How Long Should I Be on Keto Before I Start with Intermittent Fasting?

minute read | Last update: Jan 22nd2024

This article is backed by studies and reviewed by a certified dietician.

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You’ve lost some weight on keto, but you know you could lose more. A friend of yours had good luck with intermittent fasting, and you’ve decided that, rather than try one or the other, you want to do both diets at once. How long should you be on keto before you start intermittent fasting?

Before starting intermittent fasting, you’ll want to have been on the keto diet long enough to be able to track when you’re in ketosis. Since ketosis is a fat-burning process much like intermittent fasting is, the two diets can work in conjunction with one another for more pronounced results.

Now that you know you can combine keto with intermittent fasting, you can’t wait to get started. Yet before you do, we highly recommend you keep reading.

In this guide, we’ll provide you with lots of useful information so you can fast safely and effectively while on keto.

What Is Intermittent Fasting?

You’re very much familiar with keto, but not so much with intermittent fasting. What is it? To do something intermittently means you start and stop, and that’s exactly what intermittent fasting is about.

Rather than fast for days or even weeks at a time, you can eat in between your fasting windows. The type of intermittent fast you choose determines whether you get one eating window per day or several. Here are the types of intermittent fasts.

Alternate-Day Fasting

The alternate-day method switches between eating and non-eating periods in 24-hour increments. For example, you may start your week on Sunday with a fasting day. On Monday, you would eat like you normally do. Then on Tuesday you would fast, Wednesday you would eat, Thursday you would fast, Friday you would eat, and Saturday you would fast.

You could also swap that so that you’d eat normally on Sundays, Tuesdays, Thursdays, and Saturdays and then fast on Mondays, Wednesdays, and Fridays.

Most people on alternate-day fasts start at midnight on Sunday night and then fast until midnight Monday, but any period of 24 hours works. For example, maybe you wake up and eat a light breakfast on Sunday, then fast from noon Sunday until noon on Monday.

On your fasting days, you are allowed to eat, but fasters won’t consume more than 500 calories throughout the whole day.

The 16/8 Method

A very popular means of intermittent fasting is the 16/8 method. The name of this fast refers to the hours you spend fasting versus those you can eat. By following the 16/8 method then, you’d spend 16 hours of your day fasting and another eight hours eating. These eating hours don’t have to be consecutive, of course, as no one expects you to eat for eight hours straight.

With such a generous eating window though, you can surely fit in breakfast, lunch, dinner, and snacks without even feeling like you’re fasting. What also makes the 16/8 method so appealing is that you can use whatever 16 hours of the day to fast that you want. If you sleep for eight hours per night, that’s already half of your fasting time gone.

If you’re having a hard time during those other eight fasting hours, consuming calorie-free beverages is permitted. Besides water, you can also drink coffee, but make sure yours has no milk so you don’t exit ketosis.

The 16/8 method is a daily diet, unlike alternate-day fasting. If you find it’s too hard to fast for 16 hours, then you can amend the diet, making it 14/10 so you have 14 fasting hours and up to 10 eating hours.

If you want an even greater challenge, you can add more fasting hours to your diet and reduce eating hours, such as on the 18/6 diet.


OMAD stands for one meal a day. This daily diet is not for beginner intermittent fasters, as it requires you to forego food for 23 out of the 24 hours. That one hour is yours to eat whatever you wish. This can make the OMAD diet a double-edged sword for some intermittent fasters.

Sure, they’re spending 23 hours without food, but if they then eat a whole pizza or six cheeseburgers, their progress doesn’t really matter much.

Considering that you’re on keto, you probably won’t blow your diet with your one daily meal. There is no calorie limit for that one meal, so you can fill up on all your protein and fat for the day. If you have a hard time eating large quantities of food at once, you can really go hungry on the OMAD diet.

You do have the freedom to choose your eating hour. It’s recommended that you plan your eating window around the time that you’re most active so you have the fuel to get you through your day. There is no calorie limit for your one meal either.

The 5:2 Diet

Unlike the OMAD diet, which novices shouldn’t start with, the 5:2 diet is more beginner-friendly. For five days per week, you can eat without fasting. Then, the other two days of the week, you restrict caloric input but don’t stop eating altogether. Instead, you’re capped at no more than 600 calories as a man and 500 calories as a woman.

It doesn’t matter which of the two days of the week you choose to fast. If you want them to be consecutive, that’s one way to do the 5:2 diet. You can plan alternate fasting days, such as a normal day on Sunday, a fasting day on Monday, a normal day on Tuesday, and then a fasting day on Wednesday.

You can also space out the fasting days, so maybe you fast on Monday and then you don’t do it again until Thursday. What you eat is important here, as it’s recommended you split your 500 or 600 calories into two very small meals.

The Warrior Diet

Ori Hofmekler, a published author and fitness professional, came up with the Warrior Diet. You don’t ever fast outright on this diet, but rather, you switch between periods of eating more calories than others.

For most of the day, you consume fruits and vegetables in small portions. Since these foods are so nutritionally balanced, the calories you’re ingesting are usually not many. Just make sure you watch the carb content of the fruits and vegetables you choose, as fruits especially can be mini carb bombs.

In the evening, you can eat a normal-sized meal or even an extra-sized meal. Your eating window is four hours long. To truly follow the Warrior Diet as Hofmekler intends it, you should fill your diet with unprocessed whole foods.


A branch of alternate-day fasting is the stop-eat-stop diet. This diet follows the same premise but instead of fasting on alternate days of the week, you only spend two 24-hour periods fasting throughout the week. You can even fast for only 24 hours once a week.

Spontaneous Meal Skipping

The last type of intermittent fasting is spontaneous meal skipping. You’re allowed to eat three meals per day on most days except when you feel like skipping a meal or two. Maybe you get caught up in work and you don’t have time to eat or you even forget. You may listen to your hunger cues, and if you don’t feel hungry, you don’t eat. It’s as simple as that.

This is still intermittent fasting in the technical sense, but you could choose many better means of fasting. That’s especially true if you want to lose weight. The unstructured fasting that is spontaneous meal skipping can make it hard to see results.

How Long Should You be on Keto Before Starting Intermittent Fasting?

When you enter ketosis, the carbohydrate deficit you created in your diet causes your body to burn fat for energy. Intermittent fasting achieves the same goal, as without any source of glucose from food, when your glycogen reserves are gone, your body uses fat.

Yet the keto diet is not necessarily an easy one, especially if you’re someone who eats a lot of carbs (or you used to anyway). Before you think of adding intermittent fasting to keto, you should make sure you’ve done the keto diet for a while.

If you’re in the stages where you’re experiencing keto flu–which is common of those new to the diet–then it’s too soon to begin intermittent fasting. Keto flu is your body’s response to suddenly taking away large quantities of carbs. The symptoms can include dizziness, constipation, headache, lack of endurance, nausea, vomiting, exhaustion, and keto insomnia.

Most keto flu symptoms subside in a few days or weeks. Then you can consider intermittent fasting.

You do want to have a good hold on when you’re in ketosis before you start fasting. Rather than rely on your breath, we’d suggest buying ketone testing kits like those that determine your blood glucose levels. If you can guess when you feel like you’re in ketosis and then test and discover that you are indeed in ketosis, you’ll be more in tune with your body.

At this point, you’re ready to begin intermittent fasting. You’ll likely reap the biggest rewards.

What Are the Benefits of Intermittent Fasting on Keto?

If keto is healthy for you and intermittent fasting is as well, then you should expect double the health benefits, right? Most definitely. Medical and scientific experts haven’t yet done a lot of research into intermittent fasting on keto, but here are the known benefits.

Faster Rates of Ketosis

To enter the fat-burning state of ketosis, you need to consume no more than 50 grams of carbs a day. Now imagine you’re eating 0 grams of carbs because you’re fasting for the next 24 hours. You may enter ketosis even faster than you do when eating only 20 grams of carbs per day. The longer you spend in ketosis, the more fat you’re burning.

Weight Loss

That goes hand in hand with the next benefit: weight loss.

According to this Healthline report, during your first week on the keto diet, you may lose one to 10 pounds. After that, for every month you’re on the diet, you can shed one to two pounds per week, which is between four and 10 pounds of weight loss per month.

Another Healthline report notes that weekly intermittent fasting can lead to weight loss of around 1.65 pounds max. You may lose body weight at a rate of three to eight percent in a span of three to 24 weeks.

So if that’s about two pounds of weight loss a week for each diet, by doing intermittent fasting on keto, it’s conceivable that you could lose four pounds every week. However, do know that weight loss results differ for everyone. The kind of food you consume during your eating window, how much you weigh at current, how often you exercise, and your metabolism will all influence your weight loss success.

Outside of those benefits, the following are proven perks of intermittent fasting.

Better Brain Health

Skipping a few meals boosts your brain by lessening inflammation and oxidative stress. In some cases, your brain could even develop fresh nerve cells for better functioning, at least according to studies done on rats. One such study was this 2000 report from the Journal of Molecular Neuroscience.

Your brain-derived neurotrophic factor or BDNF might also increase by fasting. This protein maintains neuronal health and is responsible for the growth of neurons, so it’s quite important!

Could Ward Off Cancer

Many animal studies have shown promising results on what intermittent fasting can do for preventing cancer, including this Mechanisms of Ageing and Development report from 2005. What the studies have concluded is that the changes to your metabolism through fasting could be what lowers your cancer risk.

According to a 2009 publication of the journal Aging (Albany NY), in humans, chemotherapy side effects may be lessened if you’re an intermittent faster.

Can Activate Cellular Repair

One of the most fascinating effects of intermittent fasting is autophagy. This is a form of cell recycling where your healthy young cells eat parts of the old cells, sometimes leaving parts intact and in other cases, consuming the old cells in whole.

Although this might seem detrimental to your health, it’s anything but. Very few of the cells we’re born with stay with us for life. The lifespan of some cells can be only three weeks or so and others can live for years.

As cells age, they become less effective and can slow down the process of cellular repair and turnover. By recycling cells through autophagy, your skin cells, brain cells, and other cells can all work their best.

May Help Your Heart

Fascinating studies such as this one from Research in Endocrinology in 2014 have found that, in animals, heart disease risk factors have decreased through intermittent fasting. The animal participants had lower blood sugar, less inflammation, lower blood pressure, and less LDL cholesterol.

Although it’s unclear if the same benefits would apply to us people, it’s certainly a possibility!

Potential Longer Life

Everyone wants to live longer, and it turns out that it may not be so much about what you eat as what you don’t. A Gerontology study on rats found that the animals who alternate-day fasted had drastically longer lives compared to non-fasting rats at a rate of 83 percent!

Granted, this is another case where we’re just not sure if the benefits apply to people, but it doesn’t hurt to try.

Tips for Safely Fasting on Keto

As advantageous as intermittent fasting on keto can be, restricting carbs and calories to an extreme degree can also lead to unappealing side effects. Make sure that you follow these rules for safe fasting when on keto.

Start with an Easy Fasting Diet

Now is not the time to prove you can fast for periods of 24 hours. You need to work your way up to that. Even though you’re cutting carbs on keto, unless you’re already only eating two meals a day, then fasting is going to be another adjustment.

We’d suggest starting with something like the Warrior Diet where you can eat throughout the day but you’re limiting calories during most hours. Maintain the diet successfully for several weeks.

Then you might try the 5:2 diet, where you fast for only two days and can still consume calories. You can graduate to the stop-eat-stop diet from there, as now you’re removing those 500 or 600 calories and fasting for 24-hour periods at least twice a week. The 16:8 diet is another good one for beginners who aren’t quite ready yet to give up food for 24 hours at a time.

Keep up Your Fluids

On any intermittent fasting diet, you can drink fluids and you’re encouraged to do so. You’ll likely have to increase your water intake, consuming around four to five liters per day rather than two to three liters. All foods have water, some more than others, so drinking this much ensures that you don’t become dehydrated through the water loss from food.

Water also has the benefit of filling up your stomach and inducing some form of satiety. This can make those long fasting hours more manageable.

Know When to Stop

All along, you have to listen to your body when intermittent fasting on keto. Some side effects, such as reduced energy, are normal because your body is shifting into using fat as an energy source. Other side effects such as feeling sick or dizzy are not normal.

If you have to stop your fast early, then so be it. What matters more is your health!


Intermittent fasting on keto is a smart solution if you want to lose more weight than you can by doing either diet separately. Remember though that you have to ease yourself into intermittent fasting and above all, listen to your body!

About the author

The Authentic Keto Team is here to bring you health tips that help you with losing weight fast. We focus on a clean keto diet for beginners because we believe that is the easiest and most simple way to healthy eating. Our keto weight loss tips will not only bring you into ketosis fast but will also help you to improve your mental health, sleep problems, and wellness.

- Our goal is to make keto success easy. -

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