Why You Hit The Wall and What To Do About It

By Josh O’Leary

Humans are seriously impressive endurance machines. We can out-run horses, chase down gazelle and regularly complete 100 mile races. Most animals can’t keep up with us over long distances. Predatory cats are masters of the all-out sprint followed (hopefully) by a big dinner and a well earned nap. Prey animals – deer, antelope, buffalo etc – can put in valiant evasive endurance efforts but eventually need to stop and catch their breath. Huskies! These guys are impressive and can certainly compete with humans…. but only in the arctic! Take huskies out of their natural environment and they wouldn’t stand a chance.

Anyone who has read Christopher McDougall’s Born to Run will be familiar with this narrative. The concept that evolution has shaped our bodies into the ultimate endurance specimens and that this is what separated us from the herd of other species and drove our worldwide domination is a compelling one, especially for runners.

This is the exact narrative that was running through my head in South Africa in 2010 during my first half marathon.

“Humans are capable of incredible feats of endurance!”

“You could run an antelope to exhaustion and carry it home to your tribe if you needed to!”


But unfortunately, my legs wouldn’t move. My calves had completely seized up. Every step felt like I was wading through thick custard and still had 5km until the finish line.

I had hit the wall.

Energy Availability

In order to understand why the dreaded ‘bonk’ is so common for the modern day, weekend warrior running population, you need to first gain an understanding of how the body uses and stores energy. The concepts of ‘energy availability’ and ‘metabolic flexibility’ have been areas of intense focus of mine over the past couple of years, and what I’ve learned has changed the way I see the human body. This is why I’ve made them a critical component of the Fit To Lead online course.

Once you have put in the work to grasp the science behind these concepts, you can understand how best to fuel your body to much greater levels of endurance capacity (plus, as an added bonus, you will be much healthier). Let’s get in to it.

Your body (mostly) produces energy from one of two substrates – glucose or fatty acids. There are some special exceptions to this rule – amino acids can be broken down in emergencies, and in certain situations ketones come in to play – but for all intents and purposes, you have two main sources of energy – carbohydrates and fat.

Why do we have two main – and competing – sources of energy? Evolution is a very stingy process, generally only preserving traits that provide the animal with functional benefits that help them survive and reproduce. This should suggest to us that there is a reason that we have two, very different, sources of energy. And this reason is that these two major energy sources have different functions. We need them both, but for different activities.

To expand on this, let’s consider how carbohydrates and fat are stored in our bodies. Our carbohydrate stores are very small, we can store about 300g of carbohydrates in our muscles and around 100g in our liver. Because 1g of carbohydrate produces 1kcal of energy when it’s burned, we can store around 1600 kcal of energy in the form of carbohydrate (if our stores are full).

Fat is a different beast. Humans can store huge amounts of energy in the form of fat. Let’s take the example of an 80kg man with 10% body fat (this is very lean – fitness model lean!). 10% of 80kg is 8000g. This man is carrying 8000g of fat on his body. 1g of fat, when burned, produces 9kcal of energy. This means that this super lean dude is carrying 72,000 kcal of energy in the form of fat. To put this in perspective, the average man burns around 2000 kcal of energy a day just being alive. So this guy could live for over a month off the energy stored in his body fat.

Now we should consider some other traits of these stores that might give us some clues about their functions.

First, fatty acids cannot enter your brain. There is a semi-permeable membrane between your blood stream and your brain, which keeps nasty stuff like pathogens from getting in to your brain. Fatty acids are too big to cross through this barrier, and so they cannot provide energy to the brain. This means that the brain needs an alternative source of energy.

Second, carbohydrates stored in your muscles cannot leave your muscles. Your muscle cells lack a key enzyme needed to break these carbohydrate molecules down in to a form that could leave the cell and float around your blood stream to another part of your body. This suggests that the purpose of these stores is to provide energy to those specific muscles.

Third, your muscles can only access these carbohydrate stores in response to the release of adrenaline. Adrenaline, as you probably know, is the “fight or flight” hormone, which is released to help you get yourself out of emergency situations.

Combining all this information, we can start to see a picture of how your body is designed to function optimally. You have massive stores of energy in the form of fat and small stores of energy in the form of carbohydrates – this suggests that most of the time, most of your body wants to be burning fat. You have small and locally restricted stores of energy in your muscles, which are there to provide a short burst of powerful energy to help you sprint away from a tiger or wrestle an enemy tribes-person to the ground. And finally you have your liver carbohydrates, which can be free to provide your brain with a steady source of energy.

This is how your body is supposed to use energy. A healthy physiology has what is called “metabolic flexibility” – the ability to go from low intensity, fat burning activity to all out carbohydrate burning effort and back again effortlessly. Metabolic flexibility is a state we should all be aiming for and is becoming rare is modern society.

So how does all of this apply to you and your desire to run long distances?

Most runners have taught their bodies to run on carbohydrates. They have been told that they need to eat a high carbohydrate diet to give them the energy they need to train and compete. I mean Christ, most runners will even carbohydrate load before a big race. The problem with this, as I’ve laid out above, is that no matter how much carbohydrate you flood your system with, you can only store a small amount. This means that you are likely to run out half way through your race.

This is “hitting the wall”!

The crazy thing is that your body has this huge amount of potential energy stored away in your fat but, through your diet choices, you have taught yourself not to use it!

So now you have a choice. Would you rather fuel your runs from the near-bottomless reservoir of energy that can potentially keeping you going for days or one that has enough energy to keep you going for a couple of hours? Would you prefer to complete marathons with no need to suck down energy gels and Lucozade Sports mid race? Would you like to make it through your running career without inflamed joints and a rapidly expanding waistline?

If you answered “Yes!” to any of those questions, you have only one option, you need to become a fat burning machine.

You need to teach your cells, muscles and tissues to become super efficient fat burners. You need to encourage them to up regulate gene expression so that fat metabolism becomes their preferred choice and they have all the enzymes they need to perform this task optimally. You need to ensure that every gram of liver carbohydrate is saved for your brain to give you extra willpower in those last few miles. You need to reserve every gram of carbohydrates in your muscles for the sprint finish.

This process is called “fat adaptation” and it is a critical component of our online course. In fact, it’s so important to all areas of healthy and performance that we teach it FIRST.

There are already ultra endurance stars that have figured out how to tap in to the incredible power of fat adaptation. People like Timothy Olson – two time winner of the Western States 100 – and Zach Bitter – current American 100 mile record holder. These guys have ditched the carbohydrate-centric dogma that has gripped the endurance world for too long and are seeing incredible results.

It’s time for you to do the same.

Intrigued and want to know more? Sign-up here and we will send you a whole module from the Be Your Best In Life online course! The module will go deeper in to the detail of energy use and how best to fuel your athletic endeavors. Everything you need to know to become an endurance machine!

* indicates required

Leave a Reply