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.
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.
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.
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.