Healthy Eating

Intermittent Fasting 101

A complete introduction to the science-backed benefits of intermittent fasting and how to get started

John Fawkes
Dec 3, 2019 · 16 min read
Photo by adrian825 via iStock.

Over the past fifteen years, intermittent fasting (IF) has become increasingly popular as both a diet and a lifestyle.

Many people find that intermittent fasting not only allows them to lose weight and keep it off, but also to be more productive and spend less time thinking about what they’ll eat on any given day. Moreover, there’s a growing body of research to suggest that intermittent fasting can extend the human lifespan, even independent of weight loss.

In this article, I’ll review the research related to intermittent fasting and its health benefits, then go over the most common intermittent fasting schedules.

After that, I’ll give you a step-by-step method to get started with 16/8 fasting, the most common IF schedule, and I’ll go over a few of the issues you may encounter when you first start fasting, and how to overcome them.

The Science and Benefits of Fasting

Weight loss

Intermittent fasting is commonly touted as an effective solution for weight loss — one that allows you to lose weight without eating less. But is it?

Studies find that intermittent fasting is indeed an effective way to lose weight, both for obese and non-obese individuals.

At least one study has also found that by alternating periods of extreme energy deficit with periods of energy balance, intermittent fasting may prevent the metabolic down-regulation that normally accompanies weight loss. As such, it’s plausible that that intermittent fasting may allow for more sustained fat loss, and/or easier weight maintenance afterward, compared to other weight loss methods.

However, other studies have cast doubt on this conclusion, with one study finding that alternate day fasting produced similar weight loss results as compared to steady caloric restriction. However, this study also experienced poor compliance among the alternate-day fasting group, raising the possibility that the issue has more to do with compliance than with the physical mechanisms of fasting.

Other studies have found that intermittent fasting improves markers of diabetes and insulin sensitivity. Notably, the results were only partly due to weight loss; fasting seemed to also improve metabolic health through mechanisms that were specific to fasting.

Ultimately, fasting probably leads to weight loss purely, or mostly, by getting people to eat less. If you cut back from three to two meals a day, it’s hard not to reduce your calorie intake.

Since studies pretty much always have to rely on the subjects self-reporting their food intake, it’s hard to gauge just how much fasting reduces energy intake. But since studies that have directly monitored energy intake and expenditure have consistently found that weight loss mainly comes down to eating fewer calories than you burn.


While fasting is widely believed to improve longevity, this is very difficult to study directly in humans. The scientific evidence for fasting having longevity benefits is either based on animal (mostly mice and rats) research or is indirect, extrapolating from the other health benefits of fasting.

First off, since fasting can aid weight loss and prevent weight regain, it will naturally extend the lifespan of anyone who’s overweight to begin with. That part needs little explanation.

Fasting also promotes autophagy, a mechanism that has been shown to reduce the risk of cancer, extend lifespan, and improve overall health both in humans and a wide variety of animals. For more information on that, read Is Autophagy the Secret to Life Extension?

Studies consistently find that fasting extends the lifespan of mice, fruit flies, rats, monkeys, and other small animals. However, since this question can’t be directly, experimentally studied with humans, that leaves open the question of whether fasting per se has longevity benefits for humans.

There’s also a growing body of research to suggest that intermittent fasting improves gut health in a variety of animal species, both through effects on the gut itself and effects on the gut microbiota. The effect isn’t perfectly consistent across all species, however, and human studies are still needed to see how fasting affects human gut health.

Right now, it looks like intermittent fasting is likely to extend the human lifespan by a combination of weight loss and lowering the risk of cancer and neurodegenerative disease via autophagy, but other mechanisms, such as gut health, are still questionable and need to be studied in humans.

Styles of Fasting

There are many, many different ways to implement intermittent fasting. For the most part, studies haven’t directly compared the health benefits of one versus the other, so the choice of fasting styles mostly comes down to personal preference and how easily you can stick to a given fasting schedule.

16/8 fasting

By far the most common intermittent fasting schedule, the 16/8 fast, was first popularized by Martin Berkhan of Leangains. Just like the name suggests, you fast for 16 hours and eat during an eight-hour window every day.

This schedule is popular because it’s easy to do on a daily basis, and strikes a good balance between eating restriction, ease of compliance, and flexibility.

Most commonly people will skip breakfast and the eating window will be from noon to 8 p.m., or a little bit later. Since eating in the evening makes it easier to sleep, and many people find that fasting in the morning makes them more alert during the workday, this is the most “lifestyle friendly” option for many people.

The other advantage to this schedule is that you get used to it– after a few days, you won’t feel much hunger during the fasting period, particularly if you fast in the morning.

The 16/8 fasting schedule is arguably the easiest way to get started with intermittent fasting and tends to be the first style of intermittent fasting that most people try. The guide later in this article will focus on 16/8 fasting.

19/5 fasting

Almost identical in concept to 16/8 fasting, 19/5 fasting restricts the eating window just a little bit further, to five hours. Whereas 16/8 fasting can allow for either two or three meals a day, a 19/5 fasting schedule allows for exactly two meals a day, at the beginning and end of the eating window.

While it may not seem much different from 16/8 fasting, the 19/5 schedule can be significantly more difficult for some people to follow. This is partly due to hunger, but often more so because this schedule forces people to miss social meals — unlike the 16/8 schedule, there is no way easy way to eat both lunch and dinner.

The upside is that, by restricting yourself to two meals a day with very little time to snack in between, you’ll be able to lose fat much more easily. This schedule is therefore ideal for rapid fat loss — if you can stick to it.

One meal a day

The most extreme daily fasting schedule is probably the one meal a day schedule. This isn’t particularly popular and most find it brutally difficult, but a few people swear by it.

The advantages are similar to the 19/5 fast, but even more pronounced. It’s pretty much impossible to be overweight on this schedule.

On the other hand, in addition to the impact it has on your lifestyle, this schedule can make it difficult to eat healthy food. After all, it requires most people to eat somewhere between 1500 and 3000 calories in one sitting. If you can’t do that without eating junk food just to add calories, you shouldn’t even try this schedule.

Occasional 24–48 hour fasts

Moving away from daily fasting, some people fast for 24–48 hours anywhere from once a week to once a month. The main advantages of this fasting schedule are that it enables rapid fat loss during the fasting period, and that fasting for 24 hours even once a month can be enough to prevent gradual fat gain, and has been shown to improve cardiovascular health.

There is also some evidence that maximizing autophagy requires fasting for 24–48 hours, meaning that this method may be superior to shorter daily fasts for life extension.

Some people find this more convenient than daily fasting, while others find it less so. Either way, the big disadvantage here is that unlike with a daily fast on a regular schedule, your appetite never really adjusts; you will feel hungry during your fasts if you do this.

A 24-hour fast lasts from dinner the night before the fast to dinner the night of the fast. A 36-hour fast lasts from dinner the night before to breakfast the morning after, while a 48-hour fast lasts until dinner on the second day of the fast.

24-hour fasts take some practice, but you can get used to them. 36-hour fasts are usually more difficult since it can be hard to sleep when fasted. 48-hour fasts are difficult for almost everyone, and should probably be done once a month at most.

Alternate day fasting (ADF)

Some people fast for 24–36 hours more than once a week, in a practice called alternate day fasting. In this schedule, people fast every other day, then on eating days, their diet is unrestricted. Most commonly, the eating window is 24 hours, meaning people eat a small dinner shortly before bedtime on fasting days.

Since it’s extremely difficult to eat twice as much food on the eating days, this schedule naturally leads to fat loss even without any attempt to follow a diet on eating days. It’s also easier to stick to than you’d think — though not truly easy by any means.

Alternate day fasting is at least as effective as daily energy restriction for fat loss and cardiovascular health. One study found it to be more effective, although that study used a flawed methodology — food was provided for the ADF group, but not the daily energy restriction group.

If ADF is better, that’s likely because it’s harder to overeat and not realize it on this diet, rather than due to any physiological advantage.

Longer-term fasts

Fasts lasting longer than 48 hours are generally not considered to be intermittent fasting. Historically, they’ve mostly been done for spiritual or protest purposes, but recently a growing number of people have started fasting for longer periods with the aim of extending their lifespan.

Studies don’t support this. Aside from the obvious problems of compliance, muscle loss, and metabolic down-regulation, at least one study has found that autophagy peaks between the 24 and 48-hour marks, and begins to decline after 48 hours. And autophagy isn’t a case of “the more the better,” either; there is such a thing as too much autophagy.

With that in mind, it’s probably better to fast for 24–48 hours more often, rather than doing occasional longer fasts.

Regular vs. ad hoc fasting

Most people fast on a regular schedule — fasting every day from 8 p.m. to noon, or every Sunday until 9 p.m., for example.

However, some people fast without following a precisely set schedule, instead choosing to fast whenever their schedule permits — a 24-hour fast whenever they have a day with no social commitments, or 16-hour fasts a few days a week.

From a health standpoint, both methods will work just fine; there’s no reason you have to fast on a totally regular schedule to get the benefits. On the other hand, fasting on an ad hoc basis isn’t necessarily easier, even though it offers more flexibility.

When you don’t follow a set schedule, it becomes difficult or impossible to form new habits. Your body also doesn’t adjust its appetite to match the fasting schedule, so you’ll be hungrier during your fasting periods.

For these reasons, I recommend following a set fasting schedule for at least your first two months of fasting, in order to build your fasting habit. After that, you can experiment with ad hoc fasting if you want to.

How to Start 16/8 Intermittent Fasting

Getting started is simple, but not quite as simple as just starting to fast, right now. As with most things in life, it’s best to do some planning upfront. Here’s the five-step process to getting started.

Step one: Plan your eating window around your schedule

Although most people choose to eat from noon to 8 p.m. or something close to that, your eating window can be any eight-hour stretch of time during which you’re awake.

In choosing your eating window, there are a few things to consider:

  • Eating in the evening, and eating carbohydrates, in particular, helps you sleep. Aside from the obvious consideration that it’s hard to sleep when you’re hungry, your brain also uses carbohydrates to produce serotonin, which itself is a precursor to melatonin. For more information on sleep, check out my complete guide to insomnia.
  • Including both lunch and dinner in your eating window tends to be conducive to a good social life. If you have to pick one or the other, dinner tends to be more important.
  • Once you’ve had a few days to adapt to fasting, you’ll probably feel more mentally alert during your fasting period. This can be helpful for productivity.
  • You’ll want to eat both before and after you work out — see the next item.
  • Consider when eating fits with your work schedule — for most people who work 9–5, that means breakfast at 8 AM, lunch around noon, and dinner around six. In that case, you could include either breakfast or dinner, but not both.

Although I’ve presented this as step one, you may actually want to do the next item — figuring out when you’ll exercise — first, and then decide on an eating window. It depends on which of the two you have more flexibility about. If there are very few times when you could exercise, you should figure out your workout time first and your eating window second.

After reading the list of considerations above, you can probably see why roughly noon to 8 p.m. is a popular eating window. It checks all the boxes, assuming you can work out sometime in that time frame and assuming further that you have a somewhat typical schedule, working somewhere in the ballpark of 9–5.

You can be somewhat flexible about your eating window. As a rule of thumb, you can shift it up to two hours in either direction on any given day, but you don’t want to do that more than once or twice a week. Shortening it a little is okay as well — consider eight hours to be the maximum, and five to be the minimum.

Step two: Determine when you’ll exercise

It’s best not to exercise on an empty stomach, since you need calories and protein to fuel your workout. If you’re resistance training — that is, lifting weights — you’ll also want to eat one or two meals after your workout to fuel muscle growth.

With that in mind, you would ideally want to exercise near the beginning of your eating window, just after breaking your fast. If that isn’t possible, try to train towards the middle of your eating window.

If you absolutely have to train at the very end of your eating window, you should have a small post-workout meal or snack — if only a protein bar or shake — to fuel your recovery and muscle protein synthesis.

Note that while you probably won’t workout every day, from a habit-building standpoint it’s still best to follow the same meal schedule every day rather than trying to distinguish between training and rest days.

Once you have an eating window and a workout time, you can set your meal times, both within your eating window and relative to your workout time.

Step three: Determine meal times

Once you have an overall eating window and a workout time, you’ll need to figure out specific meal times. An eight-hour window means you’ll either be eating two meals, three meals, or two meals and one snack.

Here’s an example of a typical scenario in which someone opts for the roughly noon to eight window, but chooses to push it back a bit to fit their schedule.

Work hours: 8 a.m. –5 p.m.

Eating window: 1–9 p.m.

First meal: 1–2 p.m.

Snack or second meal: 5–5:30 p.m.

Workout: 5:30 p.m.

Dinner: 7:30–8:30 p.m.

As you can see, taking a late lunch provides this person with enough time to hit the gym after work, eat before and after their workout, and still have enough time between meals and workouts that they don’t feel rushed, and their meals are spaced fairly evenly apart.

Step four: Determine meal composition

Intermittent fasting doesn’t require you to have a strict diet, other than fasting in and of itself. Nonetheless, you want to have some idea of what you’re going to eat and when.

This could mean that you have broad calorie targets for each meal, or just a general idea of what kinds of foods you’ll eat, depending on your dieting style. Here are a few guidelines to consider:

  • You need to eat at least .6 grams of protein per pound of body weight per day. If you’re overweight, make that .8 grams per pound of lean body mass instead.
  • Protein should be spread fairly evenly throughout the day, but a bit higher in the first meal.
  • Every meal should have at least 5 grams of fat; other than that, fat timing doesn’t matter much.
  • Carbohydrates should be concentrated later on the day and/or in the meal right after working out.
  • Every meal should have at least one serving of fruits and/or vegetables, and ideally, you want 5 servings of vegetables and 3 servings of fruit a day.
  • The higher-calorie meals might make you tired, so meals should generally get bigger in the evening.
  • It’s much easier to build consistent habits if you eat the same few things over and over for most meals, particularly the earlier ones. Letting yourself have more variety in the last meal works best for most people.

Based on those guidelines, here’s how the person from the above example would likely plan their meals:

  • Work hours: 8 a.m. — 5 p.m.
  • Eating window: 1–9 p.m.
  • First meal: 1–2 p.m.— Small meal, high-protein, low-carb. Usually a burrito bowl or chicken salad eaten near work on weekdays, or bacon, eggs, fruit and veggies at home on weekends.
  • Snack or second meal: 5–5:30 p.m.—Either a protein bar or a small sandwich.
  • Workout: 5:30 p.m.
  • Dinner: 7:30–8:30 p.m.—On workout days, a large high-carb meal, such as pasta or pizza. On other days, a smaller, moderately high-carb meal, like a stir fry with rice.

Step five: Start fasting, and track your meals

Now that you have everything figured out, it’s time to get started. For at least the first two months, track your meals, including meal times, with a food tracker such as MyFitnessPal.

If you’re committed to this and get off to a strong start, you’ll find yourself falling into an easy routine with it within the first week. You’ll get hungry shortly before your usual meal times, and find yourself getting less hungry during your fasting window.

Troubleshooting Your Fasting Strategy

Here are a few issues that might come up in the first few months of intermittent fasting.

Help! I’m not losing weight!

If you haven’t lost weight for a few days, you might just be retaining water weight. That can’t last forever. You could just wait it out, or you can speed up water weight loss by cutting back on carbohydrates and sodium.

If you haven’t lost weight for two weeks or more, you’re eating too much, plain and simple. Cut back on fat and carbs, add in some lean protein and vegetables, and drink more water.

If you’re losing weight too fast

This is the reverse of the last item, except it’s even more likely to be water weight. If you cut back your intake of salt, carbohydrates, and food in general, you’ll lose a lot of weight early on. Most of that will be water weight and the contents of your digestive system; nobody loses several pounds of fat overnight.

If you still feel like you’re losing weight “too fast,” take stock of your health and energy level. As long as you feel good, you’re probably not losing weight too fast. Although two pounds a week is often touted as a guideline, it is entirely possible for obese individuals to lose weight even faster, safely, with a large enough caloric deficit.

If you really want to slow down your rate of weight loss, just add two or three hundred more calories a day, ideally in the form of high-protein foods, like meat, beans, eggs or tofu.

Afternoon energy crashes

Some people experience recurring slumps in their energy levels at a certain time of day, usually the afternoon. There are a few things that can cause this.

First, you might not be eating a large enough breakfast (or whatever you want to call your first meal). Try making it a little bigger.

Second, if your breakfast doesn’t include carbohydrates, you may be entering ketosis and suffering from the keto flu. While you probably don’t want to have a high-carb breakfast, it’s often advisable to have at least ten or twenty grams of carbohydrates with breakfast to avoid depleting your glycogen reserves and entering ketosis.

Third, you could be crashing from caffeine. Many people make the mistake of increasing their morning caffeine intake in an attempt to make skipping breakfast easier. If you’re a caffeine drinker, try cutting your intake in half.


Insomnia on an IF diet is usually caused by one of three things: hunger, lack of carbohydrates, or caffeine.

As I just mentioned, try cutting your caffeine intake in half if you’re a caffeine user. As for the first two, you might experiment with pushing your eating window back an hour or two, and/or having 30 extra grams of carbs with the last meal of the day. One or both of those almost always do the trick.

Food cravings

Nothing about intermittent fasting necessarily requires you to change what you eat, only when. Food cravings may happen because you’re hungry, but they’re usually psychological in nature.

Feeding food cravings will satisfy them for a while, but makes them stronger in the long run. The best way to deal with food cravings is to refuse to give in to them– follow your diet as written. The longer you stick to your diet, the easier it gets as the food cravings start to fade.

That said, you probably don’t want to give up your favorite foods — that’s fine, but you need to regulate when you’re allowed to eat them so it doesn’t just become “whenever I crave them.” The best time to allow yourself to eat “junk” foods is during your post-workout meal. This works on a physical level because the calories feed your muscles, and on a psychological level because it motivates you to work out.


When you’re traveling, you’ll need to be a little bit more flexible about timing. On days when you fly across time zones, allow your eating window to be anywhere from four to nine hours, and your fasting windows to be anywhere from 14 to 24 hours.

As for what to eat, you might need to be a bit more flexible about that too. Bags of nuts are usually available in airports, and are a solid option. Ultimately, though, bending the rules a bit for one day is alright as long as you don’t eat out and out junk food. Having a sandwich on an otherwise low-carb diet, for example, is fine.

Of course, frequent travelers will need to be stricter about this, since they travel often enough for it to actually become a problem. But for most people, a day of bending the rules here and there, provided the circumstances truly warrant it, will be fine.

Start Fasting Tonight

Intermittent fasting is simple in principle, but in practice… it’s also pretty simple. You can easily plan out your intermittent fasting schedule in a half-hour or less, which means you can start fasting tonight and complete your first full sixteen hours of fasting tomorrow.

If you have any questions or comments about this article, or fasting in general, ask them in the comments below.

Better Humans

Better Humans is a collection of the world's most trustworthy writing on human potential and self improvement by coaches, academics, and aggressive self-experimenters. Articles are based on deep personal experience, science, and research. No fluff, book reports, or listicles.

John Fawkes

Written by

Los Angeles-based personal trainer, online fitness & nutrition coach, and health & fitness writer.

Better Humans

Better Humans is a collection of the world's most trustworthy writing on human potential and self improvement by coaches, academics, and aggressive self-experimenters. Articles are based on deep personal experience, science, and research. No fluff, book reports, or listicles.

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