Should you work out while fasting?

When I first heard the concept of intentionally training in the fasted state I thought it was the craziest idea I had ever heard.  For years we’ve been conditioned that before going out for a run or lift weights you need a good meal–to do otherwise was counterproductive or even dangerous.  And perhaps for the conventional, carb-fueled athlete or weekend warrior this still rings true.  But for followers of this blog, many of whom are fat-adapted (or aspiring to be) and see the flaws in conventional health wisdom, fasted training may seem intriguing.

Before I adopt or experiment with a new bio-hack or strategy to optimize health and performance, I like to take a step back and think about it from a ancestral health perspective.  In other words, what would our ancestors have done in this situation thousands of years ago?  Did they have a big meal before going out on a hunt? No! It would’ve been precisely the opposite scenario: they were going out on a hunt in order to acquire food.

So on first principles, the idea of working out or training while fasting makes evolutionary sense–but does it make biological sense? There’s some pretty compelling evidence that the answer is yes, especially with regard to strength training.  It all boils down to growth hormone (GH).  GH normally fluctuates during the day but seems to be suppressed by eating.  After a period of fasting for 24 hrs, growth hormone levels spike making this a potentially opportune time for strength training.  This is precisely what is recommended by Dr Mecola in Ketofast and by the folks at LeanGains, who claim to have pioneered the intermittent fasting and training idea.

In practice, what does this look like?  Simply fast for 18-24 hrs and then–while still in fasted state–engage in some type of strength training regimen.  If you’re feeling sluggish its OK to use a little caffeine (which acts as an appetite suppressant as well as energy).  Be sure to follow this up by a period of “feasting” to reap the gains by refueling with ample sources of branched chain amino acids (BCAAs) which stimulate mTOR and thereby muscle synthesis. I have been doing a variety of this for past 6 months and have experienced great results with ongoing strength gains with minimal effort.

But what about endurance training?  It’s not as clear.  One thing is for certain: If you’re not fat adapted and you are reliant on carbohydrate fuel for energy your performance will suffer substantially if you don’t eat before (and even during) exercise.  But before you lace up and reach for the Powerbars, maybe you should see this as a huge red flag.  Think about our ancestors again: did Grok say, “hey wait, I have to suck down this gel pack before I run away from this lion!” or did he just have the ability to perform regardless of his immediate nutritional status?  I bet it was the latter…or else you and I wouldn’t be here.  Once you become fat adapted you can perform aerobic exercise just as well (if not better) with the clean-burning fuel stored in your fat cells called Ketones.

In summary, training in the fasted state is not as crazy as you’ve been led to believe.  For resistance training, it may even be advantageous to piggyback on the spike in GH after a moderate duration fast, then replete with food rich in amino acids to maximize the anabolic properties of both mTOR and GH.  For endurance/cardio workouts, fasting may seem counter-intuitive at first but will not cause too much impact once you are fat adapted; however, for more optimal performance, fueling with ketogenic foods and supplements makes more sense than relying on the endless cycle of carbohydrate dependency.