Sick of paying visits to “Bonk Town”?

You’ve planned meticulously and done the training, sacrificing time with friends and family. You’ve made it to race day in one piece, fitter and stronger than ever. As you start your race, everything is on track, but as time goes on things begin to fall apart. Your power and energy are dropping; you feel like you’ve got nothing left to give. You try to keep to your fuelling plan, but something isn’t right – Bonk Town is just around the corner. You know it’s going to be a tough slog to the finish. Sound familiar?

What is bonking?
According to Wikipedia, “In endurance sports, particularly cycling and running, hitting the wall or the bonk describes a condition caused by the depletion of glycogen stores in the liver and muscles, which manifests itself by precipitous fatigue and loss of energy.”

Do you know if you are a sugar burner or a fat burner?
A key indicator that you’re a sugar burner (and therefore more likely to experience the bonk) is the number of times you bonk in training/racing and how much you crave sugar or carbohydrates. If you are a trained fat burner, you will notice needing to carry less fuel than your comrades; bonking is rare, as is craving all things sweet or starchy.

As an endurance athlete, you want to work towards being a fat burner. This will give you the metabolic advantage.

The most precise way to determine if you are a sugar or fat burner is through Metabolic Efficiency Testing (MET).

This testing protocol will give you the data to build an accurate nutritional plan, for both everyday eating and race day fuelling – a plan that suits your physiology, prevents bonking and allows you to express your full potential while also working towards optimal body composition and metabolic health in training.

What is a MET?
MET is a test completed in a laboratory setting that measures your use of oxygen (O²) and production of carbon dioxide (CO²) to determine how many calories you burn at varying intensity levels and how many of these calories are coming from fat and carbohydrates. The tests are done both at rest and during exercise using either a bike or a treadmill.

The resting tests determine your specific basal metabolic rate as well as, assess your calorie, carbohydrate and fat requirements on a daily basis for either, maintenance, body fat loss or lean muscle gains.

The exercise test will start easy and gradually increase in intensity until you reach your VO²max (your max ability to use oxygen to produce energy).

Throughout the test, a key data point is monitored – referred to as your Respiratory Quotient (RQ). This number is displayed on a scale of 0.70 – 1.0. Your RQ is the number used to calculate gas exchange (carbon dioxide and oxygen), which is then used to calculate fuel utilisation at varying intensities.

RQ = CO² eliminated/O² consumed

An RQ of 0.70 represents pure fat oxidation
An RQ of .85 represents the crossover point (COP), at which you burn more carbohydrates than fats
An RQ of 1.00 represents pure carbohydrate oxidation

At each RQ we want to pay attention to your heart rate and calories burned. From there we can start to establish a fuelling plan for these varying intensities.

In figure 1 and 2 below, COP is represented when the green and orange line collide. The green line represents carbohydrate burning at given heart rate intervals; the orange line represents fat burning.

Athlete 1 in figure 1 is a sugar burner, who has a COP below their Aerobic/Endurance HR Zone.

After a period of fat adaptation through training (low intensity), nutritional and stress management variables, we would want to see this athlete’s COP shift to the right.

Athlete 2 in Figure 2 is a fat burner, who has a COP above their Aerobic/Endurance HR Zone.

This is a great result and is very beneficial for an endurance athlete. If this athlete wanted to optimise their potential even more, we would want to reduce the spike in carbohydrate utilisation after their COP – ideally showing a more gradual increase/curve. This would be achieved by specific fat adaptation protocols and training variables that involve intensity.

What are the benefits of being a fat burner?
Do you like visiting the petrol station? I avoid it and end up getting myself into trouble or running late. I don’t like the cost or the time required to fill up with petrol. I also don’t need the temptation for lollies, chips and chocolate! In the following example think of your body as the car, the fuel tank as your metabolism and the petrol as energy/fuel from sports nutrition. If you had access to a fuel tank with ~60,000 calories available to you to use for energy during a race, or you had a tank with ~2,000 calories available to you, which one would you prefer? The 60,000 calorie tank right?

An endurance athlete can store between 50,000 and 80,000 calories stored as fat, but only 1,000 to 2,000 calories as carbohydrates. Once an athlete’s intensity goes over that RQ of 0.7 they will be tapping into this short supply of carbohydrates (CHO). Once they reach an intensity or RQ of 0.85 – carbohydrate utilisation is amplified – making it very difficult to replace the amount of carbohydrate being burned. This is why typical sugar burning endurance athletes require exogenous sources of fuel, forcing them to continually stop in at the petrol station or run the risk of checking into Bonk Town.


For events like Ironman, this makes the logistics of carrying fuel and getting enough fuel in, very difficult.
As a fat burner, your physiological ability to burn fats for fuel at higher intensities is enhanced while minimising your body’s reliance on carbohydrate utilisation for long-term fuelling.
This means you don’t need as much exogenous fuel to continue exercising/racing at a given intensity.

This is beneficial for many reasons:

  • Stable blood sugar: less energy and mood fluctuations – also leading to better concentration and decision making
  • Metabolic health and general wellness
  • Blood supply is directed to your muscles (where it’s needed) as opposed to your digestive system
  • Minimise gastrointestinal distress
  • Less logistical nightmares of how to carry fuel during training or a race
  • Less money spent on sports nutrition products

Putting the data into practice
To establish a race pace and nutrition plan you will need:
1. Goal time/pace – give yourself a range.
2. Estimated Heart rate (HR) or Power (Watts) to achieve this pace

In the table below you will see real-world examples of two athletes MET data, summarised to show you the impact of being a fat burner vs a sugar burner for two different athletes planning for an Ironman 70.3.

In the case of Athlete 1, they are an inefficient fat burner at both rest and exercise. Based on their goal pace, and training heart rates, their goal was to sit at around 165bpm. Due to this heart rate sitting well above Athlete 1’s COP they are burning a significant amount more carbohydrates. For a fuelling plan, we want to aim to replace 50% of what is burned. So, the resulting fuelling plan would require 85g of carbohydrate per hour. Let’s say the athlete was aiming to complete the Ironman 70.3 in five hours, that’s a total of 425g of carbohydrate and if using a standard gel for fuel (containing 20g of CHO), would mean carrying 21 gels!

What should they do?
Well, they could plan to carry the 21 gels while sitting at the planned heart rate or they could adjust their goal pace/intensity to minimise carbohydrate utilisation – by sitting at a heart rate just below their COP and only requiring ~48g CHO per hour.

After this race, I would suggest this athlete completes a period of (about 12 weeks) base training (at a low heart rate) and make nutritional changes to become a more efficient fat burner at rest and exercise.

In the case of Athlete 2, they are a well-primed fat burner. To achieve their goal pace, they can sit around 167bpm. This is below their COP and would require 28.5 g carbohydrate per hour for their fuelling plan. Based on a five hour goal time, that is a total of 142g CHO and seven standard gels. Much more manageable. By needing less exogenous fuel, they minimise the risk of gastrointestinal distress, have stable energy levels and reduce the logistics. That is a huge difference – 13 fewer gels required for this athlete to fuel their optimal performance. It’s a huge cost saving – both energetically and in dollars.

Also, for athlete 2, if they wanted to push harder on race day or adjust their goal time, this would be very achievable without the risk of bonking.

What’s next?
If you’re interested in becoming a fat burner I would suggest speaking to your coach and a sports nutritionist. The next step is to find a provider for MET – I recommend Jupiter Health in Queensland and Bodyology in Victoria.

Often Universities are conducting studies and are looking for test subjects – so your local universities are a great place to start.


MET is a very specific test and data output that cannot be done via a standard VO2 Max test.
If you are unsure or would like some guidance, I’d love to help.


Katee Gray

Katee is a self confessed “Hormone Nerd” with a background in Exercise Science and a passion for Triathlon. She combines her knowledge or physiology, functional anatomy, and testing protocols from her Bachelor of Exercise Science with research from fields of hormonal balance, female reproduction systems and triathlon related studies specific to females to coach and guide endurance athletes, which ultimately led her to penning her book: “Healing The Grumpy Athlete” - Embrace your Hormones and Achieve your Athletic Potential.
For more information check out
Facebook: @Holisticendurance
Twitter: @KateePeds

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