Won’t intermittent fasting limit your athletic performance? How are you going to fuel yourself without carbohydrates? In this article I’m going to dive into my varied experiences with intermittent fasting and exercise.
But if you’re an athlete or just someone who just loves to move and use their body, won’t the act of not fueling up for multiple hours every day limit what you can do? For many years I’ve been periodically experimenting to find the answer to that question.
What Is Intermittent Fasting?
You already fast intermittently. Fasting is any period during which you go without food. Most people don’t eat between bed time and waking up in the morning, for instance. When we talk about intermittent fasting, we’re usually just talking about making this period longer. Some people approach it by having an “eating window,” during which they do all their eating. This might by eight hours, six hours, or less. Other people simply seek to wait a certain number of hours after waking up before they start to eat. If you woke up at 8 a.m., you might decide to wait six hours – until 2 p.m. – to have your first meal.
Although it may never happen during some shorter versions of intermittent fasting, it’s my experience that a wonderful type of mental clarity and mild euphoria comes over you during your daily fast after you’ve given your body a few weeks to adapt to a shorter eating window. Since you don’t have to think about eating and preparing food, this is a great time to do intellectual work.
I’m not really interested in mind-altering substances, but this is a mind altering state I can really get behind, and I can get into it almost every day if I put in the effort to limit my eating window to a few hours for a few days in a row.
Although I know from an intellectual stand point that fasting is good for me, it’s my enjoyment of the state of extended fasting that makes it something I actually want to do.
But even though I enjoy this state, I always through it comes at the cost of not being able to perform well physically while I’m in it.
The Empty Tank
I started skipping breakfast in 2008, and within a few years I wasn’t eating till 1 p.m. unless I happened to be doing very intense physical activity in the early part of the day.
Two years ago I pushed that back to 3 p.m. on most days.
And during this whole time, I would have told you that after my muscle glycogen (the storage form of sugar/carbohdrates) was exhausted sometime in the late morning, I couldn’t really perform well physically until after I’d had more carbohydrates and restocked my sugar reserves.
Over the years I’d heard about becoming, “fat adapted,” or the body making enzymatic changes to allow it to be better able to draw on body fat reserves for fuel. I’d looked at some of the science behind this and agreed it was a well-documented phenomenon, but I had good reason to believe the adaptation wasn’t significant enough to matter for athletic performance.
After all, I once went 26 days without eating anything at all (just drinking water), and at no point during that did I become fat adapted enough to perform at anything more than 50% of my max athletic effort. So yes, I thought, the body can draw on fat reserves for fuel, but not well enough to lead to truly vigorous athletic activity.
The other element that lead me to dismiss fat adaption was that its implementation demanded that a person eschew carbohydrates for fat and protein, a dietary strategy that science has demonstrated leads to disease and ill health.
A New Strategy
One day when I was reading yet another study on fat adaption in athletes, it occurred to me that while the studies all drew on low carb diets to bring about fat adaption, a severely-curtailed eating window limiting the period of time during which the body was taking in carbohydrates might do the same thing.
I’d always wondered about the early European accounts of Native American eating habits. Early explorers and colonists described them eating nothing for a day or more, and then having huge feasts that seemed glutenous.
And they weren’t just sitting around when they were fasting, but were our farming, hunting, and traveling. Some surviving hunter gatherer groups have similarly-irregular eating periods. Were they “fat adapted,”? If so, they were adapted while eating a lot of carbohydrates.
To test our whether a very-curtailed eating window could lead to fat adaptation, in 2015 I did my first experiment.
The basic idea was that I’d eat my usual fruit-and-greens based diet, but only eat it over a window of two to three hours in the evening. I wanted to separate out the lower energy levels that result from running a calorie deficit from what might occur from depleted glycogen levels, so I made sure to eat a weight-maintenance level of calories. This means I really packed in the food, and I’m not sure if your average person not adapted to large meal size could have handled it.
I found that during my first few days on this regime, my results were what I’d always experienced. I could exert myself reasonably well in the early mornings, but by the time afternoon came I was out of glycogen and couldn’t function at a high level physically. By the time a week or two had gone by, though, something really interesting had happened.
The feeling of mild clarity and happiness I’d experienced with intermittent fasting during longer eating windows intensified to the point where I found it pretty intoxicating. I felt at peace with the world and happy for no reason. I also had some interesting experiences during hikes of feeling drawn to and connected with the trees around me. I felt clear headed but like I had access to something I normally didn’t.
Even during my 26-day fast, I’d never experienced anything like it.
And slowly, my ability to exert myself physically started to ramp up. After a few weeks I was able to do some things that surprised me. I made a point of doing mentally-intense work in the mornings, walking around, running errands etc, before departing in the mid afternoon for some sort of physically-strenuous activity. This was timed for a period when I should have been completed glycogen depleted and sputtering on fumes.
Some physical activities I tested:
- A four hour hike through the woods
- Three hours of biking around Austin.
- A four-mile run.
- A crossfit workout
- Partner acrobatics (acroyoga).
During none of these did I experience what I expected: having to slow down to a crawl because I had no fuel left.
What I found was that I was never able to perform at my best, but I eventually reached the point where I was able to crank out 80-85% of my max effort without it feeling like a struggle.
There are several advantages I really loved to this kind of fat adaption/intermittent fasting.
I’ve gone through most of my athletic life being hobbled by the need for regular fuel intake. I once regularly ran long distances, sometimes going as far as 33 miles at a time, and I always had to concern myself with the balancing act between fueling up to allow for maximum performance and knowing that eating food on the run often leads to cramps, gas, or lower energy levels, even when I’m extremely judicious about quantities, fruit types, and timing.
Even through I can’t go as fast and likely not as far, I absolutely love the feeling of running on a completely empty stomach that hasn’t had anything in it for 20 hours. It feels way better than trying to do it after I’ve digested a breakfast of easy-to-digest fruit.
I also really love that I can go about my day without having my mental acuity downgraded when the last of my glycogen stores are gone. Although intermittent fasting with a longer eating window improved this, it wasn’t until I started with the super-short window that I was able to keep my mind and emotional state running at what I feel to be 100% even 19 to 21 hours after topping up with sugar.
Another big advantage from an athletic perspective is suppressed body temperature during athletic performance, which means less sweating and your body not having to expend energy on cooling itself.
The main disadvantage of this method of eating is that your adaption is very temporary. What takes days or weeks to bring about initially can be gone in a day or two of eating during a longer window.
It often happens that I want more than 80% of my max athletic ability earlier in the day, so I might eat at 1 or 2 p.m., for instance. But doing so means that it will take two to three days of readaptation to get into the smooth-sailing spot of the shorter window, where I don’t want to eat during the day, and feel at my best physically and mentally.
You might ask why I don’t just have my shorter eating window start and end earlier in the day on days when I need fuel earlier. I’ve tried this, but find that once you’ve introduced carbs to your system, the body doesn’t cycle back to drawing on fat reserves quickly, and my experience is downgraded once I’ve run through my glycogen stores. Usually I need to sleep to reset everything, and running out of fuel mid evening is no fun.
The other downside is that as far as I can tell, gluconeogenesis, the process by which body fat is converted into sugar for use in the body, doesn’t take place fast enough to keep up with very intense activities, particularly those that go on for hours.
I feel like I can hike or bike with ample energy for hours, but during longer runs I do feel like I eventually outstrip my supply of fuel and my body’s ability to generate new fuel from fat.
I feel like, even after more than a year of experimentation, I still don’t have a good handle on fat adaption and the super-short eating window I’ve spoken about here. I doubt I’ll ever give up intermittent fasting with a longer eating window, but so far I haven’t found a way to make the shorter window feel effortless and continuously maintainable due to my regular desire for all-out athletic activity.
However, because I’m so fond of the fasting state I plan to continue with my experiments to see if I can find some variation of this that’s sustainable.
Your Own Experiment
If you’re interested in running a similar experiment (and it’s certainly not something I think your average person should do, or should even be interested in), expect a significant amount of adaptation time during which you’ll feel less than your best mentally and physically.
I’d suggest you:
- Spend time with longer versions of intermittent fasting first so you get a feel for what’s involved. If you’re not a healthy eater you’ll likely have headaches or other withdrawal symptoms during fasting.
- When you’re ready to get started, limit your feeding window to two to three hours in the evening. The shorter the window the better.
- Make sure you’re eating a weight-maintenance level of calories so you can desperate out the effects of a calorie deficit from the effects depleted glycogen stores.
- Expect it to take at least a week week before you’re adapted enough to really feel like you’re thriving on this regime.