I don’t usually do this, but I’m starting this article off by answering the question stated by the title. And I’ll do that by giving you a very definite maybe. 

The research is kind of split on aerobic training while fasted vs while fed — leaning more towards fed being the way to go. However, there are some instances and circumstances where fasted aerobic training could be useful for some hunters. Which brings us to a new and better question: When and why would a hunter do fasted aerobic training?

What’s the Point of Fasted Aerobic Training?

One of aerobic training’s main goals is to increase and improve fat utilization, meaning you get better at burning fat for fuel during low- to moderate-intensity exercise. It’s important for endurance because, even untrained, you have way more fat stores in your body. And as your conditioning improves, relative intensity shifts so that you end up spending more time moving at low- to moderate-intensity. You get more efficient; movement costs you less, and it’s easier for you to utilize fat for fuel instead of working at higher relative intensities where your muscles burn carbs to get the work done. You run out of carbs fast, and they need to be replenished often.

The folks who do the science have also shown us that our body preferentially burns available carbs even at low- to moderate-intensity exercise. Available means they’re in your belly. That makes sense though, doesn’t it? Your brain and body’s main goal is survival, so why wouldn’t it burn the cheap stuff first and save your stores (of fat and carbs) for when you really need them? Say, in the case that you have to walk for days to find more food. 

This leads us to a sensible follow-up question: 

Does training fasted improve your ability to use fat for fuel because your body won’t preferentially burn carbs? 

In this case, improve means increasing your intramuscular fat stores and the efficiency with which they’re accessed for fuel. It’s a very good thing. 

Now, before we take this chat any farther, we have to understand what fasted vs fed means in research terms. Fasted means no food overnight for 8 to 12 hours and exercise first thing in the morning. Fed means you have food in your stomach to varying degrees. It could mean you had a meal a couple of hours before training. It could mean that you ate carbs throughout your workout.

Armed with our definitions, let’s move forward to talk about what aspects of your physiology improve fat utilization.

What are the Main Drivers of Efficient Fat Utilization?

God bless The Magic School Bus. Ms. Frizzle (fellow ginger, by the way) and her band ultracurious kiddos taught Millennials like me that the mitochondria are the powerhouses of the cell. That shit’s been burned into my brain since 1997. And it’s important to understand that fact for our current conversation. 

The number and quality of mitochondria in your slow-twitch muscle fibers is one of the main drivers of efficient fat utilization. Why? Because every step necessary to convert fat to ATP (Adenosine triphosphate is the full name. It’s the main source of energy in your body) occurs within the mitochondria. The more you have, and the better they work, the more efficient you become at utilizing fat for fuel. It’s important to note that mitochondrial quality is just as important as mitochondrial density. Training improves both. It increases the number of your mitochondria while also helping your body discard mitochondria that are no longer pulling their weight.

The process of converting fat to ATP is called beta-oxidation. Efficient beta-oxidation is reliant on catalyzing enzymes. An enzyme called CPT-1, located in the mitochondrial membrane, shuttles fatty acids into the mitochondria. Once fatty acids are inside the mitochondria, four enzymes catalyze the reactions that convert them into ATP. Aerobic training increases the activity of some of these enzymes, including CPT-1, making the reactions more efficient and speeding up the process of turning fat into usable energy. 

All of this mostly takes place in slow-twitch muscle fibers. Slow-twitch fibers naturally have more mitochondria and aerobic enzymes, and are much more trainable to increase the number and efficiency of each. Slow-twitch fibers also have denser capillary beds than fast-twitch fibers. Capillaries are the smallest blood vessels in your body. They transfer oxygen and nutrients into your muscles and remove metabolites from your muscles to be discarded or recycled. Aerobic training improves your ability to recruit slow-twitch muscle fibers, making it easier for you to develop your mitochondria and aerobic enzyme activity while increasing the density of your capillary beds. Each adaptation contributes to improved endurance. It’s a nicely designed system.

Slow-twitch muscle fibers also have greater amounts of intracellular triglycerides. These are fat stores within your muscles, and aerobic training increases them, making fat more easily accessed for fuel. This improves efficiency, and subsequently, endurance.

Now, we have to ask, what’s the main training variable necessary to develop mitochondria, aerobic enzyme activity, slow-twitch fiber recruitment, and intracellular triglycerides?

The answer is consistent accrual of aerobic training volume over time, regardless of whether there’s food in your system or not.

Knowing all of this begs the question, what does happen when you train fasted?

What Happens During Fasted Aerobic Training?

Well, you do utilize more fat for fuel. Your body increases fat usage – which is called lipolysis – to spare carbs and keep them available for work at higher intensities. As part of this deal, there’s increased activation of your AMPK pathway, which is responsible for mobilizing fuel for use in the body. It does this for carbs and fat. In the case of fasted training, it essentially says, “Hey, we’re moving and we don’t have anything in our stomach. I gotta pull from the fuel stores.” 

But here’s the thing. There’s limited evidence that increased lipolysis and AMPK are chronic adaptations – meaning that they last. Mostly, it seems, they’re acute. Which means that they are ramped up just because you don’t have fuel available at the time. However, there is some nuanced evidence that fasted aerobic training leads to chronic fat fuel efficiency.

In some studies, increased AMPK signaling during fasted exercise increased gene expression associated with fat metabolism, metabolic flexibility (the ability to use carbs or fat as fuel), and how adipose (fat) tissues interact with hormones associated with fat metabolism. It seems like repeated exposure can help these things stick. But this stuff is all minor. Training fasted to try to get these adaptations would be like trying to put extra icing on a cupcake that’s already delicious. Accruing aerobic volume over time is what improves the main drivers of fat utilization.

Besides, the studies showing these results were pretty short. The longest was six weeks, the shortest was only one day.

There are also some negative effects of fasted training.

Training stamina and quality decreases. Makes sense, right? You just don’t have as much fuel to use, that limits the effort you’re able to expend. It also limits resources available for recovery. These factors combine to limit the amount of training volume you can handle, which is very no bueno. Total aerobic training volume accrued over time is the biggest factor for increasing fat utilization and carbohydrate sparing. You’ll use fat more efficiently and burn fewer carbs just by increasing your aerobic training volume over time. And the best way to do that is by having some food in your stomach.

That leads us to the next part of our discussion. 

What Happens When You Train Fed?

Honestly, a lot of nice stuff.

Your perceived level of exertion decreases. That means work doesn’t feel as hard. Consider that in the context of long aerobic workouts, which are necessary for full development of your aerobic system. If you’re out pounding pavement for two hours trying to stay in Zone 2, it will feel much harder to maintain your pace. This creates a mental battle to maintain your pace (which is sometimes beneficial, more on that later) that costs you even more energy, making you less efficient. It becomes harder to get in the volume necessary to improve your conditioning.

Training fed also decreases neuromuscular fatigue. Having an adequate amount of carbs in your system stabilizes your glycogen levels. This essentially signals your central nervous system that it’s okay to keep firing your muscles because the risk of exhaustion decreases. It’s a survival mechanism. Your brain’s main goal is to keep you from putting yourself in situations that could kill you. It puts the brakes on when it senses that you don’t have enough fuel.

Speaking of mechanisms, adequate glycogen levels help your muscles contract by ensuring that calcium is released into the muscle cell. Without enough glycogen, calcium release is limited and muscles don’t contract with as much force or for as long. 

Then there’s the big thing we need to handle all of the training necessary to improve our conditioning – recovery. 

There are a lot of things that training with adequate carbs on board does for your recovery. The largest being that it limits the overall physiological cost of the workout. You deplete yourself less, so you have more resources onboard to use for recovery. Carbs eaten before and during aerobic workouts also limit your overall stress response by dampening cortisol, reducing the inflammatory response, and limiting the fatigue of your central nervous system. They do more than that, but those points fit for the scope of this article. And, truthfully, they all harken back to the first point – limiting the overall physiological cost of training. Less stress = more resources to use for recovery.

So, What Makes the Biggest Overall Difference for Fat Utilization?

Here’s the answer in one sentence:

Accruing a lot of quality aerobic training volume over time.

Folks who do end up with a well-developed aerobic system. This gives the more intramuscular fat stores, more and better mitochondria, more aerobic enzymes, and a better cardiorespiratory system. All of these factors contribute to more efficient fat utilization. Eating carbs before or during a workout doesn’t limit them; it enhances them.

With all this knowledge onboard, we have to answer one more question. Does it ever make sense for hunters to do fasted aerobic training?

Should Hunters Ever Train Fasted?

I’ll echo what I said during the introduction – maybe in some cases.

Before I get into those cases, I’ll tell why it could be valuable.

When we’re in the field, we often hunt hard in a big calorie deficit for days or weeks at a time. Not only does this take a big toll on our bodies, but it’s also mentally challenging. Getting used to moving for long periods while unfed can help train a hunter to deal with the psychological discomfort. And since there is some evidence that fasted training can enhance how muscles use fat for fuel, we can put some extra icing on the cupcake for those who already have well-developed aerobic systems and are going into gnarly environments.

So, if a hunter has a well-developed aerobic system from accruing consistent aerobic training volume over the course of months or years, a handful of fasted sessions spaced out throughout the summer could be useful. 

However, if a hunter is in the process of developing their aerobic system, fasted sessions are a bad idea. The cost of the session will be too high, their recovery will be limited, and it will make it harder to accrue the quality training volume they need.

Because here’s the thing, to get the benefit, a fasted training session must go for 60 to 120 minutes. An aerobically tuned hunter could handle that if they’re normally well-fed and replenish wisely after the session. If a hunter’s not there yet, not only will the training session be lower quality, but a 60- to 120-minute fasted session will likely put them too far behind the recovery curve. So, the chance of them adapting to the session are slim, and they likely won’t have the gas to bounce back, limiting their ability to put quality effort into their next few training sessions.

Now, here’s one more question that’s likely on your mind.

Will I program fasted aerobic sessions for Packmule members?

If they meet the above criteria and everything else is in line with their training, lifestyle, and nutrition, yes. But only at the short end of the time spectrum and only a handful of sessions throughout the summer.

To Fast or Not To Fast?

Fasted training to improve fat utilization seems logical, but the evidence to support it for most hunters most of the time just isn’t there. Hunters with well-developed aerobic systems could use it as a little extra icing to work on their mental game and to prepare to be under fed in the field. But the best thing to do long-term is aerobic training with some carbs onboard.

References

1. Bartlett, J. D., Louhelainen, J., Iqbal, Z., Cochran, A. J., Gibala, M. J., Gregson, W., Close, G. L., Drust, B., Morton, J. P. (2013). Reduced carbohydrate availability enhances exercise-induced p53 signaling in human skeletal muscle. Journal of Physiology, 591(18), 4537–4551. https://doi.org/10.1113/jphysiol.2013.257857

2. Hansen, A. K., Fischer, C. P., Plomgaard, P., Andersen, J. L., Saltin, B., & Pedersen, B. K. (2005). Skeletal muscle adaptation: training twice every second day vs once daily. Journal of Applied Physiology, 98(1), 93–99.

https://doi.org/10.1152/japplphysiol.00163.2004

3. Impey, S. G., Hammond, K. M., Shepherd, S. O., Sharples, A. P., Stewart, C., Limb, M., Smith, K., Philp, A., Jeromson, S., Hamilton, D. L., Close, G. L., Morton, J. P. (2018).

Fuel for the work required: a theoretical framework for carbohydrate periodization and the glycogen threshold hypothesis. Cell Metabolism, 28(1), 1–14.

https://doi.org/10.1016/j.cmet.2018.06.012