As I’ve mentioned many times before, the post-workout period is prime opportunity for protein synthesis. For that reason I usually do a high protein meal in the first 30-60 minutes to capitalize on that benefit, with some carbs if I’ve been going hard and really burned through glycogen (although not always). 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 fat burning and muscle mass and organ reserve to general cell reproduction and healing in the body’s systems.
As I’ve mentioned before in relation to intermittent fasting (IF), fasting is known to significantly increase HGH secretion. This makes perfect sense, as ancestral wild humans needed to be able to remain efficient, preserve lean tissue, burn through fat stores, and maintain high activity levels when food was unavailable. If you’re hunting on an empty stomach, you need to be able to maintain the health and viability of your muscles, bones, organs, and cognitive function. If you can’t, you die, fail the hunt, or both.
Intense exercise, particularly resistance training, also triggers a rise in HGH.1 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!), like protein, can also be helpful from time to time if you want to maximize human growth hormone.
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 and eat protein post-workout the rest of the time, doesn’t negatively impact nitrogen retention and protein synthesis. Just the body can survive without refilling glycogen stores immediately after every 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.
Plus, one of the functions of HGH is protein conservation.2 A 1990 study in elderly people with low growth hormone status found that those receiving enough GH injections to give them “youthful GH status” experienced increased lean mass gain, fat loss, thicker skin, and better bone density.3 I’m not saying that fasting after your workouts will give you those kind of results, but it can be a helpful intervention to keep the ravages of aging away.
Ketones themselves, which you generate during a fast or after a hard workout, are also protein-sparing. 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. Protein intake over the course of several days is more crucial than squeezing in 32.8 grams of protein 15 minutes after your workout.
I’m not suggesting fasting after every workout, but do I think it’s worth doing occasionally. 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.
But let’s go into some use cases for post-workout fasting. When does it make sense to skip a meal after a workout?
When you’re trying to heal an injury.
This seems to work especially well for long term or chronic soft tissue injuries. You know, those nagging tendon or ligament or muscle strains that just don’t seem to get better. We know that growth hormone is used to regenerate tissue and speed up recovery from injuries.4 We know that fasting improves wound healing, and the same healing factors should also apply to injuries.5 It’s all damaged tissue that needs to be regenerated.
This could be useful for someone with a long term or “nagging” soft tissue injury.
When you want to boost fat metabolism.
If fasting is a quick way to boost fat adaptation, and workouts in general improve fat adaptation, fasting after a workout will supercharge that adaptation. If I were a newbie trying to get into ketosis really quickly, a short fast followed by a hard workout and even more fasting would be how I’d get there.
This could be useful for someone trying to speed up their adaptation to a high-fat or keto diet.
When you want to create more mitochondria.
One important role of growth hormone is to produce more mitochondria—to “grow” them. Mitochondria are the power plants of the cells. They create ATP out of the food we eat. Post-workout fasting boosts human growth hormone, creating the impetus for more mitochondriaal biogenesis. Once you’ve made the mitochondria, of course, it’s important to start eating normally again so they can actually start working for you.
This could be useful for someone preparing for a competition or event that’s still months away.
When you want “insurance.”
The reasons for post-workout fasting are mostly theoretical. I mean, there are anecdotes (including my own experiences) attesting to its efficacy, and we’ve identified plausible physiological mechanisms, but there aren’t any controlled trials in humans looking at the effects of post-workout fasting. A justifiable reason to fast after a workout on occasion is because it doesn’t do any harm and it probably has some good long term effects on health and longevity.
This could be useful for someone on the older side who wants to improve longevity and health span.
When you’re already fat-adapted and want to speed up fat loss on a temporary, short-term basis.
This is an advanced tactic for fat-burning, metabolically-flexible beasts. This won’t work as a long-term weight loss strategy. This is an acute intervention, a quick push for a pound or two of fat loss. And once you get it, you can’t do it again for a long time. It won’t keep working.
This could be useful for an advanced trainee who needs to lose a pound or two of fat.
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 I said earlier, you should not fast after every workout. Personally, this is something I do once or twice a month—tops. If you do it too often, you’ll lose the benefits. The combo of post workout fasting is a fairly big stressor. It’s a stressor you can recover form and adapt to, but not in perpetuity. You also need to feed. You need to come back to camp with an elk haunch more often than not.
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.