Should You Work Out When Intermittent Fasting?

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Running ShoesNot sure if you should work out on fasting days?

And if exercise will increase your hunger and cause you to cheat on your fast? Or whether the time of day that you work out will make a difference?

These are all important questions to us intermittent fasters. The last thing I feel like doing is too much exercise so that I overeat and undo the benefits of my fasts. That sounds about as much fun as running a marathon backwards.

If you’ve read our previous article about how chronic cardio exercise can increase appetite and cravings for high calorie foods, I’m sure you’ll be interested to know whether you’re doing the right thing by adding exercise to your fasting schedule. Thankfully Krista Varady looked at the effect of mixing exercise and intermittent fasting on weight loss in a recent study and has answered these questions for us.

If you watched the documentary ‘Eat, Fast and Live Longer’, Krista Varady was interviewed about her study where the participants fasted every second day (Alternate Day Fasting or ADF) by only eating 25% of their daily calorie needs on their fasting days (similar to the 5:2 method).

Krista Varady Weighing a Subject

She found that the subjects did not overcompensate for these calories by overeating on their non-fasting days so they managed to lose weight successfully. And in this study they looked at ADF again but added exercise into the mix, to see what happens.

They split 64 men and women into four groups:

  • One fasted as well (ADF) as well doing exercise
  • One group only fasted.
  • One group only did exercise
  • And the last group did nothing at all.

The exercisers did 25 minutes of moderate exercise on a stationary bike or elliptical machine three times per week and gradually increased to 40 minutes per session over the 12 week study.

Exercising on a sationary bike


They asked them to rate the following factors:

  • How hungry they felt after exercising and for the rest of the day.
  • How full they felt after their fasting meal.
  • How satisfied they felt after eating.
  • Their level of ‘restrained eating’ (the conscious restriction of calories in order to lose weight).
  • Their level of  ‘uncontrolled eating’ (their tendency to eat more than usual due to a loss of control).
  • The amount of ’emotional eating’ (eating when not hungry for emotional reasons).
  • They also took note of what they all ate on their non-fasting days.


What Did They Find?

There was no difference in appetite in the fasting with exercise group regardless of whether they exercised on a fast day or a non-fast day. There was also no increase in the urge to cheat on a fast day if they chose to work out on that day.

The amount of ’emotional eating’ decreased in the fasting with exercise group. However there was no difference in hunger, satisfaction and fullness in the fasting with exercise group. Strangely these all improved in the group that only fasted (with no exercise) which was a very interesting discovery.


In summary they found that with ‘moderate level’ cardio for up to 40 minutes:

  • You can work out on a fast day and it’s not likely to lead to overeating.
  • Your hunger should not increase if you work out on a fast day.
  • Your feeling of fullness after eating should not decrease if you work on a fast day.
  • Your feeling of satisfaction after eating should not decrease if you work on a fast day.
  • Your level of ‘restrained eating’ and self control around food should improve!
  • The amount of uncontrolled eating (binge eating) and emotional eating should also improve!
  • There should be no difference and time of day you exercised and the amount of food consumed on that day.


So In Summary…

This was an interesting study because all of the benefits mentioned above also applied to the group who only fasted and did no exercise at all.  Not only that, but the fasting only group also had a reduction in hunger and an increase in satisfaction and fullness and the exercise group did not!

I should also mention that aside from these benefits in ‘eating behaviours’ the study showed the fasting with exercise group had a higher rate of weight loss than the ADF-only group that did not exercise, but only by 7%. So it seems intermittent fasting and working out only has a small advantage over intermittent fasting without exercise.

So I guess the take home point is that if you do choose to add exercise to your fasting schedule (and obviously I recommend that you do for the other health benefits) you’ll probably lose weight fast but keep an eye on how much exercise you do and whether it changes your eating habits. Otherwise you could be undoing the 7% benefit that it has over intermittent fasting without exercise.

Remember, the exercise in this study was only done three days a week and would not be classed as heavy exercise and can be replicated with by other activities such as fast paced walking cycling, playing golf, swimming etc. They estimated the calories burned in each exercise session to be around 150–250 calories, so keep that in mind when planning your workouts.

My advice is to keep a food diary for the first two weeks of a new exercise regimen so you can look over your eating habits and see if there are any changes and adjust your workouts accordingly.

What has your experience been with exercising on fast days? And what type of exercise do you usually do? We’d love to hear your comments below!


8 thoughts on “Should You Work Out When Intermittent Fasting?

  1. Juliette,
    I am Kristina’s sister, she commented on your The-ifasters-Method page. I am too on my 4th week of 2-24 hr fasts per week. I am a Zumba Instructor and teach @ 5 hrs per week. One of these classes is a Zumba Toning class which I consider a Low/Medium Cardio with Weight Training. As she mentioned I think we have both lost a few lbs and have just started measuring for Inches lost. I have been trying to add in 30 minutes of weight/cross training (i.e. machine weights verses free weights) twice a week. I have a fear like the previous commenter about not having enough energy so I do Lunch to Lunch as my fasting days are also on days that I teach Zumba (Mon-Tues & Thurs-Fri). After running into your website after first reading Eat Stop Eat by Brad Pilon I have incorporated the 25% eating method at the end of your fast day. I have found it a challenge as I have two meals that day. However, it seems to get easier as I go. I haven’t done the cutting of calories as she has on her non fasting days but I feel that at first I tried to compensate (unconsciously) the first time and was sooo sick. I am assuming if I asked for advise it would probably come back as less cardio and more weight lifting but since I have students that rely on me I am not sure how to proceed. What do you think?

    1. Hi Rebecca, Well it sounds like you can’t do less cardio because it’s your job! That’s ok, because with aerobic exercise your energy levels should remain constant throughout the week (check out this study here ) regardless of when and how long you fast. We can actually fast for up to 3 days before our body starts to run out of energy stores. So unless you’re running marathons, you’re fine with what you’re doing and you can change your fasts to dinner time to dinner time if you like. I recommend through that on the days that you’re doing weights (and Zumba toning) that you eat within an hour afterwards so you can restore muscle (which is great for fat burning!). Otherwise just fast on the other days of the week when you’re not doing weights, so you dont have to worry about it. And yes you’re right, with that much exercise don’t count your calories on your non fasting days, just listen to your body’s signals for when and how much to eat. Hope that helps.

  2. Hi Juliette,

    I have completed about 2-3 weeks of IF. For the first 2 weeks I have used the 16/8 fast – seeing about 1kg dropped. For the past week I then closed my eating window 5 hours instead of 8 and fast for the rest of the day and dropped 2.4kgs. But since yesterday using my 5 hour window I have stayed at the same weight – which I am thinking may lead to a plateau effect. In my 5 hour window I consume two meals between 12pm and 5pm and workout either before I break my fast or after I eat my last meal – generally either a 30min cardio session or 1 hour zumba class – 7 days a week. My meals are protein and veg, no carbs. Today I broke my fast at 11:30am and consumed 1 cup of risotto and tuna as I had the shakes between 9 and 11am this morning which made me feel a little better but not as good as I usually feel. Could my body be in starvation mode due to maybe having a calorie deficit or would I need to drop my calorie intake even more? Or should I increase my feeding window to ensure I get the calories I need to start burning again? Please help.

    1. Hi Michaela, It depends on how many calories you’re consuming after your fasts. If you’re not getting your daily calorie needs (which can also be from a slow cumulative effect over time that can slow down your metabolism dramatically, hence the plateau in your weight loss) plus you’re exercising a lot your body will eventually make you crave calories (and carbs!) if you’re not eating enough. So yes that would be ‘starvation mode’ unfortunately. Not to worry though! Instead I recommend you change to two 18 hour fasts a week instead and only eat 25% of your calorie needs after the fast on those two days. Then on the other days make sure you eat as close as possible to your calorie needs. Don’t worry about cutting out carbs, as long as you’re sensible with carbs and sugar you should be able to eat freely on your non-fasting days. This ‘cycling’ of calories over the week means your metabolism will stay at it’s peak and burn fat like crazy when you are fasting. At the moment your body probably won’t be burning fat when you’re fasting because it thinks it needs to hold on to it. That’s how intermittent fasting works! By changing your calories dramatically in intermittent periods. It’s not a calorie restricted diet. Calorie restriction = slow metabolism and probable weight gain further down the track. Another tip – ditch the cardio and go for a quick session of weights and/or High Intensity exercise three times a week. That’s all you need. If you do want to exercise daily do moderate intensity (mentioned in this article/study e.g walking, cycling at moderate pace for 40 min max). Good luck and let us know how you get on!

  3. My problem is that I don t feel I have enough energy to go through a normal workout, on fasting days, i.e. gym on the last 2 h of fasting before having a meal. I usually do 50 min cardio (25 on elliptical with resisance and speed) and 25 on a bike (peak parcour), at the end I usually a toning routine for arms. But half way through I feel dizzy and simply not able to continue the cardio…it’s frustrating. Is there something I do wrong? Should I exercise after the meal? Thank you 🙂

    1. Hi Georgina, this study looked at moderate intensity cardio exercise (like fast paced walking or cycling) for 40 minutes with an estimated 150–250 calories burned in each session so your usual workout sounds like it would definitely exceed that amount. Having said that, many ifasters find they can get through their usual workout with no problems at all because metabolic rate and energy levels should remain the same. Maybe you’re not getting enough calories on your non-fasting days? So you might have an overall calorie deficit and therefore low energy? Are you restricting you calories on your non-fasting days? Are you working out too much? Remember 3 days a week (of the right type of exercise) is all you need. Otherwise try cutting down your workout so it’s approx 150–250 calories (I recommend with no weights) on fasting days or do just your workout after you have your meal.

  4. I’ve noticed higher intensity exercise does increase my hunger but it often depends on the time of day it’s being done. If I run for 30+ minutes it tends to make me hungrier about two hours later. I will time my workout to be before my planned small meal for that day (if a fast day) or if a non fasting day I will workout whenever I can. If I was to take a walk or less intense exercise then it doesn’t affect my appetite hardly at all and if it does, it suppresses my appetite for the rest of the day.

    1. Hi Michelle, Thanks for your comment. It’s great that you now know your body well enough now that if you do a 30+ min run you’ll probably be hungry afterwards. So yes, it’s a great idea to go for a run just before the end of your fast (which also means extra fat burning potential as a bonus!) or if it’s not possible just do low intensity exercise instead on that day.

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