What Can we Learn from Continuous Glucose Monitoring – Part 1

A key aspect of the Athlete Nutrition Coach Program is helping the athlete understand what type of energy source they will be using for any given intensity – either at rest or for high intensity training or racing.

James Morton’s phrase “fuel for the work required” encapsulates this process quite simply which we discussed in our previous blog Fueling Your Training For Peak Endurance Performance.

The obvious aspect of fueling performance is the ability to fuel high intensity exercise and racing. How do you get to the end of a 3 hour race and still have fuel in the tank for the sprint finish?

However, for the athlete looking to improve their aerobic capacity and look for those gains which will drive endurance adaptations in that realm the question is “How do I improve my FatMax” 

We know that we burn fats and carbohydrates at different proportions when at rest and during exercise. Fat oxidation predominantly at rest with an increasing proportion of carbohydrate oxidation as intensity increases. 

The graph below indicates this relative contribution of fats and carbs for an athlete with anaerobic threshold of 288Watts – taken from an INSCYD test

Fat and Carb Oxidation Rates – INSCYD Test

Following the green line you can see the rate of fat oxidation increases gradually until it peaks at 187Wats and then drops away

The red line indicates carbohydrate oxidation rates which increases with intensity and beyond the anaerobic threshold it is the sole contributor of fuel.

For the athlete who wants to become a better fat burner (which will also have the knock on effect of becoming more efficient at burning carbs) there are several interventions which can help to increase Fatmax.

And a tool we are using with some athletes is a Continuous Glucose Monitor.

But glucose is a carbohydrate you ask – yes, but what we can see from high circulating glucose at rest is an INDICATION that an athlete is burning predominantly carbohydrates as fuel instead of fats. Something we often see with athletes who rely heavily on high intakes of processed carbohydrates and refined sugars.

If we constantly see sharp spikes and dips in blood glucose without any exercise we know the body is having to manage this high glucose by secreting insulin. Over time, an over exposure to insulin can lead to metabolic disturbances, inflammation and hormonal imbalances – hello hangry athlete!

Conversely – constantly dipping below the recovery range of blood glucose especially when trying to recover from exercise will impede glycogen replenishment and therefore ability to recover and perform on subsequent training days.

Of course – you don’t need a CGM to tell you you’re craving sugar, not sleeping well and dealing with inflammation. And luckily neither do we – but it does visualise it well.

Our Endurance Performance Nutrition Programs are designed specifically to manage nutrition and training load optimally, be that at rest or during high intensity training. Each program is bespoke to the athlete and will strongly depend on the adaptations you are seeking. 

And if your goal is away from training, i.e. reducing body fat, eradicating sugar cravings, dealing with hormonal imbalances or managing digestive issues we can help you manage this sustainably. All we ask is you have a target in mind when you speak to us.


Jeukendrup, Asker & Achten, Juul. (2001). Fatmax: A new concept to optimize fat oxidation during exercise?. European Journal of Sport Science – EUR J SPORT SCI. 1. 

Impey SG, Hearris MA, Hammond KM, Bartlett JD, Louis J, Close GL, Morton JP. Fuel for the Work Required: A Theoretical Framework for Carbohydrate Periodization and the Glycogen Threshold Hypothesis. Sports Med. 2018 May;48(5):1031-1048. 

Hearris MA, Hammond KM, Fell JM, Morton JP. Regulation of Muscle Glycogen Metabolism during Exercise: Implications for Endurance Performance and Training Adaptations. Nutrients. 2018 Mar 2;10(3):298.