Intermittent Fasting: Definition, Benefits, & Schedules

By Charlie Huntington, M.A., Ph. D. Candidate
​Reviewed by Tchiki Davis, M.A., Ph.D.
Intermittent fasting is a form of dieting that offers health benefits way beyond weight loss. Read on to learn the definition of intermittent fasting and how to unlock its power.
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It is commonly said that losing weight is simply a matter of calories in versus calories out. Eat less than you usually do, and you’ll lose weight. Exercise more than usual, you might see the same effect. But what if changing when you eat, not what or how much, would do the trick?
Intermittent fasting, as many research studies attest, delivers on this promise. In fact, potential weight loss is just one of the substantial health benefits of practicing intermittent fasting. From a stronger immune system to anti-aging properties, intermittent fasting has something to offer most anyone interested in improving their overall health. Read on to learn the definition of intermittent fasting, its benefits, and the different schedules for intermittent fasting.
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What Is Intermittent Fasting? (A Definition)

Intermittent fasting is restricting all of your eating to a specific period of time in the day. Most people eat throughout the day, starting with breakfast and ending with dinner, or maybe that last handful of evening snack while you’re winding down with a good TV show. A person practicing intermittent fasting condenses all their eating into a shorter window of time. For example, when I practice intermittent fasting, I typically do all my eating between lunchtime and the early evening. The rest of the time, I don’t eat, even if I feel hungry.
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Intermittent fasting is a diet only in the sense that it involves making conscious choices about your food intake. It is not limiting what foods you eat, or how much you eat. Some intermittent fasters eat multiple full meals during their eating window, while others might graze throughout that time. The goal is to eat your regular amount of calories, but within a shorter timespan.

Does Intermittent Fasting Work?

Let’s get right to the point: intermittent fasting works! It is an effective practice for improving mental and physical health in a variety of ways (de Cabo & Mattson, 2019). Here are some of the backed-by-science benefits of intermittent fasting:  

Benefits of Intermittent Fasting
  • Burning fat for fuel. Once your body stops getting glucose to burn, it uses fat as fuel instead (de Cabo & Mattson, 2019). This is the primary way that intermittent fasting leads to weight loss.
  • Cellular repair. While fasting, your body engages a process called autophagy, in which it recycles dead or unused cells that otherwise clutter the body and cause problems (de Cabo & Mattson, 2019).
  • Decreased inflammation. Certain key biomarkers of inflammation go down among fasting individuals (Wang et al., 2020).
  • May reduce depression. Individuals in treatment for depression saw slightly larger improvements in their symptoms when they fasted (Fernandez-Rodriguez et al., 2022).
  • Improves insulin resistance. Individuals with diabetes can lower their blood sugar levels through intermittent fasting. After extended fasting, some people have no longer qualified as diabetic (Welton et al., 2020).

In addition to the scientifically documented benefits, many people report feeling more alert and better able to concentrate when they are late in their fasting period. Scientific research is inconclusive about whether there are cognitive benefits like this to fasting (Gudden et al., 2021), but anecdotally, it appears that people are more productive when they are fasting.

Is Intermittent Fasting Healthy?

For most individuals, most of the time, intermittent fasting is perfectly healthy. In fact, scholars think fasting is so helpful because it recreates an experience that was once common for humans: needing to survive without food.

Think about it: our hunter-gatherer ancestors often went days without a successful kill or discovering a new food source. Their bodies had to develop ways to fuel themselves and operate more efficiently during those periods (de Cabo & Mattson, 2019). We can activate those same internal processes by choosing not to eat food for longer periods of time.

This might be why many people report feeling more alert and productive while they are fasting. Your very hungry, fasting-not-by-choice ancestor needed to stay focused when out on the hunt! It’s kind of a neat trick for modern-day you - increase your focus by creating the illusion that food is scarce.  

Perhaps the most powerful internal process activated by fasting is called autophagy, which literally means “eating oneself”. In autophagy, your body gets rid of or repurposes damaged and dysfunctional cells, clearing space for healthier cells. The accumulation of dead cells that never get recycled or removed is one way that the body ages; since fasting reverses this process, some people say that fasting is an anti-aging technique (Mattson et al., 2017).
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Believe it or not, fasting can be so effective in cleaning up cells that it has even been suggested as a treatment for dementia (Yoon & Song, 2019). A likely cause of dementia is the accumulation over time of damaged and dysfunctional brain cells, which eventually get in the way of different parts of the brain communicating with each other. By clearing out those unnecessary cells, fasting can reduce and maybe even prevent dementia (Yoon & Song, 2019).

Intermittent Fasting and Ketosis

Many of the positive effects of intermittent fasting come about because fasting causes the body to enter what is called ketosis. The body’s first choice of fuel is the glucose that it ingests or creates from the food you eat. When there isn’t enough glucose to use for fuel, the body starts to convert stored fat into ketones, which it can then use for fuel. This process is called ketosis, and it’s why people who fast typically lose body fat and weight over time.
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For a more visually stimulating and in-depth description of how fasting works, I recommend watching this video:

Video: How Intermittent Fasting Affects Your Body and Brain  

Intermittent Fasting for Weight Loss

It should be clear by now that intermittent fasting could induce weight loss, and there are many, many research studies showing this is the case (Cho et al., 2019; Welton et al., 2020). Intermittent fasting each day for as little as two weeks can start weight loss, but most research studies have featured longer periods of fasting (Welton et al., 2020). 
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Many dieters find it psychologically demanding or unpleasant to count calories or part with their favorite “indulgences”. A major advantage of this approach is that it does not involve calorie restriction or removing any particular food from your diet.

How to Intermittent Fast

To practice intermittent fasting, commit to a certain period of time in which you won’t eat. For example, you might plan not to eat all morning, and then have three meals between lunchtime and the early evening. During the hours you aren’t eating, it is fine – actually, a very good idea – to drink as much water as you want. It’s also okay to drink coffee while fasting. The goal is just to avoid consuming any calories during the fasting period.

Examples of Intermittent Fasting Schedules and Diets

Intermittent Fasting 16/8​
Perhaps the easiest place to begin with intermittent fasting is to go 16 hours without eating. For example, I often follow a pattern of fasting from about 7:30pm to 11:30am each day. Therefore, my eating window starts at 11:30am and ends at 7:30pm. For most people, this can feel like “skipping breakfast”, although the goal is still to eat as many calories as they usually would during the eating hours.

Intermittent Fasting 20/4
The longer you fast, the more your body experiences the benefits of fasting, such as ketosis and autophagy. That means many people attempt to limit their eating window to four hours a day, spending 20 hours in fasting. A common way to do 20/4 intermittent fasting is to break one’s fast in the mid-afternoon, then finish eating in the early evening.

Intermittent Fasting OMAD
For the truly strong-willed, there is OMAD, or “One Meal a Day”. This is just what it sounds like – trying to cram all your caloric intake for the day into a single meal, or a short period of about an hour.
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OMAD fasting is effective in the short-term for weight loss, but not very sustainable in the long-term. It’s hard to get a full day’s calories into your body in just an hour! Although many people practice 16/8 fasting or 20/4 fasting daily or almost daily, OMAD fasting should be undertaken only temporarily.

Intermittent Fasting for Women

The effects of intermittent fasting are not the same for women and men. One study found that women’s blood sugar levels went up, while men’s went down, after three weeks of fasting (Heilbronn et al., 2005). Female bodies may be more sensitive to fasting than male bodies, with longer periods of fasting possibly disrupting the production and regulation of key hormones (Meczekalski et al., 2014). For these reasons, it is sometimes suggested that women fast less frequently, or for shorter periods of time, than men.

Tips for Intermittent Fasting

I have been practicing intermittent fasting on and off for several years, and I have experienced many of the benefits described by the scientific research. If it’s so effective, why do I only practice it “on and off”? Although it gets easier over time, intermittent fasting is a demanding practice. Here are some tips for how to successfully intermittently fast:
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  1. Start modest. At the start of my first sixteen-hour fast, I was scared of how hungry I might get. (The hunger passed and things turned out okay.) If I had aimed for even longer, though, I might have had a bad experience and been turned off by the idea. I recommend easing into fasting: Start with one sixteen-hour fast in a week. Increase the frequency of your fasts, or the length of your fasts, gradually.
  2. Accept your hunger. Almost every intermittent faster I know only feels hungry for a small fraction of their time spent fasting. Expect to feel hungry – and expect that the feeling will pass.
  3. Limit temptation. If it’s hard for you to resist snacks at the office or the pastry shop on the way to work, making changes to your routine that minimize your exposure to food during your fast can help. For example, I might work at the office in the morning without bringing any food; this makes it easier to break my fast when I get home around lunchtime.
  4. Break your fast judiciously. What you eat to break your fast makes a big difference. During your fast, your body has been using low glucose levels to its advantage. If you eat something sweet or carb-heavy to break your fast, your energy levels will soar – and then plummet. So break your fast with a smaller meal that incorporates vegetables or a good source of protein, such as dairy or chicken. This ensures a smoother transition into the eating window.
  5. Watch your stress levels. Months of fasting acclimated me to feeling hungry and carrying on with my day. But being hungry remains a stressor, and on particularly stressful days, choosing not to fast becomes a good self-care decision. I recommend not restricting your eating window on busier and more stressful days.
  6. Consult with your doctor. About a month ago, I started taking a morning medication that should be taken with food – a change in my life that clashed with my intermittent fasting practice of not eating food through the morning. Intermittent fasting, at least on that schedule, is off the table until my body adjusts to the new medication. On that note, you might want to check with your doctor before you start intermittent fasting, especially if you are taking any medications that don’t go down well on an empty stomach.

Articles Related to Intermittent Fasting​

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

Books Related to Intermittent Fasting​

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

Final Thoughts on Intermittent Fasting

Intermittent fasting is a simple, straightforward technique for accessing a variety of health benefits. While it is difficult at first to sit through one’s hunger, many people, including me, have experienced the rewards on the other side.

If nothing else, I invite you to try intermittent fasting simply as an experiment in changing your routine. Prior to intermittent fasting, I had no idea that I could conduct my regular morning routine without eating anything. Now, I recognize hunger as a vitally important signal from my body – but not one I have to respond to immediately.
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That said, if you are considering trying intermittent fasting, I encourage you not to go into it with any particular set of expectations. The benefits to your health, your focus, or your waistline may not be easy to discern at first. Like so many techniques for promoting health, intermittent fasting needs time and commitment to show its effects.

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