Why Fast? Part Five – Exercise

In previous installments, I’ve discussed the powerful effect of fasting on weight loss, particularly with respect to adipose tissue. I’ve explained how intermittent bouts of going without food have been shown to increase cancer survival and resistance and improve patient and tumor response to chemotherapy, and I went over the considerable evidence suggesting that fasting can provide the life extending benefits of caloric restriction without the pain of restricting your calories day in, day out. And last week, I highlighted how fasting may have protective and therapeutic benefits to the brain.

As such you might be thinking that I only recommend fasting to the sedentary, the aged, and the infirm. Surely I wouldn’t go so far as to recommend to the active, the athletic, and the jacked that they engage in vigorous physical activity without having eaten a solid square meal beforehand – right? I mean, no good can come of a fasted training session, as the gym bros with the sweet ‘ceps are so quick to intone.

So, Sisson, what’s the deal? Can we exercise in the fasted state and live to tell the tale?

Yes. And there may even be benefits to doing it. There’s actually not a huge amount of literature on the subject out there, with the bulk of it studying Muslim subjects during Ramadan and getting mixed, sometimes negative results. I’m wary of using the negative results of the Ramadan training studies to color our opinion of fasted training for the population at large for three major reasons: first, Ramadan restricts daytime food and water intake during the fast. If you’re sedentary, you can probably get by without guzzling water, but if you’re an athlete, or even just someone who dabbles in a bit of lifting, some walking, and maybe a few sprints, your performance and results will suffer without adequate hydration. And I’d say a complete and utter absence of water during daylight qualifies as “inadequate hydration,” wouldn’t you?

Second, since eating and drinking are limited to pre-dawn and post-sunset hours, Ramadan often means sleep deprivation. Studies show that sleep onset occurs later than normal, sleep duration is lessened during the month (PDF), daytime sleepiness increases, and general performance of daytime tasks decreases. We’re already aware of the importance of sleep for general health, but inadequate sleep can also translate to poor athletic performance.

Third, the subjects in these studies most likely aren’t on a healthy Primal eating plan. Heck, they’re probably not on a conventionally healthy whole foods diet. While it would be nice to believe that these Ramadan fasters were feasting on fresh lamb, high quality extra virgin olive oil, extra-thick pastured labneh, grass-fed breadless shawarma, and pomegranate salads, they were likely eating the same junk that everyone in the industrialized world eats. And as such, they were probably poorly equipped to shift smoothly and easily to the fat based metabolism required by fasting. Sure, they switched over to burning their own body fat out of necessity and a sheer lack of calories, but it wasn’t the easy, seamless transition that Primal eaters typically enjoy at the drop of a hat. For the carb-addicted, fasting is mentally, physically, and spiritually taxing. For the fat-adapted, fasting often just happens. As we often say around here, we eat WHEN – When Hunger Ensues Naturally. For folks with easy access to the fat-burning switch, skipping a meal (or three) doesn’t ruin the day and preclude exercise.

Right off the bat, then, I’ll say this: don’t even consider fasting and training if you’re not going to hydrate, sleep, and become fat-adapted.

Now that we have those caveats out of the way, let’s look at some of the purported benefits of exercising in a fasted state, as shown in the literature:

Improved Insulin Sensitivity

Intermittent fasting improves insulin sensitivity, as I mentioned before in the fasting and weight loss post. A recent study found that this effect is heightened when combined with exercise (in this case four days of endurance training each week). By the end of the study, subjects who fasted had lower body weights (the only group not to gain weight), better body-wide glucose tolerance, and enhanced insulin sensitivity. Furthermore, only fasted training significantly improved muscular adaptations to training.

Improved Recovery from Endurance Exercise

Three weeks of overnight-fasted endurance cycling (with caloric restriction to boot) improved post-workout recovery, maintained lean mass, lowered fat mass, and maintained performance. There was unfortunately no control group, but this study does show that fasting doesn’t hurt (and it may help).

Another study suggested that fasted endurance training may quickly re-activate the muscle protein translation that was negated in athletes who had eaten carbohydrates before training.

Improved Recovery from Weight Training

A 2009 study found that subjects who lifted weights in a fasted state enjoyed a greater “intramyocellular anabolic response” to the post-workout meal. Levels of p70s6 kinase – a muscle protein synthesis signaling mechanism that acts like an “indicator” of muscle growth – one hour after a fasted workout doubled levels compared to one hour after a fed workout (in the same group). In other words, fasting boosted (physiological indicators of) post-workout muscle growth.

For a further look, check out Martin Berkhan’s take on the study. Also note his recommendation that 10 grams of BCAA (branch chain amino acids) taken before the workout should boost the enhancement without taking you “out of the fast.”

Improved Glycogen Repletion and Retention

What happens when you train in a low-glycogen state? If you’re used to running on full glycogen stores, your performance might take a hit when you have to shift toward a more oxidative, fat-based energy pathway. That’s understandable. Another thing that could happen is you learn to make do with less glycogen by, well, making do with less glycogen. This is elementary stuff, folks. Just like your muscles adapt to imposed stressors by getting stronger, your body adapts to low glycogen training by learning how to train under low-glycogen conditions, thus sparing glycogen for when it’s really needed and boosting performance when glycogen is actually available. It’s the classic “train low, race high” idea that I’ve discussed before. It’s the specific adaptation to imposed demand (SAID) principle, only in this case the “imposed demand” is a low-glycogen, low-food environment.

A recent study exemplifies this phenomenon, pitting a group of untrained, carb-fed cyclists against a group of untrained, overnight-fasted cyclists and comparing both groups’ muscle glycogen content and V02 max. Who won? The fasted group improved their V02 max by nearly 10% and their glycogen content by over 54%, while the fed group improved V02 max by just 2.5% and glycogen by a paltry 2.9%. Lesson? Don’t eat 1.5 grams/kg body weight in cereal-based carbs pre-workout, and definitely do not eat a delicious shake of waxy maize during your workout (unless you really really like cereal and corn starch slurries).

What do you notice? Fasting does not instantly imbue its adherents with super powers. It’s not supposed to. Improved performance during a given training session isn’t really the point of fasted training. The point of fasted training, as I see it, is to maintain performance while enjoying the metabolic benefits, like improved recovery, higher glycogen stores, better insulin sensitivity, and improved muscle response to exercise. The point is that fasted training won’t kill you, won’t eat your muscles, and it might even improve adaptation to exercise by forcing you to train in a “less optimal” state, which can boost performance down the line. The Olympian isn’t going to be well-served by doing the main event on an empty stomach, but he just might benefit from occasionally training on one.

Mind and Matter Matter

The success of your training, whether it be lifting heavy things, running, sprinting, rowing, cycling, or climbing, isn’t wholly dependent on your physical state. The amount of glycogen in your muscles and liver, the mobility of your tissues, the structural size of your muscle cells, the distribution of the fiber types within those muscle cells, the V02 max – these all matter and help decide the amount of weight you’re going to put up, the time you’re going to hit, the miles you’ll be able to check off, and the number of pullups you’ll complete, and fasting will obviously have an effect on these and other markers. But just as important is your mindset, your personal approach to fasted training.

Me, I like a good long hike in the morning with maybe just a cup of coffee in me. It gives me exactly the kind of steady energy I want without negatively impacting my performance (which doesn’t really matter on a pleasant hike) or my enjoyment (which does). However, I don’t like playing Ultimate Frisbee on an empty stomach. I can do it, but I feel like it impairs my performance – and when I play Ultimate I play to have fun and win (as PrimalCon attendees are soon to find out). As far as lifting goes, I’ll sometimes do it fasted, but I’m a big fan of fasting after a strength workout. I do so to milk the post-workout growth hormone surge and because I’m just not that hungry immediately afterward. If immediately stuffing one’s face was required for optimal gains after a workout, you’d think we’d all be ravenous after lifting heavy things, but we’re not. I can do sprinting on an empty stomach, but I hit the wall quicker (probably due to the depleted glycogen).

Don’t let the results of a study (or my words) dissuade you from doing something that seems to help you. If fasted resistance training has you hitting PRs (or at least feeling like you could if you wanted), keep doing it and disregard studies that suggest “THIS IS IMPOSSIBLE. YOUR GLYCOGEN-DEPLETED MUSCLES WILL SURELY DISSOLVE INTO THE ETHER.” If fasted resistance training has you lagging, eat something the next time and disregard studies that suggest “YOUR POST-WORKOUT MUSCLE PROTEIN SYNTHESIS AND INSULIN SENSITIVITY WILL SKYROCKET TO THE HEAVENS ABOVE.” In the long run, it may not matter. People have gotten in great shape eating six meals a day or just one.

Whatever you do, don’t fall prey to paralysis by over-analysis, as did one of the Worker Bees. This guy got way too deep in the fasting literature. He was reading PubMed articles, scouring online weightlifting forums for anecdotes about fasted training and running multiple self experiments with his eating and training. He become so enamored by the idea that working out in a fasted state would elicit superior metabolic and performance effects that he found himself unable to workout if he’d eaten anything at all. And it wasn’t a physical inability; it was a mental hang up. He became frozen, stuck and often unable to reap all these wonderful benefits he spent so much time reading about, all because he felt guilty working out if he’d had so much as a few pieces of beef jerky, a couple eggs, and a banana. Don’t be that guy. He has since seen the light and now realizes that something is better than nothing, that even “non-optimal” training can still be effective. But he wasted a lot of time getting there because he obsessed over studies performed on people who were not him which suggested some (often obscure) benefit to working out in a fasted state.

Do what works for you and if you find that fasted training qualifies, so be it. But don’t think it’s a requisite of Primal living. While I absolutely recommend that people play around with it, and most people find that Primal eating makes it easier, fasted training is not required.

What are your experiences with fasted training? Have you tried it? What benefits, if any, have you noticed? What about particular activities, like sprinting, lifting, or jogging – how do they respond to fasting?

Here’s the entire series for easy reference:

Why Fast? Part One – Weight Loss

Why Fast? Part Two – Cancer

Why Fast? Part Three – Longevity

Why Fast? Part Four – Brain Health

Why Fast? Part Five – Exercise

Why Fast? Part Six – Choosing a Method

Why Fast? Part Seven – Q&A

Dear Mark: Women and 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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