat regularly the rest of the time.

Benefits of Fasting

I have a confession to make, dear reader: this is the part of the article I’ve been most geekily excited to write. As an unabashed fan of scientific articles, and a practitioner of intermittent fasting, I truly enjoy how regularly a new article comes out extolling the benefits of intermittent fasting. So let’s dig in!

Fasting has profound effects on our metabolism and how our cells operate and regenerate, which can reduce hypertension, arthritis, and neurodegeneration (the breaking down of nerve cells, such as in the brain) (Longo & Mattson, 2014). Fasting also strengthens the immune system and makes our bodies more stress resilient (Longo & Mattson, 2014). Some studies have found that fasting is even as effective as the typically-prescribed drugs in treating seizure disorders and conditions such as rheumatoid arthritis (Hartman et al., 2012; Muller et al., 2001).
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Through processes I’ll describe in more detail below, fasting helps the body stay young. It literally refreshes your cells. This means that it may be able to help with degenerative diseases like Alzheimer’s and Parkinson’s and support people in overcoming strokes (Gudden et al., 2021). While these effects are more easily seen in older adults, it is possible that fasting can help with cognitive functioning in younger adults as well (Seidler & Barrow, 2021). To learn more about how fasting affects the brain, you can watch this video:

Video: Why Fasting Bolsters Brain Power

One of the coolest potential applications of fasting is in fighting cancer. Cancerous cells derive their sustenance from what the host body eats, so fasting deprives the cancerous cells of the sustenance it needs to survive. This is why some doctors are now recommending that people fast while undergoing chemotherapy and other kinds of cancer treatments (Nencioni et al., 2018).

Fasting for Weight Loss
Fasting is a well-established way to lose weight. Studies have found that people who fast, whether intermittently or for very long periods of time, lose weight (Cho et al., 2019). Since intermittent fasting in particular may be more sustainable over time than diets that focus on restricting overall calories, it is often described as one of the most effective ways to lose weight (Welton et al., 2020).

Fasting and Blood Sugar​
Having high blood sugar can lead to many health problems, including diabetes. Fasting is a very effective way of lowering blood sugar levels and reducing insulin resistance – so effective that people with diabetes who fast consistently can end up not needing to take insulin or even qualifying as diabetic (Albosta & Bakke, 2021).

How Does Fasting Work?

Okay, that was all really cool, right? But how does fasting work? After your body has gone about 12 hours without any new food intake, it begins to run out of stored glucose from your last meal. At this point, a metabolic switch is thrown, and your body goes from storing fat and burning carbohydrates to turning the fat in the body into fuel (Anton et al., 2018). Why would your body do this? The goal of this metabolic switch is to preserve the body’s muscle mass while continuing to function without new energy inputs.

Fasting and Ketosis
Fasting activates the process of ketosis in the body. Ketosis is the conversion of fat cells into ketones, which the body can use as fuel. This is the primary mechanism by which fasting seems to promote weight loss. Ketones serve as the fuel for our bodies when we are fasting or exercising for long periods of time.
As I noted above, we don’t want our bodies using up muscle or brain cells as fuel when we’re hungry. In fact, it is exactly when our bodies are deprived of fuel that we need to be alert and effective. It’s a matter of survival, and ketosis makes that possible.

Autophagy in Fasting​
But fasting doesn’t just make us into fat-burners. It also activates a process called autophagy. In autophagy, enzymes in your body break down poorly functioning or dead cell matter in your body, recycling those cells or using the material to make new cells (Longo et al., 2021). Autophagy is often called an anti-aging process, because it is the accumulation of those dead and useless cells over time that makes our bodies less efficient and functioning over time. So fasting can quite literally help you stay young and fresh.

Tips for Successful Fasting

It may be intimidating to consider trying out fasting. There are many ways to fast and even your doctor may not know exactly what is best. I recommend proceeding with caution and being gentle with yourself. Fasting is not how we are used to living our lives. Here are some tips for how to successfully introduce fasting into your lifestyle:

1)    Start modestly. Start with one sixteen-hour fast in a week. Pick a day that will be low stress, where there are few demands on your time and energy. Choose a food for breaking your fast that you will look forward to, but not one that you might be tempted to eat before the fast is done!

2)    Break your fast judiciously. Although I want you to enjoy the meal you break your fast with, the nature of what you eat definitely matters. If you eat something carb-heavy to break your fast, your digestive system will rush to process that new source of energy, giving you a big spike and dip in your energy levels. So, break your fast with a smaller meal that includes a good source of protein, such as dairy or chicken, and ideally some fiber as well. This ensures a smoother transition into the eating window.

3)    Embrace your hunger. You might not believe it when you are first starting out, but hunger doesn’t last. In this regard, it is like most sensations we have – temporary. Almost every intermittent faster I know only feels hungry for a small fraction of their time spent fasting. So, expect to feel hungry – and expect that the feeling will pass. If you have a mindfulness practice, consider using it to acknowledge, but not become controlled by, your hunger.
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4)    Consult with your doctor. For many of us, eating regular meals is important. Or perhaps you take a morning medication that wouldn’t go down well on an empty stomach. It’s always a safe bet to talk to your doctor before trying something like fasting.

Articles Related to Fasting

Want to learn more? Here are some related articles that might be helpful.

Books Related to Fasting

To keep learning, here are some books to explore:​​

Final Thoughts on Fasting

Fasting is a time-honored human tradition, with origins in spiritual traditions and abundant modern research documenting its physical health benefits. If you are not someone who has tried fasting before, it stands to benefit you in many ways.
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At the same time, fasting is not for everyone. If you have, or have had, a complicated relationship with eating, I suggest moving very slowly with fasting. Fasting is not sustainable, or even worth the effort, if it causes you emotional distress or discomfort. I hope this article has given you the information you need to make an educated decision about whether or not to include fasting among the health-boosting practices in your life.

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