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Intermittent Fasting on a Keto Diet

Intermittent Fasting on a Keto Diet

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Intermittent Fasting, or “IF”, is a relatively new craze that is used as a supplement to your diet. It revolves around the timing of your food intake, and can have some benefits in the long run. There are quite a few people misinformed on fasting, so we’ll clear that up and explain how intermittent fasting can be useful.

I want to be clear that intermittent fasting should be used for CONVENIENCE only, not for better weight loss results. There are no studies on intermittent fasting that show clear weight loss benefits, but there are studies that show health benefits. We’ll get into those later. The thing that matters the most is HITTING YOUR MACROS. If you need to learn how to calculate your macros, visit our Keto Calculator.

Restricting yourself even more than you are is pointless – it’s not worth it if it makes you unhappy. Most people are fasting because of the few health benefits and the major convenience it has for their schedules. It has been reported (no studies backing it up) that it can help you if you are in a plateu – and I am talking about a plateu of 3 weeks or longer.

There’s 2 basic terms we need to understand here: feeding, and fasting. You’re in a feeding state when you are eating your food (naturally), and you are in a fasting state when you are between your meals.

The Approach

There are a few approaches when it comes to intermittent fasting.

  • Skipped meals. This is when you skip over a meal to induce extra time of fasting. Usually people choose breakfast, but others prefer to skip lunch.
  • Eating windows. Usually this condenses your entire macronutrient intake between a 4 and 7 hour window. The rest of the time you are in a fasting state.
  • 24-48 hour cleanse. This is where you go into extended fasting periods, and do not eat for 1-2 days.
Skipping breakfast with fasting

I don’t recommend that you go straight for a 1-2 day fast, but begin by restricting yourself to certain eating windows. Typically people restrict themselves to the hours of 5pm – 11pm. People often refer to their fasting windows by numbers: 19/5 or 21/3, for example, means 19 hours of fasting and 5 hours eating or 21 hours fasting and 3 hours eating, respectively.

Once you have the hang of eating on a schedule, you can try short periods of 18-24 hour fasting. Then you can judge if intermittent fasting is for you.

Whether you decide to do it every day, once a week, or twice a week is up to you – do what makes you feel best and listen to your body.

How does it work?

Fasting can help with overeating and calorie counting
The whole point of intermittent fasting is to allow ourselves to increase the amount of food we can intake at one time. Our bodies naturally can only take in a certain amount of food at once, so we are creating a sort of limit on our calorie intake.

This is also a great method for people that overeat. I tend to see people that forget to count the snacks that they have throughout the day, and wonder why they are putting weight on.

Your body will adjust itself to fasting, and you will find yourself not as hungry as you used to be. This allows you to properly record and maintain their nutrient intakes.

In this fasting state, our bodies can break down extra fat that’s stored for the energy it needs. When we’re in ketosis, our body already mimics a fasting state, being that we have little to no glucose in our bloodstream, so we use the fats in our bodies as energy.

Intermittent fasting is using the same reasoning – instead of using the fats we are eating to gain energy, we are using our stored fat. That being said, you might think it’s great – you can just fast and lose more weight. You have to take into account that later on, you will need to eat extra fat in order to hit your daily macros (the most important thing). If you’re overeating on the fats here, you will store the excess anyway.

Like I said, it’s more for the convenience for timing, rather than the overall weight loss effects that it may have. There are other benefits, though, and we’ll discuss these too.

Benefits of Intermittent Fasting

There are a number of benefits shown that come from intermittent fasting. Some of these include blood lipid levels, longevity, cancer, and testosterone levels. The main two I want to focus on are the basis of our every day lives.

Fitness

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People are always saying that if you don’t utilize the importance of pre and post workout meals, you are going to be losing muscle when you work out. This isn’t necessarily true, and it is even less so when you are adapted to ketosis.

Fasting while training can lead to a number of benefits in the long run, including:

  • Higher metabolic adaptationsStudies show that your training performance will increase in the long run when you are exercising in a fasted state.
  • Improved muscle synthesis – Studies show that muscle gains are heightened when you train in a fasted state, and utilize proper nutrient intake.
  • Improved response to post-workout meals – Studies show that the speedy absorption of nutrients after a fasted workout can lead to better results.

There’s been numerous studies on fasting while training, including one on Muslim athletes during Ramadan. It concluded that there is no effect on performance in training while fasting – so you have no need to worry.

Mental Clarity

Once your body is keto-adapted, your brain can effectively run on ketones, which are derived from fat breakdown in the liver. Fat is considered one of the most energy-efficient fuels for your body to run on, and your brain is a huge consumer of energy.

Most of the high carb advocates are running a campaign on the deprivation your body gets when you do not continuously refuel on grains and fruits.

Gain focus through fasting
They want you to be carrying around an apple and a granola bar wherever you go, but the beauty with keto is that you don’t.

Even if your body is out of glycogen (which it most likely is if you’re in ketosis), it can rely on the abundance of fat from the foods you eat and stores you have. That means your powerhouse of a brain can run full power all the time. Less mental fogginess, and more focus.

As you get used to fasting, you should start to fast naturally. Meaning, only eat when you’re hungry. Don’t plan your fasting – let it happen naturally.

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39 comments

  1. Do you do intermittent fasting? Let me know your experiences :)

    • I have been intermittent fasting for a few months now, longer than being keto-adapted, because I feel better eating this way. I wake up fasted and do cardio exercise, typically a thirty minute run and a 1km swim. I then stay fasting until about 16:00 in the afternoon when I drink a spirulina and supergreen concoction to set me up. My ‘rule’ is that I then eat when I’m hungry, be it immediately or a couple of hours later, for maybe a six hour window. I eat a large meal for dinner and that is usually it until the next day. Being keto-adapted takes precedence over fasting so on the occassional day that I need to eat, for whatever reason, I do so as long as it is on my low-carb terms. The point in fasting is to feel better and for it to compliment my life; if my life gets in the way (going out for lunch or weight training earlier in the day rather than the evening) then the fasting takes a day off. My main concern is your comment that it is important to get all my macros in. I have started tracking my macros more carefully and realised that I am probably skirting the bottom values of my calorie requirements. This is not intentional – I eat when hungry and I can eat like a beast at times. Most of the time though I can’t squeeze everything in during the window without feeling sick. How desperate are my macros as long as they stay in proportion and over my minimum calorie threshold? I am assuming that if I haven’t consumed enough then my body will scream for energy; as long as I am feeling strong and energetic should I just go with it? Doesn’t it mean that I will just burn through bodyfat quicker and then need to eat more according to my energy needs?

      • Just by reading that some days you don’t eat much and others you eat like a beast would make me say that it’s fine. The human body doesn’t use calories on a clock like we thing – it may not use all the calories you eat over a 24 hour period – it can roll over. That means that those days you can eat like a beast – your body is calling out for it. The body has natural caloric cycles (which is also messed up for many people, but it doesn’t sound so for you) which means over time you are most likely balancing out on your calories anyway. I wouldn’t worry about it too much honestly – long as you’re fat adapted :)

  2. Are BPCs allowed during fasting or is water only allowed?

    • BPC is allowed as a replacement breakfast, as it’s a liquid and just to get a slight start to the metabolism. I wouldn’t suggest drinking more than 1 though, if you need more coffee just make it a regular – black.

  3. Craig, is it necessary to wit until fully keto adapted to do intermittent fasting (16/8) or can we start just being in ketosis?

  4. Hey there if I run first thing in the morning – run for 60 min then some weight lifting. Do you recommend Tasty Bpf prior???

    What is a sample of your day while fasting?

    • Yeah you could drink BPC prior. Normally you can run fasting, you might want to drink the BPC prior to lifting. My 30 Day Meal plan breaks down how I eat right now (or a similar example of how I eat). You can check it out if you’d like.

  5. Hey Craig I am a small person (105lbs 5’3″) and I’m in the Keto world for the health benefits and mental clarity. I don’t really get hungry anymore but I just recently got over the headaches, nausea and fatigue. I had the BPC and then waited the 12 hours and had a spinach salad with roast beef, ham, vinaigrette, hummus wrapped in cheese (if you haven’t tried this it’s delicious, just load hummus onto a slice of cheese, roll it up, and noms).

    The problem was that I got really weak during the hours of not eating (without being hungry or anything, just weak) even though I was drinking water almost non stop. I don’t want to give up because I also felt a huge burst of energy when I ate again and more mental clarity than ever before on this diet.

    What can I do to stop the weakness without having to eat?

    • Ainsley, how long have you been doing IF? You need time to adapt to it. If you are not fully fat adapted then it might not be advised to do IF since you will get weak. I can’t think of anything that would cause this besides not being fat adapted or because you’re body is still adjusting.

  6. Hi, Can you tell me when i fast should i still eat my full days calories in the time slot for eating? It seems a lot to cram in over 3 or 5 hours. Or have i missed something somewhere. Thanks this really is an awesome site.

    • Hey Rebecca, yeah you have to eat all your days food in those few hour window. Usually it’s not as hard as it seems honestly. I’ll do it in 1 small meal to break my fast and 2 bigger meals in my window.

  7. Hi Craig,

    Been trying to find a straight answer does IF help with fat loss? You mention in this article that “there are no studies on intermittent fasting that show clear weight loss benefits” so does that mean you feel there are no benefits? In your 30 day guide and IF on Keto diet articles you mention it can be used to help with plateau’s and help you burn your own fat stores instead of the fat you get from food this would tell me there are benefits? If you are IF and burning your own body fat that would have to be benefits as long as you don’t regain the fat by over feeding. Let me know what you think. Thanks.

    • Les, there are no actual studies that show it helps with fat loss. You shouldn’t use it for fat loss, either. Keto alone will burn bodyfat, and in combination with IF it might have a slight advantage but I certainly wouldn’t use it if your intent is fat loss. Like I said in the article, only use it for convenience sake over anything.

  8. Hi Craig,

    First of all, what a wonderful site man. Great job!

    I am new to Keto but have been doing IF for about 5-6 months. Recently started following the 16/8 leangains protocol having previously followed the 20/4 protocol which led to weight gain… I have not lost weight even after cutting my calories down significantly and that’s why I am trying to incorporate a ketogenic way of life to get ripped. My stats are – (F/35/143lbs).
    I just have a couple of questions.
    1. Would it be wise to still do my morning fasted training and break my fast with BPC around midday then just eat one meal around 6pm? I take amino acids after my workout leading to my lunch.
    2. How much calorie deficit can I cut down to to lose more body fat? Currently eating 1800-2000 calories (pure keto – 65% fats, 35% protein & 5% dietary carbs)

    Appreciate your wisdom man! Thanks.
    Tahlita

    • Hey Tahlita,

      Everything sounds good to me but I suggest going lower on the protein. You should only need around 1g protein per pound of lean mass to get proper muscle growth during workouts (I link the studies in some of the workout articles). The BPC sounds fine to me though, you just want to cut back on the protein a bit. Too much protein will just convert over to glucose and put you out of ketosis – keep your eye on that so you can become properly fat adapted.

      As for your caloric deficit, it kind of depends how much you’re working out. 2000 sounds good but I can’t always tell you if that IS actually good, because I don’t know what you’re doing in the gym and whatnot.

      Hope that helps and thanks!

      • From what I’ve heard, the guideline is 0.8 to 1g protein per KILOGRAM lean mass. For a 143# woman, I would estimate lean mass of no more than 130# (60kg) and it’s actually probably significantly less unless Tahlita is a highly trained athlete. This would mean a protein intake of 48-60 grams should suffice.

        I personally don’t think of my keto diet in terms of % calories from each macro, I simply add up the grams of protein (I keep mine to 65 grams max), and a max of 20g carbs. The rest is fat.

        I take no responsibility for the 0.8 to 1g/kg number; that comes from other researchers.

        • For most sedentary people, 0.6 – 0.8g of protein per lean POUND of lean body mass is just fine for muscle conservation during sedentary lifestyles. But, if you are working out, you will need to intake a bit more protein. The optimal levels (from over 30 years of research on bodybuilding and muscle growth) is around 0.8 – 1.1g protein per lean POUND of body mass.

          While I do like to tell people that keeping track of grams is more important, some people have it set in their mind that needing a specific ratio is necessary too. While that does mathematically normally fall within the ranges given, I do agree with you that grams are most important to track.

    • You might want to reconsider 5% calories from carbs. On a 2,000 calorie diet, this comes to 100g carbs which may keep you out of ketosis.

      • Jon, this means 5% calories from carbs. So if you break down 2000 calories into 5%, that is 100 calories in total. Each carbohydrate is 4 calories each, meaning that 5% calories from carbs on a 2,000 calorie diet would be 25g total carb. Hope that helps.

  9. Craig:

    A combination of intermittent fasting, being ketogenic, the Paleo diet & exercise saved my life when I was young I was a Determined but not particularly good, Golden Gloves boxer. after my short, painful “career” as a boxer I took up long distance running, 5-19 miles 4 days a week & floor exercises.

    I work in the music business & kept this up til I was about 31. At that point the sex, drugs, & rock & roll got to me, I quit working out, my diet went to hell & I was drinking , smoking & doing drugs. The drugs ended in the late 80′s but everything else continued until I was 57.

    At that point I weighed 223 pounds – as a boxer I weighed 135 at 5’8 -& I was diagnosed with extreme high blood pressure, cholesterol level of 340, diabetes type 2 & a liver enzyme count that was heading straight for cancer.

    Luckily for me I spent my formative years on hippie communes in the 60′s & I picked up a lot if info on herbal medicine $ diet. When I was confronted by the vast array of meds they were going to put me on, I refused them, did some research, went on the Paleo diet. & started studying & taking Ayurvedic herbal medicine instead of meds. 3 years after that I started intermittent fasting.

    I’ve now been IFing for 4 years. I eat once every 20-24 hours & am totally fat adapted. I can go without eating 29+ hours & then do an intense workout of a couple of thousand crunches, 500 squats & I rotate lifting 30-50 pound dumbbells or boxing 10 hard rounds on the heavy bag.

    Too make a long story much shorter, today I’m 64, weigh 141 pounds- what I weighed at 20 – I now have low blood pressure,, my cholesterol level is 177, my diabetes type 2 has been in remission for 5 years & my liver enzymes are completely normal.

    The point of writing all this is to emphasize how correct Craig is about intermittent fasting, it has worked wonders for me. LastlyI want to point out that it is NEVER too late to start repairing your health. & I believe intermittent fasting, the Paleo diet & exercise are the keys to aging healthily & not becoming a diseased invalid just because your getting older.

    regards,

    Stephen Gordon

  10. Correction on my post, I ran 10 miles not 19 & I don’t go 29 hours before working out its 20-24 hrs. Those were typos.

    Stephen

    • Stephen,

      That is an amazing story! I’m really glad to hear you have everything under control at your age and seem to be in the best shape you’ve been in the last 40 years. Good on you for taking control, choosing what you eat, and doing what’s best for you.

  11. I appreciate the kind words, Craig but it really isn’t amazing.It was a matter of being confronted with dire physical straits & when a doctor tells you your going to die if you dont do what he tells you it catches your attention. I was lucky enough to have enough of a herbal/health/diet/exercise background that I had a foundation of knowledge to start from. Plus I do believe we are our own best doctors if we listen to our bodies.

    But most people have been brainwashed all their lives about Big Pharma/Agriculture & dont realize the damage thati is being done by our western way of life & diet. & thats why sites like yours are so valuable for people. You show people another, much healthier path to follow.

    That is invaluable because the knowledge you share can save lives if people are willing to change their ways.

    Lastly, I want to be clear that i didnt get to this point overnight. It took years to repair the damage I had caused to myself. It was not a crash diet that took off the weight it was a much longer, healthier & effective process. The key is to not get discouraged & to keep plugging away & after awhile Intermittent fasting, ketogenic paleo, autophagy & exercise just become your routine way of life. At thus point I cant ever imagine going back to 3 squares a day & eating carbs other than fruit & veggies.

    & I know if a fat, lazy, 57 year old like myself can turn it around, ANYBODY can do it…

    • Steph,

      Even if you say it’s not amazing it still really is. It takes quite a lot of motivation, encouragement, and support for most to do this way of eating/way of life and that’s no small task. I’m really happy to have heard your story (and even more happy to hear your successes of it). I do agree that is takes a lot of time to repair damage, but if you’re willing to stick to it, a much healthier you can come out the other end.

      Really appreciate the kind words :)

      My best!

  12. I IF almost every day (the only exception being for an event or if I have company) and I feel pretty great doing it. I follow a HFLC diet as well, so I suspect that I am in nearly a constant state of ketosis (unless my meal contains large amounts of protein all at once). From New Years to the end of April I have lost 33 pounds without exercising much, although I plan to change that now in order to gain strength. So far, my strength doesn’t seem to have diminished. I definitely notice an increase in cognitive function, as well as the willingness to physically do things more often.

    IF has also been really beneficial to my college schedule, because I am usually in classes until about 2:30 every day; I will typically wake up, do homework, go to class, and eat between 2:30 and 5:30. Then I will fast again until the next day. I drink no coffee, and my A+ grades get curved up above 100% because my classmates are struggling. Probably all the bagels and toast they are eating for breakfast!

    • Haha, probably! Glad to hear IF is going so well for you and that you’re losing weight with pretty much no effort at all. Keto and IF are really useful together in my eyes because of the convenience factor. Definitely keeps my energy levels up too!

  13. Hi Craig,
    Awesome site! You should be paid.

    I recently started the keto diet. Not to lose weight, but to help cure my IBS. I have found that not only has the mere food on the diet been incredibly healing, but so has the timing of when I eat. When one goes on a keto lifestyle, they are less hungry throughout the day, and therefore eat less often. This has helped heal my insides. Fasting gives your digestive system time to cleanse itself. Eating carby foods only makes you hungry more often, and therefore you eat more and more often, not allowing an already damaged digestive system to heal. (not to mention all the crap load of additives sold in the so called “food” of the standard American diet) Fresh organic food is where it’s at.

    I would encourage anyone with IBS or SIBO to try this diet, and I would recommend your website as a good source of recipes, tips, and nutrition. I would maybe add to the reasons to try the fasting periods as a way to help heal and cleanse one’s digestive tract (that way it detours from the weight loss aspect).

    I typically get up very early in the morning, do an intense 30 minute workout of hit & strength training, then have just a few tablespoons of coconut oil & a cup of spinach for “breakfast”. I don’t eat again until 1ish and use a time window from then until about 8:00 (if I get the munchies) to get in my macros for the day. My energy is consistent, and I am still able to build muscle in my fasting state.

    Eating this way has helped me recover to a state of almost no symptoms! And I have been doing this for only about a month and a half.

    • Natalie,

      That’s an awesome story – I’m really happy to hear that this is doing so well for you. I am aware of fasting and also using keto for many other auto-immune diseases and other problems. I will end up getting into that once I finish everything I have to say on keto, but until then just keep your eyes open! I really appreciate all the kind words :D

  14. Hello craig!
    I bought your 30 day meal plan and I have a question? have you taken 3 weeks to start IF for a health reason or just to ease us into it? I’m more of a jump head first kind of person, and know I will be successful, but if you recommend for some health reason I follow the plana nd wait 3 weeks I will, let me know.

    Also, if I am fasting and going to the gym daily, would you recommend me going to the gym during my window, or before it? I usually do HIIT and some workouts with my body weight(squats, pushups, etc) or should I try it out and see which my body responds better to? I do not know!

    Thanks :))

    • Leslie,

      Since IF doesn’t really have much of an impact on weight loss, I decided to just ease people into it to see if it works for them. I tend to find I have more energy when I IF, so I wanted to let people experiment to see if they find the same. If you want to plunge head first into it, you could work out right before your window and eat 20-30g of protein (with no fats) right after your workouts. You could alternatively eat fats an hour or so before a workout to fuel it, then eat some protein after. Really up to you :)

  15. I’ve actually been stalling. Stuck at 148 pounds at the lowest. Im fluctuating between 148-150 for the last few weeks. Maybe its time I start IFing. I’m still not sure how it works after reading everything here.

    So I can just wake up, not eat all day and around 8 or 9, I can eat all the macros I’m suppose to eat? Eat everything in one meal?

    I should probably also recalculate my macros. I got this Body fast percentage reader, but I don’t think its very accurate. I have ketone sticks, that states I have excess ketones. I bought the Ketone Meter, but its broken and I couldn’t test it. I’ve been eatting alot of erythitol, so….I hope thats not stalling me.

    • Andy, yep that’s how it works. If you’re stalling, I don’t know if IF will make a huge difference. It might, but you may want to look into other things you are making too. If you’re using a lot of erythritol that means you might be making a lot of treats – try to cut back and see if that helps :)

  16. Good evening Craig I’m starting an intermittent fasting on Monday I saw a recipe for tastier bulletproof coffee which included chocolate peanut butter and coconut oil my question to you is is it okay to use that coffee while intermittent fasting?

    I’m also going to purchase your book my question in regards to that is your site secure can’t tell? THANKS!!!

    Gena

    • If you’re going to do intermittent fasting, it depends how you want to do it. If you want to do it properly, then you shouldn’t even include bulletproof coffee into your morning routine. You technically are fasting if you eat 12 hours apart, so if you eat at say 7am and 7pm, then that is technically fasting – but you might see more benefit if you cut out that bulletproof coffee. Breaking a fast takes about 50 calories, so drinking BPC alone (1 tbsp. butter, 1 tbsp. coconut/mct oil) would break a fast.

      As for the book, when you go to the checkout, it uses PayPal to do so – while my site isn’t secured with SSL, PayPal most definitely is – and my website doesn’t see any of the information you use the pay for the book :)

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