Dear Mark: Post-Workout Fasting

Empty BowlBefore I jump into this week’s Dear Mark post I wanted to direct everyone’s attention to Andrew Rubalcava’s site, Go Healthy Go Fit. Andrew just published an interview with me. Here are just a few of the questions I answered:

Who have been your top 3 favorite bloggers over the years?

How did you get involved in physical competition such as your experience as a triathlete?

If you could give a few words of advice for those who are just beginning to enter a world of health and fitness, what would you say?

Check out the interview here, and check back on Thursday when I’ll be publishing Andrew’s guest post on how to stay healthy no matter what type of lifestyle you lead.


For this week’s Dear Mark I thought I’d follow up on the issue of post-workout nutrition from a couple of weeks ago. I mentioned in Dear Mark: Muscle Building and Carbs that I fast once in awhile after a workout. I’ve gotten a lot of comments and messages about this point since that post. So, here’s my take on PWO fasting as promised!

As I mentioned in the earlier Dear Mark, after a workout is prime opportunity for protein synthesis. For that reason I usually do a high protein snack in the first 30-60 minutes to capitalize on that benefit. (Carbs, as you recall, aren’t necessary for the enhanced synthesis, and I don’t recommend them in any form after a workout that’s under an hour’s time.) However, I also occasionally choose to fast after a workout to maximize another physiological benefit – the rise in human growth hormone (HGH), which critically influences everything from bone density to muscle mass and organ reserve to general cell reproduction in the body’s systems.

As I’ve mentioned before in relation to intermittent fasting (IF), fasting is known to significantly increase HGH secretion. (Grok’s survival depended on the ability to hormonally kick start efficiency and preservation modes during the lean days.) Research has also consistently confirmed that intense exercise, particularly resistance training, also triggers a rise in HGH. While I can (and do) take advantage of each individual method, combining the two opportunities can maximize my body’s HGH release.

Furthermore, insulin suppresses HGH. Skipping the carb snack and subsequent insulin upsurge goes a long way post-workout. But skipping anything that might even mildly raise insulin levels (that heightened sensitivity in the muscles, you know!) can be better yet.

And I should add that I do this without worrying about a blow to muscle mass. Fasting occasionally post-workout, provided I maintain a high protein diet the rest of the time, doesn’t negatively impact nitrogen retention and protein synthesis. Just as I mentioned last week that the body won’t suffer if you don’t force reloading of glycogen stores with a shot of glucose after a workout, I’ll go out on a limb and say that you’re unlikely to waste away if you don’t fuel muscles immediately with protein as well. The fact is HGH has its own protein conservation potential. And, while the post-workout period is an optimum opportunity for protein synthesis, it’s not the sole time your body is able to use protein and provide for the muscles’ needs.

While I’m not suggesting fasting after every workout, I think it’s worth doing occasionally. (After all, Grok probably wasn’t successful with every hunt.) In the spirit of IF and recreating patterns of our primal ancestors’ lives, varying your eating/exercising/fasting practices ultimately allows for maximizing the hormonal and upregulating benefits of different physiological scenarios.

Finally, yet another means of naturally encouraging HGH release is getting a good night’s sleep. After a good fast (and a great workout), I find this part comes the easiest. Honestly, it’s the best sleep I get.

As always, thanks for your questions and comments. Keep ‘em coming!

DeathByBokeh Flickr Photo (CC)

Further Reading:

The Hype Over Human Growth Hormone

1 Meal vs. 3 Meals

How To: Intermittent Fasting

About the Author

Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.

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49 thoughts on “Dear Mark: Post-Workout Fasting”

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  1. Hi Mark, Great post as always.
    Where I work out is between 45 min to 1.5 hours (depending on traffic… oh Los Angeles) from my house so I tend to work out and then just wait until I eat dinner (generally 2-3 hours after I finish my work out). Would it be a good idea to start bringing a protein rich snack in the car to eat while I’m on my way home?

  2. Thank you for the post Mark. I know thhis should answer some of the questions out there. I too enjoy fasting every now and then after a workout. Only occasionally though as I also thouroughly enjoy chowing down after a good hard workout too.

    The SoG

  3. Very nice blog! I was so hungry after my two hour hike yesterday! I had fish, cereal, yogurt, ravioli and a salad. Just went on and on. Plus I was really tired, had a small breakfast and stayed up late the night before. I like this idea of an IF occasionally after a workout. I slept very well last night.

  4. Hey Mark,

    I just wanted to say thanks again for the interview!

    I also had a question regarding IF post workout. I tend to hit the gym sometimes pretty late at night. If there isn’t enough downtime between your workout and bedtime, is it a good idea to just not eat until the morning or should you at least eat something?

    And thank you for letting me write the upcoming post for Thursday! I’ll make sure to let my readers know.

    All the Best,

    Andrew Rubalcava

  5. Loved the interview, and glad to hear you’re a fan of Jack Lalanne. I was worried only us old fogies (76 years young) had heard of the original fitness guru!

  6. Hey Andrew,

    I’m not sure if Mark covered it, but I think Art DeVany will work out and occasionally skip dinner. If you do this intermittently, it will not work against you as both he and mark have stated, sometimes Grok just can’t get the bear no matter how hard he tries.

    I’m glad this post confirms what I’ve been doing. On hard workouts (ie. glycolytically demanding) I’ll have a post workout meal, but on Alactic or just heavy lifting days, I can go with a bit of a fast pwo.

  7. The main idea in the post today is to confirm that it’s OK not to eat after a workout once in a while. I don’t do it all the time, but I also never freak out (like my body builder friends) if I skip a PWO snack or meal – or two. I can’t cite any research as to an exact ideal amount of fasting time, so I’ll say anything and everything will work in this case: 1.5 hours or 3 hours, overnight if you’ve worked out late. I just don’t go for the high carb PWO meal anymore.

    Remember, Grok ate intermittently, sporadically, fractally, and pluri-voluminously. His genes (and ours) were designed to respond ideally to all forms of feast/famine (provided the food choices are in alignment) and the types of exercises he did.

    1. I’ve just started documenting my own experience with IF as I prepare for a physique show. So far I love the results, and will try a 2 hour post-workout fasting window to see if that accelerates fat loss. Thanks for the post!

  8. Hi Mark, Love your blogs soo helpful! I’ve been low carbing for only 5 weeks, I’m having between 15-30gms carbs. I have had major blood sugar issues in the past which are improving little by little since switching to low carb. I’m wondering whether fasting after a workout for a person with blood sugar issues is a good idea?

  9. Hey Mark,

    I had a similar question. If one is already partially ketotic going into a workout, and one works out intensely, say 20-35min (ie. a long-ish crossfit WOD)would blood sugar be a bit “high” right after the workout? since the body will be converting fatty acids to ketones, how will that read on, say, a blood sugar meter for a diabetic?

  10. Michelle, you could always go the protein route after your workout. Giving your body a substrate to use for gluconeogensis as needed (and gluconeo burns fats) should address the low glucose issue.

    As for the follow-up question, I can only speculate as to what a meter might read PWO. Ketosis itself generally causes a drop in glucose and/or gluconeogenesis – not a rise. There are a few variables that will affect this: are we talking type 1 diabetic? (different mechanism), how much glycogen was in the muscles and liver before the workout, how long the subject has been in ketosis, how hard the workout, etc. But someone working out under ketosis should not have anything close to a “high” glucose reading.

  11. I try to follow alternate-day fasting as much as possible, a 36/12 hour fasting/eating cycle (often more like 40/8). I frequently train on two consecutive days: early afternoon on the fast day and a similar time on the eating day, before breaking the fast mid-afternoon. Not only is this not counter-productive, it possibly even gives me a boost. I really tear into a workout on an empty stomach.

    Most people I tell about this have a fit when they realise how long I go without food before training. Some can’t even believe I skip breakfast.

    1. I think it depends a lot on the person. I can fast for ten days easily most of the time. Now ten years ago I couldn’t. Not that I do fact that long more than once a year. I’m just saying going without food really bothers some people, but others not do much. I usually do 18/6 IF, but I also just started low carb at the end of last year. I haven’t tried going longer than that mainly because I’m often told I don’t eat enough. I’m happy with 1000 cal a day mist days. It’s hard for me to eat 1,500 calories a day. So yeah some people freak out when you talk about fasting only because I think they go into starvation mode just st by hearing about it. I drink coffee for breakfast then eat my eggs and bacon at lunch. I’ve never been a breakfast person, but the BP coffee helps me switch back into ketosis after naturally dropping out by morning since I don’t eat after 5 PM.

  12. The best sleep I ever have is midway through my fast, sleep is so good.

  13. Great post Mark,
    I enjoyed reading your interview with Andrew.

    I’m also a fan of Jack Lalanne all my life, since i was little, i’d watch him on T.V. and exercise with him, it was fun!

    Fasting causes my best night sleeps, actually get better night sleeps on a day of fast.

    I more than appreciate this post, you’ve answered alot of my questions i’ve always had on fasting!

  14. Great post. As I say, muscle builds 24/7….well if you give it the correct stimulus of course. Also there might be MORE benefit to pre-workout amino acids in their role for muscle repair vs post workout. Post workout shakes are for people who like to get fatter and call it mass (not muscle) gaining….I did it for many years of my younger life.

  15. Hey Mike,

    Wow, I’m so glad you said that! I’ve been using BCAA’s (powder) pre workout for months now and I’ve really seen the difference!

    Thanks bud!

    All the Best,

    Andrew R

  16. With all due respect, I’m a bit disappointed with the article. Where is the evidence (research) to back up these claims? I’m not convinced that fasting after a workout or before one is optimal for muscle retention/building. I mean, anything is possible, but saying that our pre-ancestors sometimes had to do it proves absolutely nothing. Give us some substance, not mere speculation.

  17. Add me to the list of fasters, not only after a workout (intermittently) but almost always before.

    90% plus of my two weekly 30-minute intense sessions are begun at a point where I’ve not eaten for 18-30 hours (I’m still in fat loss mode — I’ll be looking to cut that to every other time). I love working out fasted, and during the time I’ve been doing this my bench press, as one indicator, has gone from around 100 pounds to warm up at 135, then 185, then 205.

    I’m usually hungry when I begin the workout, but hunger is completely killed 5-10 minutes in. It never returns for at least two hours after completion, and I never force feed. I eat when hungry. However, about every other time, I’ll extend the time time before eating to as much as 4-6 hours.

    My results (so far with more to come) speak for themselves:

  18. encdxkcjsototkvqwell, hi admin adn people nice forum indeed. how’s life? hope it’s introduce branch 😉

  19. As for PWO nutrition, sometimes I eat and sometimes I don’t. Good reasons for both, I guess, but remember that in PB workouts, you really don’t need to refill glycogen stores the way you do if you are training longer, harder and higher heart-rates. When there’s no need for extar glycogen, there’s no need to eat extra. meanhwile, HGH improves when you DON’T eat PWO.

  20. Hey Mark, I’m glad I just came upon this article because I have a few theories regarding pwo nutrition and you seem have similar views.

    1) Number one is the HGH double whammy. Another theory I have is that while fasting blood is supplied to the brain and muscles, blood is essential to repair of damaged tissues. Now eating a big carb heavy meal would, imo, divert the blood from those damaged muscles and into the digestive system further impairing muscle repair.

    2) I have read the six part post over at about what happens to your body when you fast, which deals extensively with tissue repair and anti inflammation. Two bonuses of IF that are a big factor in why I IF that I feel would be negated during a critical time if I break my fast immediately post workout.

    3) Finally, on an adaptive level, Grok probably would have failed on a hunt fairly regularly. Especially when he was just learning how. This, IMO, would cause our adaptive mechanisms to insure that next time Grok went out hunting, he would be able to run faster and throw his spear further in order to see to it that he doesn’t fail again.

    Now I am by no means a scientist and it’s easy to make arguments for anything if you try hard enough. But I was just wondering what your thoughts would be on some of these points.

  21. Joey, I agree with all three points. Not being a scientist sometimes means you get to think logically and not have to back it up with some made-up science project!!

  22. I would like to see solid information regarding the HGH production when fasting after a work out.

    Alfredo E.

  23. One thing I wonder about regarding post work out carbs…doesn’t elevated insulin suppress cortisol? How does one factor that in as a mitigator to the catabolic effects of the elevated cortisol? As of now, I still have a banana as part. I have wondered about whether to ditch it or not.

  24. Hey Mark,

    I read different approaches to preworkout nutrition while fasting. Some say your first meal after fasting should be the preworkout meal while others say the first meal should be post workout meal. What do you think ?

    One more thing, right now I follow the traditional 6 small meals per day and do my cardio on empty stomach in the morning. How would you place the cardio if I do IF and if my eating period is between 17.00 – 24.00 *


  25. Post-workout nutrition is grossly over-hyped pseudoscience. Scientifically, there is no magic nutrition window for muscle growth, which actually continues 3+ days after each workout. Plus, the body has more than enough protein available to recycle for muscle needs for an hour or two (actually, much longer than that).

    The notion of a “magic window” for post-workout nutrition is just based on misunderstanding a small portion of research taken grossly out of context.

  26. Hello Mark,

    I was wondering, if I were to fast PWO when I did eventually eat does it really matter too much what level of protein I eat? Should I strive for a fast digesting protein as I would PWO or just go with a high quality protein regardless of the pace of digestion? And also after that fasting period are you still a fan of hardly any carbs or none at all?

  27. Hey, when you fast post-workout, are you being in the ‘fed’ state the first half of the day? Or do we fast, workout and continue fasting?

  28. Hi,

    Sometimes if I skip a meal I get a bit light headed – last night I did an hours resistance intense workout class and came home at 9 and wanted something to eat – I get a bit grouchy if I don’t eat! So I just had some steamed veggies and some yoghurt after, and I woke up the next morning feeling really hungry. My question is, would this have had any benefits like IF if I did eat something last night?

  29. Hi,

    Sometimes if I skip a meal I get a bit light headed – last night I did an hours resistance intense workout class and came home at 9 and wanted something to eat – I get a bit grouchy if I don’t eat! So I just had some steamed veggies and some yoghurt after, and I woke up the next morning feeling really hungry. My question is, would this have had any benefits like IF if I did eat something last night?

    Read more:

  30. Whether it is ok to fast PWO or not depends on many factors.

    1) How long are you working out ?

    2) How many calories are you consuming daily ?

    3) What is your Body Fat % ?


    If your goal is to build muscle, then you want to be eating on a surplus and not on a deficit, so you will have to get proper nutrition, proper macros and supplements if necessary to assure muscle health and growth.

    If your goal is to slim down, tone muscle/maintain muscle, then you have adding some heavy lifts to your workout, no more than about 20-30 minutes, low rep but heavy. Combined with proper protein and fat, for those who are on keto type diets. You can’t have your cake and eat it. If you want to lose weight, you can’t also want to body build muscles 😀

    If you are a newbie you might get away with it,by what’s called newbie gains.

    One thing to keep in mind never go on a too high calorie deficit (VLC Very low calorie diet) combined with long workouts, cardio, etc….. Muscle loss is the least of your concern but other bigger problems like screwing your metabolism and immune system. If your body fat % is on the low side and have stubborn fat to lose (few pounds) then I don’t recommend working out and fasting. Remember that if you work out and expand more calories than you ingest, combined with a VLC and low body fat %, you are going to have your body hold on to your fat and your weight loss will STALL you might even gain weight, and you will lose muscle mass… If you want to maintain muscle / tone,whilst keeping slim you can use the CKD (Cyclic Keto) whereas you go intermittent low carb days followed by carb days, etc…..cycling your glycogen stores preventing overspill and gaining fat. But you won’t build muscles on any diet,
    on ANY deficit. Fasting PWO on occasion won’t hurt you,
    providing the rest of the day you eat well, you eat enough calories, and get proper supply of proteins based on your LBM, you should preserve lean mass. Otherwise you are asking for inevitable lean muscle loss.

    For optimal recovery and growth it is advised to have a PWO meal, ideally both carbs and protein, but if you don’t want the carbs, at least the protein ! If by fasting you mean working out evening and fasting over night until breakfast (which is probably around 12-14 hour fast) that’s not too good, hinders recovery and in the long run muscle loss.

  31. Mark sorry bad writing, do not speak English, I have a doubt, weight only 57 pounds and I’m 1.82 meters tall, my body fat percentage is very low even then I do not eat carbs after training?

  32. A few years back I did intermittent fasting and upper-lower body split workouts with free weights.

    I cycled eating and training, so one day I would eat, the next train upper body, then eat, then lower, and so on. (I ate a *lot* on the days between workouts!) I had no trouble gaining strength (and also muscle mass). I don’t know if I would have gained more without the IF, but I was not unhappy with the results. It was also very time-efficient.

    I’m considering doing something similar again. These days I do full-body workouts and train three times a week M/W/F. I might just fast two days a week, and mix it up so that some eating days are on training days, and some are in between.

  33. Hey Mark, I heard you say on the Rogan podcast you most recently did, very briefly on this, in effect “If I am trying to gain muscle I’m not eating after lifting”. Googled more and found this post which is quite old! the way you said it on the podcast made it sound like you had more definitive thoughts on this than this blog suggests, you have updated your thinking here?

  34. Hey everyone!
    Im searching for more information about exercising after que fed window (eating in the morning, training at night)! I’ve been reading a lot but not very useful information! If someone could suggest me anything, would be great!