Intermittent Fasting & Keto: The Ultimate Guide

Intermittent Fasting & Keto: The Ultimate Guide

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Intermittent Fasting & Keto: The Ultimate Guide

Posted 4 years ago

Luke Jones

Luke Jones


Tony O’Neill, PT, DPT, MSc, RDN

Tony O’Neill, PT, DPT, MSc, RDN

Author and Scientific Reviewer

Expert Approved

After a quick glance at the headlines on most health-related sites, you’d no doubt notice that intermittent fasting (IF) and the Ketogenic diet are two of the most popular nutrition trends around right now.

Both are commonly used by health-conscious consumers looking to manage their weight and support their general wellbeing, and they each have a solid body of research behind them as standalone dietary interventions.

Nevertheless, you may be wondering whether it's effective (and safe) to practice both at the same time...

In this article, we take a deep dive into all things IF and Keto, exploring how they work, their potential benefits, and how to successfully combine the two. 

Keto Diet 101

Without going into the nitty-gritty details, ketosis is a metabolic state in which the body shifts its primary fuel source from using glycogen (stored carbohydrates) to burning fat and ketone bodies for energy. 

This metabolic shift occurs in response to a decrease in carbohydrate intake, which can be triggered by fasting (not eating for a prolonged time period), or by following a Ketogenic diet - commonly referred to as Keto. 

Keto is a low-carb, high-fat, moderate-protein diet that has seen a resurgence in recent years due to a number of potential benefits.   

Learn more: Ultimate Guide to the Keto Diet

3 Science-Backed Benefits of a Keto Diet

1. Enhanced Weight Loss

Weight loss is one of the primary reasons people turn to Keto. Indeed, studies have shown that following a high-fat diet can result in a reduced appetite, helping you stay fuller for longer [*]. 

Research has also shown that Keto can result in faster initial weight loss when compared to high carb diets, although this may be partly due to a loss of water and glycogen stores [*]. 

Interestingly, Keto has been associated with increased weight loss around the midsection when compared to other dietary interventions. In addition to aesthetic concerns, visceral belly fat is linked with inflammation and chronic disease, so keeping it at a minimum is ideal [*]. 

2. Epilepsy & Neurological Disorders

Keto was first used in a clinical setting as a supporting treatment for epilepsy [*]. The diet has since shown promise with other neurological conditions like Parkinson’s and Alzheimer’s [*], with animal studies suggesting that Keto may help to slow cognitive decline [*], although more research is needed.

3. Improved Blood Markers

Aside from body composition, many people turn to Keto in an attempt to help manage various health conditions. 

Studies show that there may indeed be some benefits related to heart health, with Keto potentially leading to a dramatic reduction in triglycerides [*] and an increase in HDL (good cholesterol)[*].

A lower carb intake and more moderate protein consumption may also result in more stable blood sugar levels [*]. One study showed that after following a Keto diet for 6 months, 95% of participants with diabetes were able to reduce or completely eliminate any glucose-lowering medication [*].

Intermittent Fasting: The Basics

There are many ways to practice intermittent fasting (IF for short), but the basic premise is simple:

It starts with separating the day or week into fasting and eating windows. You then simply shorten your feeding window, and extend the window in which you consume little to no food..

Unless you’re prone to a midnight snack, you already practice a short period of fasting overnight between dinner and breakfast. 

The idea is that extending that fasted period on certain days of the week can bring about some interesting benefits. This could be as simple as using a 16-hour fasting window each day or opting for longer fasts once every week or two. One of the big plus points for fasting is that it’s easy to customize to your lifestyle. 

Although IF may seem like a relatively new dietary trend, fasting is something that human beings have practiced for thousands of years. 

Like many other predaceous animals, early humans would have naturally gone without food for prolonged time periods in between successful hunts. While this might seem less than ideal at first glance, natural periods of fasting would have likely put these early humans into ketosis. This may have imparted benefits like appetite reduction and mental clarity, which could have actually assisted them in getting through to their next meal and enjoying another successful hunt.

And it wasn’t just hungry hunter-gatherers who used IF. There’s evidence of intentional fasting being practiced for spiritual and health reasons as early as the ancient Greeks. 

7 Potential Benefits of Intermittent Fasting

1. Weight Loss

Like Ketoketo, IF is widely used to help decrease fat mass and increase lean body mass. As the body switches to using fat as a fuel source, studies have demonstrated that fasting can lead to a decrease in belly fat without having to restrict calories, [*] [*]

There’s also evidence that IF can increase fat loss through an increase in fat oxidation (burning fat as a fuel source) and an overall decrease in appetite [*]. 

For many people, this could prove to be a much more sustainable approach to weight management compared to a typical calorie-controlled diet.   

2. Longevity

After approximately 24 hours of consuming no food, it’s thought that a bodily process known as autophagy kicks into high gear [*]. 

This is essentially your body's way of performing cellular housekeeping - cleaning out old, damaged cells in the muscles and organs, which can then be replaced by healthy, new cells. While the mechanism is still not fully understood, many scientists believe autophagy is a key process when it comes to longevity. 

As you may have figured out by now, our standard eating window means we rarely reach that 24-hour threshold, so we could be missing out on some of these benefits. Indeed, one animal study showed that subjects who followed an intermittent fasting protocol lived on average 33% longer than those who didn’t fast [*]. 

3. Reduced Inflammation 

Chronic inflammation has been called by many top health professionals ‘the root of all disease’. The good news is that intermittent fasting shows promise when it comes to reducing blood biomarkers linked with inflammation, such as CRP [*]. 

4. Cognitive Function

Animal studies show that fasting can lead to the generation of new neural cells and an increase in Brain-Derived Neurotrophic Factor (BDNF) expression - a protein that’s been described as “miracle-grow for your brain'' [*].

5. Heart Health

Heart disease is currently the number one killer in the western world. There’s promising research when it comes to fasting and markers of cardiovascular health. A study of alternate day fasting showed a decrease in cholesterol, LDL cholesterol, and triacylglycerol concentrations after an 8-week period - similar in some respects to results seen with Keto [*]. 

6. Cancer

Animal studies suggest that alternate-day fasting may have a protective effect against cancer [*]. This may in part be due to the process of autophagy that we touched on above - clearing out old, damaged cells and making way for new, healthy ones.

Additional preliminary research is beginning to point to the protective effects of fasting during chemotherapy and radiotherapy, as healthy cells are protecting from harm in a fasted state while cancer cells lack the flexibility and actually become more sensitive to the treatments. Research is currently being done to determine the safest ways to provide these benefits of fasting to individuals undergoing treatment for different cancers. [*] [*] [*]

7. Insulin resistance

A 2014 study showed that intermittent fasting was effective in reducing visceral fat mass, fasting insulin, and insulin resistance in type 2 diabetics [*]. 

Combining Keto & Intermittent Fasting: Benefits + Challenges

Although they may seem different on the surface, Keto and IF share many similarities. They both encourage the body into a Ketogenic state, and have the potential to increase fat burning. There’s also a lot of crossover between the potential wellness benefits, from heart health to insulin sensitivity. 

You may be wondering whether it's safe and effective to combine the two -- extending your fasting window, and consuming a high-fat diet during your eating window. 

The good news is that because both diets work via similar underlying mechanisms, they have the potential to complement each other. Keto and IF act synergistically towards the same end goal - making the body more adept at burning fat as a fuel. You might also find that the common challenges of each diet are canceled out when you combine them together: 

  • Restricting food intake through IF may make it easier to get into and stay in ketosis, as this kick-starts the depletion of carbohydrate levels. 
  • A Ketogenic diet may make intermittent fasting feel easier as your body becomes more fat-adapted, and you feel more satiated from consuming higher-fat foods. 
  • If you’re fairly new to Keto, you may find that IF helps you stick to the diet, as you have fewer meals to think about and prep.

That being said, there are some potential drawbacks to  be aware of:

  • You will likely find it challenging if you are brand new to both Keto and fasting, as it's a big shift from the norm. You may be better off starting with one thing at a time. 
  • Fasting is not suitable for pregnant women, children, and people with eating disorders.

Which Fasting Protocol is Right For You?

There are numerous fasting protocols to choose from, each with their individual benefits and drawbacks. Here are some of the most popular: 

Lean Gains - 16:8

As the name suggests, the 16:8 fast involves a 16 hour fasted window, followed by an 8-hour feed. This can be achieved by compressing your normal mealtimes, or simply by skipping breakfast and having two meals a day. It’s a great introductory protocol suitable for both beginners and more experienced fasters, and can be performed on a daily basis. 

Warrior Diet / OMAD - 20:4 

The Warrior Diet or One Meal a Day protocol involves fasting for 20 hours of the day. It’s a popular choice after adjusting to the 16:8 protocol. Aside from helping you get deeper into a fasted state, this protocol is popular because it can save time and allows you to binge on one big meal in the evening. Some practice OMAD daily, but others prefer using it on a weekly or monthly basis. 

Alternate Day Fasts

Alternate day fasts are one of the most commonly studied protocols in the fasting literature. As the name implies, it involves a day of eating, followed by a day of no food, or a day of reduced calories. Alternate day fasts are often used by people seeking significant weight loss, or to help manage a health condition. Because the fasting period is typically around 24 hours or longer, autophagy should kick in, meaning you may get some of the proposed longevity benefits of fasting. 

5:2 Diet

The 5:2 diet is less of a true fast, and more of a “Fasting Mimicking Diet”. It involves five days of normal eating, followed by 2 days eating less than 500 kcal. This can be effective for weight loss, but because you do not enter a completely fasted state, may not provide some of the other potential benefits of fasting. 

Extended Water Fasts

Water fasts of 24+ hours are typically used in an attempt to manage or combat long term illnesses like autoimmune conditions, or for significant weight loss. Long fasts are not suitable for everyone, and should only be completed in a supervised medical setting. 

How to Supercharge Your Keto Diet With Intermittent Fasting

If you're interested in merging both dietary protocols, below you’ll find a sample daily schedule combining Keto with 16:8 (the most beginner-friendly form of fasting).

Day 1: 

8:00 PM - Dinner - Keto Alfredo Chicken Bake

10:30 PM - Bed

Day 2:

7:00 AM - Wake, Meditate, Walk or Yoga

12:15 PM - Breakfast - Low Carb Overnight Chia Almond Pudding

4:00 PM - Lunch - Keto Lemon Chicken Meal Prep

6:00 PM - Evening Workout

8:00 PM - Dinner - Keto Turkey Chili

After day 2, simply repeat this daily schedule, following the same approximate eating and fasting windows, and substituting in whatever Keto-friendly meals appeal to you.

Intermittent Fasting Tips & FAQs

How long should I fast for? 

If you are completely new to fasting, start with 16:8, or even 14:10. You may then occasionally start to sprinkle in longer fasts, but listen to your body (and your doctor). 

Can I drink liquids during a fast?

Yes, no-calorie liquids like water, unsweetened and milk-free tea and coffee are allowed on most fasting protocols.

Can I take supplements while fasting?

You can take supplements when fasting, but be aware that certain fat-soluble vitamins may not be absorbed as well by the body compared to having them with food. To combat this conundrum, you could potentially shift your supplement-taking regime, taking your usual morning supplements with your noontime meal instead (or whenever you break your fast).

Is fasting suitable for everyone? 

No. Fasting is not suitable for pregnant and lactating women, children, people with a history of eating disorders, and those with certain health conditions. If you have a chronic health condition or you're otherwise concerned about whether or not fasting is right for you, please consult a medical professional before attempting your first fast. 

Can I work out while fasting? 

Yes, many people enjoy training when in a fasted state, although you may need to reduce the intensity of your workouts if you are new to fasting or completing an extended fast. 

How can I manage hunger while fasting?

It's completely normal to experience hunger pangs in waves. Although this can feel uncomfortable at first, drinking some water or tea and waiting it out will often result in the wave passing. 

Will fasting slow down my metabolism? 

No. While fasts longer than 3 days can reduce metabolism [*], shorter fasts have actually been shown to increase metabolism [*]. 


Both Keto and intermittent fasting have a wide range of potential benefits, and there is an extensive amount of crossover between the two. For most people, they can be safely and effectively combined. Just remember to ease in and listen to your body.

If you need further guidance to help determine your ideal fasting protocol, check out our handy guide to choosing your IF schedule. To help you stay on course you can also use a diet tracking app like Carb Manager to record your health metrics, build your weekly meal plans, manage your custom intermittent fasting program and much more.

If you experiment with them both, let us know how you get on in the comments section below, and feel free to reach out with any questions. 

Please note that as with any significant diet or lifestyle change, we recommend working alongside a registered health professional, especially if you are currently on any prescribed medications or undergoing any medical treatments. Fasting is not appropriate for everyone and should be avoided by those with a history of eating disorders, pregnant and breastfeeding women, and anyone under 18. 

Comments 27

  • SuperMacadamia328813

    SuperMacadamia328813 13 days ago

    Thank you for this article. Very informative.

    • RemarkableCauliflower290494

      RemarkableCauliflower290494 3 months ago

      Very informative. I am a beginner IF can I take MCT oil in my coffee before I break my Fast or after?

      • SpectacularKale492484

        SpectacularKale492484 4 months ago

        It's all a bit confusing, isn't it? The idea of snacks on a Keto diet is counter intuitive when you get into fasting and Keto eating which removes all cravings.

        • Nanomaus

          Nanomaus 5 months ago

          Practising IF Version One meal a day together with Keto since one month. 8KG lost (17.5 lbs) and feeling more energetic and mentally fit at the same time.

          • UpbeatArugula256659

            UpbeatArugula256659 9 months ago

            Great information

            • GorgeousRadish317224

              GorgeousRadish317224 10 months ago

              IQMIX Sugar free drink mix is this allowed during fasting. It does have electrolytes and that’s what I was concerned with.

              • WellnessWarrior

                WellnessWarrior 7 months ago

                Anything more than 10 calories will break a fast

            • StupendousKale398028

              StupendousKale398028 a year ago

              Can you have a coffee in the morning during the 18:8

              • Swinginamiss

                Swinginamiss a year ago

                Great article, a lot of the information stated about fasting's IF's and EF's, I researched for myself about 3 years ago and I am happy to see that the information shared here has not changed at all, with the exception that Fasting is being studied more by the medical field, which is a plus. SAM

                • Justwants2travel

                  Justwants2travel a year ago

                  I think I know the answer to this but if I try this, does that mean no Crystal Light or Propel water? Those are my two go-to's for liquid. Thanks.

                  • Col-lean

                    Col-lean a year ago

                    Yes, sadly, They are a no-no. You'll adapt! After a while they will taste sickenly sweet and you will prefer water.

                • ChiroGirl32177

                  ChiroGirl32177 a year ago

                  i’ve been doing the 16:8 with a window of 1-9 5 days a week

                  • FortuitousKetone302805

                    FortuitousKetone302805 a year ago

                    I have found the 16:8 fasting protocol to be very doable.

                    • Ambler007

                      Ambler007 a year ago

                      I have been following this program for the past month Keto & 16:8 fasting. Love it, so much energy! This is my new lifestyle, not a diet!

                      • FabulousCauliflower322959

                        FabulousCauliflower322959 2 years ago

                        I’m doing 16/8 today . At 1:45 noon I will be eating chicken bites homemade with broccoli. & at 5:30 pm I’ll be eating a salad at 8:00 at night I end it

                        • Keto4Lif3

                          Keto4Lif3 2 years ago


                          • RuthHazel

                            RuthHazel 2 years ago

                            Great info, thank you!😊 I’m doing 16-8 with my first meal at noon, dinner at 6 p.m. and nothing after 8 p.m. Can I use 2-tsps of sugar free Coffeemate and 2 packets of Splenda with my morning coffee and remain in ketosis or does that break my fast?

                            • Leslie1952

                              Leslie1952 2 years ago

                              This is not a clean fast and will break the fast.

                            • Martin7625

                              Martin7625 2 years ago

                              For keto I found the Splenda monk fruit is 0/0/0 and has no bad taste.

                            • UpbeatKetone873445

                              UpbeatKetone873445 2 years ago

                              Replace the chemicals with Stevia which is plant based. Splenda and Aspartame should be avoided not because it would break the fast but because of other health issues.

                          • Rick

                            Rick 2 years ago

                            Can I drink diet coke while fasting?

                            • Col-lean

                              Col-lean a year ago

                              Sadly, no. The artificial sweetener tricks your brain into "preparing" you body to process sugar and starts a whole process that is counteractive to the benefits of fasting. That's why we as a society haven't become skinny over the last several decades drinking no. Cal. Bevys!

                            • Leslie1952

                              Leslie1952 2 years ago

                              No. Nothing flavored. It tricks your body into releasing insulin

                            • UnbelievableCauliflower321738

                              UnbelievableCauliflower321738 2 years ago

                              the sweeteners in diet coke are poison

                          • Littlerosey902

                            Littlerosey902 2 years ago

                            Can I have unsweetened almond milk in my tea during my fast

                            • Leslie1952

                              Leslie1952 2 years ago

                              No. Breaks the fast

                          • deb7805

                            deb7805 2 years ago

                            Do I consume my regular macros during my eating window if doing 16:8