Intermittent fasting and a ketogenic diet have both become popular in recent years. Whether you’re aiming to lose weight or improve overall health, these techniques both offer unique benefits.
But is fasting a good idea for those who are following a keto diet?
Intermittent fasting can be an effective tool to maximise results on a keto diet. Studies show benefits such as weight loss and improved mental clarity. However, fasting isn’t for everyone, and it’s important to approach it with caution.
Personally, I tried many different styles of fasting throughout five years of low-carb eating, only to discover that it wasn’t right for me. You do not have to fast to lose weight or be healthy! In fact, I've lost more weight (and cm's) the past 6 months than I did in the 18 months before that.
Fasting is something that exacerbated my binge eating cycle, regardless of how I tried to approach it. Now, I no longer fast - I eat breakfast every day! I enjoy 4 meals a day, full of protein, healthy vegetables and a small amount of fat (I don't need high fat if I'm eating regularly). I've boosted my metabolism, built muscle and no longer crave the crappy stuff.
However, it’s hard to ignore all the testimonials around combining intermittent fasting and keto. For every story like mine, there’s a success story out there - and if you’re reading this article, you’re probably wondering whether fasting is a good choice for you.
I’ve created this blog to help you understand more about the pros and the cons of fasting on keto, including:
What intermittent fasting is and how it works
Common types of intermittent fasting
Benefits of fasting on keto (with studies)
How to tell if fasting is right for you
I’ll also share some tips for intermittent fasting on keto, including what you can eat, how to break a fast on keto, and how you can minimise the risks.
Read on to learn more about intermittent fasting and keto.
What is Intermittent Fasting?
Intermittent fasting is an approach which restricts when you eat, rather than what you eat. While the intervals may vary depending on your chosen method, intermittent fasting involves cycling between periods of eating and not eating (or “fasting”).
The goal of intermittent fasting is typically to accelerate weight loss or break through a weight loss ‘plateau’.
However, others practise intermittent fasting for overall health or cognitive benefits, rather than for weight loss purposes.
Approaches to intermittent fasting vary, from eating within a certain time window each day to having ‘fasting days’ during the week. Intermittent fasting may be practised on a temporary basis, a cyclical schedule, or may become an everyday eating pattern.
The most common method is 16/8 intermittent fasting, which involves a 16 hour fast in a 24 hour window. While it sounds like a long time, it essentially means you eat during an 8-hour window - such as 10am to 6pm - then fast again until the next morning. If you’re able to avoid eating before bed or skip breakfast, it’s easy to extend your usual overnight fasting time.
Other methods include the 5:2 method (where you eat normally five days out of the week then fast two days), or alternate day fasting (where you fast every other day).
Intermittent fasting can be practised on any diet, but is particularly popular for those following a ketogenic or low carb diet. It’s often recommended as a way to overcome a ‘plateau’ when losing weight, but it’s important to weigh up the pros and cons.
Below we’ll explore the benefits and downsides of intermittent fasting.
Pros & Cons of Intermittent Fasting
Intermittent fasting has gained popularity amongst people wanting to lose weight. However, this isn’t the only benefit. Studies have shown that it can actually improve your body, brain and may result in you living longer.
Of course, it’s very important to note that these studies are not stating anything definitive, and in all cases more studies are required to prove a link.
What are the Benefits of Intermittent Fasting?
Here are some of the documented benefits of intermittent fasting:
May Assist Weight Loss
If you want rapid weight loss, or your weight loss has stalled, then intermittent fasting can help you reach your goal. How does it work?
Fasting causes your body to release more norepinephrine, which is a fat burning hormone. That will increase your resting metabolic rate so you will burn more calories when you’re in a resting state.
A study published by ScienceDirect showed that intermittent fasting caused weight loss of 3-8% and was able to reduce the waist line of participants by up to 7% over a 3-24 week period.
May Reduce Risk of Diabetes
Intermittent fasting will reduce your insulin and blood sugar levels which means you’ll have a lower risk of diabetes. Studies in humans found that during intermittent fasting blood sugar was reduced by up to 6% in less than 3 months and similarly fasting insulin was reduced by up to 31% in the same timeframe.
May Reduce Risk of Cancer
Although there is not enough information to definitively link intermittent fasting and reduced cancer risk in humans, there have been several studies done on the topic, with results suggesting there may indeed be a link.
A study in animals showed a noticeable trend linking fasting diets and cancer prevention and treatment, suggesting the diet could help to reduce the chances of developing cancer as well as improving the chances of recovery. Similarly, a study done on humans suggests that short-term fasting could help improve cancer treatment.
Can You do Intermittent Fasting on a Keto Diet?
Yes, you can combine intermittent fasting and a ketogenic diet. In fact, this combination is popular since fasting can help you reach ketosis more quickly.
The goal of the keto diet is to enter into nutritional ketosis, a metabolic state in which your body burns fat instead of carbs. However, it can take some time and effort to get there. This is where intermittent fasting comes in.
Fasting helps accelerate this process by using up any stored glycogen in your body first, so that your body shifts into fat-burning mode more quickly. Additionally, intermittent fasting can help reduce cravings and increase energy levels, making it easier for people on a keto diet to stay on track with their nutrition goals.
A ketogenic diet also helps make intermittent fasting easier. This is because the high fat content of a keto diet is very satisfying and keeps you feeling full for longer. If you’re struggling to stick to intermittent fasting, a low-carb and high-fat diet can help avoid cravings and make fasting easier.
Pros & Cons of Fasting on Keto
Fasting isn’t right for everyone, just like a ketogenic diet. You should always check with your doctor or another medical professional before you start fasting, especially if you have a health condition.
Discover the pros and cons of keto fasting below.
Benefits of Combining Keto & Intermittent Fasting
Intermittent fasting accelerates the process of entering into nutritional ketosis by depleting glycogen stores first. This means your body reaches ketosis and begins burning fat as fuel more quickly than normal.
Benefits of intermittent fasting on keto include:
General Benefits of Intermittent Fasting
Intermittent fasting on a keto diet can still reap all the same benefits as intermittent fasting in general, including improved weight loss, lowered risk of diabetes and the potential to reduce your risk of cancer.
Improved Cognitive Function
Keto and intermittent fasting have known improvements on physical health, but there is evidence to suggest that it can improve your mental function as well. Dr Richard Isaacson, the director of the Alzheimer’s Prevention Clinic at Florida Atlantic University is currently studying the potential link between a low-carb, fasting diet and improving the mental capacity of those with Alzheimer’s.
Faster Weight Loss
When you combine both keto and intermittent fasting you may experience much faster weight loss. This is because intermittent fasting aids ketosis, as well as limiting the calories you’re consuming. A combination of intermittent fasting and a keto diet can be beneficial for those who have Type 2 Diabetes.
Helps Make Keto Lifestyle Easier
Many people find it difficult to maintain keto all the time, especially while out and about. Introducing intermittent fasting can actually help to resolve this problem. You’ll no longer need to worry about what and where to eat during your fasting windows, which can simplify sticking to keto when socialising or travelling. For instance, you can avoid wondering how to stay low carb when travelling by booking a flight during your fasting window.
Downsides of Combining Keto & Intermittent Fasting
It should also be noted that combining fasting and keto may not work for everyone. Not all intermittent fasting styles will be suitable for your individual needs, so if you’re not having any success, consider a different method or altering your fasting window.
You may find that combining keto and intermittent fasting isn’t ideal for you. It’s very possible to do keto without fasting, or to practise intermittent fasting without a low-carb diet. Ultimately, you should ensure the diet you choose helps to support your health and wellbeing.
In all cases you should consult with your medical professional before starting a new diet plan.
Not Suitable for All Health Conditions
Intermittent fasting is not recommended if you:
Are under the age of 18
Are pregnant, breastfeeding or trying to conceive
Have an eating disorder
Are an athlete or training intensively
Have certain medical conditions
Anyone in these categories should consult a medical professional before fasting to ensure it’s safe.
If you have Type 1 Diabetes a combined keto and intermittent fasting diet may not be healthy, as it could lead to Diabetic Ketoacidosis. This is where ketones can build up to a dangerous level.
For those with any kind of health condition, even if fasting is approved by your doctor, be sure not to overdo it; start slow with shorter fasts and work up from there if needed.
May Have Side Effects
It’s important to remember that everyone’s body responds differently to intermittent fasting, so it’s important to listen carefully to your own body as you try it out. Some people may struggle with fasting and find that they overeat in between fasts, while others report feeling moody or fatigued.
If you begin feeling lightheaded, dizzy, or experience extreme hunger pangs, discontinue the practice immediately and speak with your doctor about any health concerns.
It’s important not to push yourself if you’re feeling unwell, and seek medical advice before continuing with the practice long-term. Alternatively, it isn’t a requirement to fast on keto, so there’s no shame if it simply isn’t your cup of tea. You can still reach and maintain ketosis without fasting.
May Be Harder to Maintain
Another drawback is that you may struggle to maintain the lifestyle. Research has proven time and time again that when you start a very restrictive diet you are likely to gain back some, if not all of the weight. A combined keto and IF diet will make it very difficult to maintain a social life and you may find that you have to miss out on socialising.
One benefit of a ketogenic diet over caloric restriction for weight loss is that it’s easier to maintain. You can still eat a variety of wholesome, tasty foods on keto without going hungry, and even enjoy a glass of low carb wine!
However, adding intermittent fasting into the mix can introduce new obstacles. If the challenges of intermittent fasting are going to tip you off the keto wagon, it’s better to stick to what’s working for you.
What Types of Intermittent Fasting is Best for Keto?
There are different types of fasting protocol you can adopt, each with its own benefits and drawbacks. Let's take a closer look at the different types of intermittent fasting that can be used as part of a ketogenic diet.
They include Lean Gains, Warrior Diet, Alternate Day Fasts, 5:2 Diet and Extended Water Fasts.
16/8 Method (Lean Gains)
The 16/8 method is one of the most popular types of intermittent fasting. This type of fasting involves restricting your eating to 8 hours per day and abstaining from food for 16 hours. For example, you may choose to only eat between 12 pm and 8 pm every day, allowing yourself 16 hours without food in between meals.
This type of fasting is relatively easy to stick to, since it doesn't require any drastic changes to your lifestyle. You can compress your normal meal times into the eight hour period or skip breakfast and just have two meals a day.
This is best for beginners.
The 5:2 diet requires you to eat normally for five days per week, then restrict your calorie intake to 500-600 calories on two non-consecutive days (usually Mondays and Thursdays).
The 5:2 diet has been shown to be effective for weight loss and can also help improve insulin sensitivity in individuals with prediabetes or type 2 diabetes. You can achieve effective weight loss using this method but as you’re not entering a completely fasted state you may miss out on some of the other benefits.
Alternate Day Fast
The alternate day fast is another form of intermittent fasting that involves abstaining from food on alternating days. On fast days, you should not consume more than 500-600 calories per day; on regular days you can eat whatever you like within reason (no binge eating!).
Alternate day fasts have been shown to be effective for weight loss but can be difficult for some people due to the drastic nature of the diet change required by this type of fasting plan.
You’ll eat during a four day period and then fast for the remaining 20 hours. This method will help you achieve a deeper fasted state and is popular because you can save time and then binge on one big meal in the evening. Other followers of this method prefer to use it weekly or monthly.
Extended Water Fast
Extended water fasting is an extreme fasting method. During extended water fasting, all food and drinks other than water are eliminated from the diet for multiple days, sometimes up to a week.
While this type of fasting is cited to deliver rapid weight loss results, it can also be dangerous due to potential nutrient deficiencies and electrolyte imbalances caused by the lack of food. Additionally, it is definitely not recommended for people with certain medical conditions or a history of disordered eating.
While preliminary studies show some benefits to extended water fasts, this isn’t definitive in humans, and water fasting can be dangerous, especially if you have health conditions like diabetes. If you're considering an extended water fast, it's crucial you speak to a medical professional beforehand.
Keto Egg Fast
A ‘keto egg fast’ Involves eating only eggs, cheese and butter for 3 to 5 days. It’s easy to see why this method is popular with keto beginners who may struggle to put together well-rounded low-carb and high-fat meals.
The keto egg fast is one of several popular ‘fasts’ that aim to temporarily restrict which foods you eat, rather than the timing of your meals. While these aren’t technically intermittent fasting, they’ve become common as a way to ‘kick start’ ketosis.
While eggs are a great keto-friendly food, a keto egg fast restricts many food groups which are needed for optimal health, even on a ketogenic diet. Keto beginners will be especially susceptible to ‘keto flu’ side effects on a crash diet like this.
Let’s take a look at why intermittent fasting is beneficial for a keto diet and how to best go about it.
How to Start Intermittent Fasting on a Keto Diet: Top FAQs
We understand that you may have questions around how long you should fast, what you can eat, and many other topics. We’ve answered these in detail below to give you more information about fasting on keto.
How Can I Safely Fast on Keto?
When trying intermittent fasting for the first time, start slowly and listen closely to your body for signs that something isn't right. If you experience side effects or fasting isn’t working for you, consider changing to a different style of fasting, or altering what you eat during non-fasting periods.
Alternatively, fasting may not be the right choice for you. Be sure to consult with your doctor and a nutrition professional if you’re considering fasting, and if you experience side effects.
You’ll also need to ensure you’re getting enough nutrition, such as protein and healthy fats, during your non-fasting windows. This may mean investing in some quality keto-friendly protein powder to supercharge your meals and ensure you get enough nutrition to support your body through the fasting window.
Of course, you’ll need to limit your carbohydrates as well - if you’re not adhering to a ketogenic diet properly, fasting won’t help you maintain ketosis.
Finally, make sure you consume enough low-carb electrolytes while on a keto diet plus intermittent fasting. Electrolytes play an important role in maintaining proper hydration while following this type of eating plan, and help to reduce adverse effects.
How Long Should You Fast for Keto?
Everyone’s body is different, so it may take some trial and error to work out how long you need to fast for. There is growing evidence to suggest that shorter fasting windows - around 12 hours in length - can be even more effective than a 14 or 16 hour fast.
The most common form of intermittent fasting is 16/8 fasting, with an 8-hour eating window and a 16-hour fast daily. However, my own results vastly improved when I adopted a 12-hour intermittent fasting model instead, at the recommendation of Laurie at Body Transformation Studio.
Laurie, an expert in Metabolic Precision Nutrition, says that a longer fasting window can impede muscle repair and excessively increase cortisol, which impairs metabolic rate. When factoring in stress and a busy lifestyle, this means a 16-hour fasting can place excessive strain on the body and impede results.
This means that a longer fasting window may not be the secret to better health or overcoming a weight loss plateau. If you’ve found a 14 to 16 hour fast ineffective, a shorter fasting period may be the answer.
Ultimately, how long you fast for will need to factor in what’s right for your body. If you’re unsure, working with a professional can help you find the right balance for you. 
What’s the Difference Between ‘Clean’ and ‘Dirty’ Fasting?
A ‘clean’ fast only permits you to drink unflavoured water, ‘Dirty’ fasting means you can drink beverages that contain fewer than 50 calories, including those with zero-calorie sweeteners, as well as adding flavourings like lemon slices to your water.
Clean fasting means you can drink sparkling, still or mineral water, but you can’t add flavourings like lemon slices, fruits or herbs. Many fasting plans allow you to drink black coffee and tea because they will not trigger an insulin response. However, caffeine does act as a diuretic, which can dehydrate your body.
‘Dirty’ fasting allows a small number of calories, which can allow you to add a small amount of creamer or milk and sweetener to your coffee. Typically you’ll need to stick to under 100 calories, and avoid carbohydrates which will impact blood sugar.
Note that depriving yourself during strict fasting can exacerbate cravings and perpetuate a binge-fast cycle. If this is the case for you, you’ll definitely want to evaluate your eating patterns with the help of a professional before you continue fasting.
What Can You Have While Fasting on Keto?
Different fasting styles will have different restrictions on what you can eat or drink, and in what quantities. For most intermittent fasting styles, you should only have water when fasting, but some permit other drinks such as tea or coffee.
Sparkling or still water is fine, and you can add lemon or lime if you’re not fasting strictly. If you have to drink outside of water, then it’s best to opt for white, black or green tea, herbal teas or black coffee without any additives.
Opinions vary on whether low carb and zero-calorie sweeteners are okay when fasting.
How Do I Manage Hunger while Intermittent Fasting on a Keto Diet?
If you’re truly hungry and have a physical need to eat, you should listen to your body and never make yourself unwell when fasting. However, struggling with hunger during intermittent fasting can indicate you need to eat more protein and fats during your non-fasting window.
This is one reason that keto and intermittent fasting complement each other - a high-fat diet will help give your body long-lasting energy and satiety. Compared to a carb-heavy diet, getting quality food will stabilise your blood sugar levels and make fasting easier.
Getting enough sleep, lowering stress and drinking less alcohol reduces your appetite so you’ll find a fast much easier. While it might be tempting to indulge in some low-carb wine, strict fasting does require you to avoid alcohol as well, and you may experience more side effects if drinking while fasting.
Maintaining hydration and electrolyte levels will also help you avoid feeling hungry. Depending on what style of fasting you choose, drinks such as tea and coffee may be permitted, but others will require you to stick to water or other zero-calorie drinks, like sugar-free electrolytes.
You shouldn’t push yourself if you’re feeling dizzy or ill, so tips like distraction or sipping water are only intended to help with the mental challenge of fasting. It’s also important to pay attention to your mental and emotional wellbeing when fasting, so persistent cravings or binge cycles are a red flag to look out for.
If you’re struggling with hunger when fasting, don’t assume this is normal - reach out to your doctor or other medical professional for support.
How Can I Break a Longer Fast on Keto?
It’s best to eat proteins and healthy fats when you’re easing yourself back into eating after a fast. If you’ve been on a longer fast, it’s especially important to avoid carbohydrates afterwards, because your insulin levels would have decreased while your sensitivity has increased.
You should avoid eating an especially large and heavy meal after fasting, though, even if you’re hungry. This is because overdoing it can lead to bloating and could cause stomach pain. Some people don’t have this issue, so feel free to break your fast with a big meal if it doesn’t cause you any digestive problems!
If you find it difficult to stomach a keto bacon-and-eggs breakfast after fasting, some lighter alternatives include:
Plain Greek yoghurt with low carb toppings, such as nuts, seeds and berries
Keto friendly protein shakes or smoothies with greens
Low-carb protein bars or granola bars
Low-carb bread topped with avocado and feta cheese
Can You Work Out While Intermittent Fasting?
Yes, you can still exercise while you're on an intermittent fast as long as you’re feeling well. You just need to do so safely. When you work out, consider the timing; for example, you could choose to work out just before, during or after the window when you can eat.
Some people prefer to work out on an empty stomach so working out before you can eat will be more suitable whereas others like to exercise after they’ve refuelled.
You need to pay attention to the nutrients you consumed the day before you worked out, for example if you’re doing strength training then you’ll need more carbs the day before you work out whereas that’s not the case with cardio or high intensity interval training (HIIT).
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