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Measuring carbohydrate and fat burning rates during exercise

How can you measure the number of carbohydrates and fat you use during exercise?

As part of our metabolic and VO2Max Testing, we measure Respiratory Quotient (RQ) or Respiratory Exchange Ratio (RER).

RER highlights what your body is using for energy and is the ratio of CO2 exhaled (L/min) divided by the amount of O2 inhaled (L/min).


When you are using solely fat, the RQ is 0.7, whereas 100% energy from carbohydrates has an RQ of 1.0. A ratio of 0.85 indicates a fuel mix of 50% carbohydrates and 50% fat.

As intensity increases, the RER will go up, and so does your reliance on carbohydrates.

And this data can be instrumental!

But how?


If you can calculate how much fat and carbohydrates you are using at different intensities, we can calculate how many carbohydrates you will need for training and racing.

It’s not just about fuelling

1. Race pacing

We can also highlight at what speed or power you will be using roughly 90g of carbohydrates per hour through Metabolic Testing. Knowing this is important because 60-90g/h is the upper ceiling of how many carbohydrates your body can use.

If we can determine that you burn 90g per hour when you produce 240w on the bike, this will be the power that you could theoretically sustain by fuelling at 90g per hour.

At an intensity (speed or power) above this point, you will not be able to keep up with the carbohydrate usage, meaning you will be tapping into your muscle and liver glycogen stores, which will eventually run out. Carbohydrate usage can easily be more than 250g per hour! So you’re not going to last very long at these intensities. For shorter distances, this is not going to matter too much, however, for long runs, rides and especially during races, you want to make sure you don’t bonk i.e. run out of fuel.

It would help if you also accounted for your glycogen storage capacity (how many carbohydrates you store), which will affect your pacing and fuelling.

2. Intensity reference points – your aerobic and anaerobic thresholds

RER is another way to pinpoint important measures of intensity, aerobic and anaerobic thresholds.

Knowing your aerobic threshold is essential as it guides your low-intensity training. It represents the point at which you begin to produce lactate (a product of burning carbohydrates) and represents the top zone 2 (usually) or where you should be performing the majority of your endurance work. If you’re a long course triathlete or ultra runner, this is also the intensity you will usually race at.

Your anaerobic threshold, also known as your lactate or ventilatory threshold 2 (there are more names haha), is the point you solely rely on carbohydrates as a fuel with an RER of 1. The anaerobic threshold is crucial as it marks the tipping point (more of an area) where you enter the severe domain where anything above means you will be producing lactate faster than you can clear it. Above this threshold is where your high-intensity intervals will lie.

To Sum Up

  • RER helps us determine whether you’re burning fat or carbohydrates as a fuel

  • Knowing this can help with fuelling strategies for training and racing

  • RER can also help with race pacing

  • RER is also a measure that helps us determine training zones and intensity reference points

Measure your RER with metabolic testing

Out metabolic efficiency test measures your carbohydrate and fat burning at different intensities. From this test, we can help you with your fuelling strategy, as well as guide you with your training zones.

Our process follows a simple formula to remove any guesswork and provide you with a clear plan for your training.

  1. Test your metrics (VO2Max, AeT, AnT, Economy and fat burning) and build your metabolic profile

  2. Establish your training parameters. Provide you with heart rate or power-based training zones based on your results

  3. Identify strengths and weaknesses to provide targeted training and nutrition recommendations around you and your physiology

  4. Fuel – Help you understand how to eat for your training

If you’re interested in booking your VO2Max Testing, send us an email for more information or book now.


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