The Facts about Fasted-State Training, Part II

More speechifying about exercise while unfed. Will it help prevent bonking during long efforts?

Welcome to the second part of our exploration of fasted-state training. In part one, we looked at using fasted-state training for weight loss. We concluded that it most likely doesn’t work unless you use it as a distraction to eat less during the day.

Now, let’s look behind curtain number two: Can endurance athletes use fasted-state training to shift their metabolism in a way that will help ward off bonking? The answer is a definitive “Probably!”

Science, anecdotal evidence, and common sense align!

Fasted-state training is a time-honored tactic for increasing oxidative capacity (muscle’s ability to use fat for fuel) in the endurance sport world.

Generally speaking, your body uses fat stores for easy to moderate efforts. It uses blood sugar and glycogen for intense efforts. Even thin athletes have body fat to last them for days. Sadly, we only have about 90 minutes of blood sugar and glycogen. When you run out, going really hard becomes really harder. We call that “bonking.”

But the shift between these two energy substrates isn’t binary. If that shift featured a light switch, it would have a dimmer.

Theoretically, fasted-state training encourages your body to push the dimmer over towards the body fat burning system so that it habitually spares your glycogen supply. That way, you can go longer and harder without bonking.

Studies confirm that habitual fasting may increase your body’s aptitude for fat burning. A pivotal 2011 Journal of Applied Physiology study split cyclists into two groups. One group exercised in a fasted-state. The other group carbed up pre-exercise. Both trained for six weeks, doing 60-minute or 90-minute rides, four times a week, at 70% VO2 max—which means at a legitimate intensity, if you don’t speak fluent VO2maxese.

By the end of the study, the fasted-state athletes showed a higher capacity for burning fat when exercising in a fasted state.

Considering there weren’t any performance differences between the two groups, this might seem kind of useless. But if you check your premise, you’ll see promise in these results.

During long, hard efforts, you burn more carbs than you can take in. If you’re properly trained and super smart about your feeding, you might be able to absorb 120 grams (480 calories) of sugar per hour. For most people, it’s more like 60 grams (240 calories) to 90 grams (360 calories) at most.

And, if you’re working hard during an endurance event, it’s reasonable to say you’re burning 600 calories per hour. Maybe more.

Do the math and you’ll see why completely running out of blood sugar and glycogen is super likely. As this happens—if you’ve trained your energy dimmer to slide towards fat burning—your glycogen should last longer.

The study doesn’t directly prove this. After an hour to 90 minutes, most of the participants probably had glycogen left. The study didn’t push the duration of training until everyone’s tank was empty—which is when performance really suffers. However, it’s reasonable to extrapolate that the fasted-state group would have taken longer to reach that depleted state.

Suddenly, the study seems kinda helpful.

Research aside, I know plenty of athletes who claim fasted-state training helped their endurance—myself included. Based on the science and on this anecdotal evidence, there’s no harm in trying it as long as you have a few guardrails in place.

Practical recommendations.

Aim for intense efforts less than 90 minutes in duration. You don’t want to bonk when fasted-state training. You just want your sugar reserves to be low so that your body looks to fat for fuel.

One blocker here might be if you’re on a low-carb diet or have too much of an overall caloric deficit. If you try a 90-minute fasted-state effort and completely run out of juice part way through, you might want to revisit your diet. Similarly, if you find yourself blowing up 30 minutes into a HIIT workout, that’s probably why; you need to eat more.

Long, moderate efforts in a fasted state should be okay, if not ideal. Word of caution though, whenever you leave the house for two plus hours of exercise, always bring a bar, gel, or banana with you—even if you plan to workout in a fasted state. There’s always a chance that you’ll miscalculate and bonk.

The one thing I don’t recommend is a long, deliberately hard effort in a fasted state.

You may have noticed that I’m putting a lot of emphasis on avoiding the bonk. I’m doing this for two reasons. The first is that I have a couple British friends who read NPB. “Bonk” has a more ribald meaning in the United Kingdom, so I write it as often as possible to amuse them.

The second reason is that, once you bonk (hehehe), the beneficial aspects of your workout decrease rapidly. While there might be some adaptive benefits, you won’t make performance gains—because you won’t be able to perform. You’re also increasing fatigue and therefore risk of injury.

Fasted-state training isn’t a must-do. You can still get great results and have a finally-tuned metabolism without it. But a lot of athletes use it and dig it, so don’t be afraid to give it a try. When you do, please let us know how it goes!