Continuous Glucose Monitoring: The physiology behind a game-changer

It’s rare that a product hits the market with potential to transform the way cyclists think about nutrition, exercise and recovery. But continuous glucose monitoring (CGM) looks set to do just that. Scan the history books and change in cycling is more evolutionary than revolutionary.

From STI shifters and clipless pedals to power meters, the well-trodden path is a product taking a foothold with the pros before slowly trickling down to us recreational riders. That’s been ripped apart by a young start-up called Supersapiens and their ground-breaking CGM device – a product backed by a galaxy of entrepreneurial, academic and sporting stars. But what exactly is CGM and how can you use it to improve your performance both on and off the bike?

A Brief Background in Continuous Glucose Monitoring

James Witts gives you more background on Supersapiens here, but as a sector, CGM systems were introduced in the early 2000s with the first models being retrospective, meaning that data was only available at the end of the sensor’s life cycle. 

Related – What is Supersapiens?

That changed in 2008 with the creation of sensors, primarily for diabetics, that streamed live data. They worked by using a wired glucose oxidase enzyme co-immobilized on an electrochemical sensor that’s worn on the arm for up to 14 days. That sounds technical – and it is. These are highly engineered and incredibly accurate monitors that’d withstand the rigours of normal life for two weeks. Now, thanks to Supersapiens, they’ll withstand the rigours of sport, too. 

This type of patch sensor is the size of a €2 coin and has a short pin (4mm long) that must be inserted into the subcutaneous tissue of the upper arm. Once in place, the CGM sensor continuously measures glucose concentration in the interstitial fluid just below the surface of the skin producing an ambulatory glucose profile. And, as you’ll find out, it’s this glucose profile that’s of great importance for sustained good health and optimal athletic performance…

THE OFF HOURS

What a rider does in the hours before and after training (known as Off Hours in Supersapiens) has a significant impact upon the stability of their blood-glucose profile. That’s because blood glucose is a moving target; it ebbs and flows hour by hour depending upon what you eat and when you exercise. And it’s this flux that’s of critical importance because the aim is to avoid steep rises and sharp drop-offs in blood glucose at all times.  

Why? Because extreme peaks and troughs have been linked to increased oxidative stress, inflammation, increased morbidity and cellular effects that impair exercise performance. As an example, roller-coaster blood-sugar levels can lead to mitochondrial dysfunction. That’s not good because as an endurance cyclist mitochondria are your best friend. They are the energy-producing power stations that reside within your muscle, and it takes many weeks of long endurance rides to increase the population of mitochondria within your legs. More mitochondria = more energy = better cycling performance. So clearly anything that interferes with their function is not a good thing. 

This is where CGM can help by making the ‘off hours’ more conducive to good health and optimal recovery. By following the simple rule of minimising spikes in blood glucose throughout the day, you limit the amount of oxidative stress placed upon your aerobic engine. Diets that have a higher glycaemic index (foods that are broken down rapidly and cause a sharp increase in blood-sugar levels) accelerate the roller-coaster effect, as well as raising the risk of cardiovascular disease, accelerating the ageing process and increasing inflammation. In contrast, a lower glycaemic-index diet is linked to healthier outcomes. All you have to do is experiment with the type, quality and timing of your meals then sit back and watch how the CGM relays data about the speed at which you’re digesting each meal. 

Related – Training and Heat

THE ON HOURS

Where CGM will have an even profounder impact on athletic performance is within training and competition, which will become increasingly apparent in endurance and ultra-endurance sports. In these longer events the challenge has always been to finish with just enough fuel to manage your performance expectations. But, seen through a professional lens, if like Mark Cavendish you want to win a sprint, then you must have adequate glycogen stores to launch a 250m maximal effort.

If you’re a GC rider like Tadej Pogacar, you must manage daily energy levels for several weeks to ensure you fuel enough to conserve your finite internal energy supply. But, whether you’re a pro or recreational rider, how do you know when to eat while training and racing? What fuel should you choose while doing high-intensity intervals? Is the intensity of your fat-burning ride appropriate for fat to be liberated from storage and burnt as fuel? The answers to these questions and more can be found via experimentation with CGM. Which is why, in my mind, it enjoys game-changer status. 

Chasing Ventoux: Rouleur's 3-month training plan – Part One

CUSTOMISED NUTRITION

I live in Germany and often drive up and down the Autobahn network. When I’m on the road and I see the light next to my fuel gauge is on, I start searching for the nearest fuel station. That light tells me I don’t have much time before my car will run out of fuel and it’s a good idea to fill up as soon as possible if I want to reach my destination on time. Now imagine if you could have access to such an early warning system while training or racing? This is the promise of CGM and, right now, protype wearables and cycling head units exist that perform exactly this function. Hopefully, they’ll be hitting the mass market soon.

You might contest that you’re up to speed with sports-nutrition protocols, that you already have proficient refuelling and recovery strategies in place. That’s great but one of the biggest benefits of CGM is that it’ll really pin down individual differences. For instance, not everyone processes the maltodextrin in many sports drinks at the same rate; when it comes to something that’s slower burning, individual variations are multiplied further. CGM shines a light on how your digestive system processes food, and you can use this information to customise every aspect of your pre-, during- and post-ride nutrition. 

PRE-RIDE FUELLING EXAMPLE

A basic pre-event fuelling plan for a stage racer looks like this…

> Around 3.5hrs before the start, a rider will have a large carbohydrate meal.

> Around 1hr prior to the start, they’ll consume a small carbohydrate snack. 

> From 3.5hrs to the start it’s recommended that fluid consumption is water only.  

And that’s it. But let’s dig down deeper. How much carbohydrate should be included in the 3.5hr meal? Should all riders on a team consume the same volume and mix of carbohydrates before the start? Does a domestique on a flat stage need to eat more than the protected GC rider? Or what exactly is the best 1hr pre-race snack that’ll elevate blood sugar at a slow rate and avoid any major troughs (rebound hypoglycaemia) before the start? 

It’s the same process for a recreational rider and their Sunday-morning long ride, and it’s by working with a CGM device and a little trial-and-error that answers to these and many more questions can be found. The future of sports nutrition will be this level of customisation with CGM playing a central role.

IN-RIDE FUELLING EXAMPLE

In much the same way that pre-exercise nutrition can be individually tweaked by using CGM so too can on-the-bike fuelling. The basic guidelines of 60g glucose + 30g fructose per hour work well. But individual differences and where a rider is in their training programme requires greater precision. Although there are differences between interstitial glucose and blood glucose, the day isn’t far away where CGM data will be relayed to your sports watch or cycling head unit and a little red light will go off indicating that it’s time to eat or drink. Or, linking to earlier, just like going to the petrol station. 

Knowing what you need to eat and drink (and when) under race conditions will give you a better chance of finishing your event strong. Of course, some programming needs to be done to account for the lag between interstitial and blood readings while gut transit time also needs to be accounted for. But a future where bonking doesn’t occur is tantalisingly close. And I for one cannot wait for this disruptive technology to become mainstream.

Chasing Ventoux: Rouleur's 3-month training plan – Part Two

IMPORTANCE OF GI AND GL

Central to understanding your own glucose profile is knowing what foods deliver high, medium or low spikes in blood glucose. Although the fat and protein content of a meal will have an effect on the magnitude of blood glucose peaks and troughs, it’s carbohydrates that play the biggest role. The Glycaemic Index (GI) of carbohydrates is simply a score of 1 to 100 where a food with a higher number is digested faster than a lower scoring alternative. It’s a simple system that’s based on the glucose standard. That is: glucose scores 100 because it’s digested the fastest of all carbohydrates while root vegetables can be as low as 20. The GI scale is a good starting point for anyone interested in experimenting with their diet and the effect of various food combinations on CGM readings. 

However, understanding the concept of glycaemic load (GL) and applying it to your CGM journey will elevate performance to the next level. GL was developed to simultaneously describe the quality (GI) and quantity of carbohydrate in a food serving or main meal and is calculated by multiplying the GI by the amount of carbohydrate in grammes (g) provided by a food serving and then dividing the total by 100. 

GLFood = (GIFood x amount (g) of available carbohydrateFood per serving)/100

Although it’s a little more time consuming to calculate GL, it’ll give you a better indication of how the type and quantity of carbohydrate you consume impacts upon your blood-glucose levels. Then the CGM system will report the live data, ensuring you’re on your way to fine-tuning your personal intake. Just remember that steep spikes and sharp drop-offs in blood glucose should be avoided. It’s time to eat your way to better biking…