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  • Edgar S.

CGMs: Boon or Bust for Endurance sports?

cgm on cyclist

This article reflects my experiences, perceptions, and philosophies as a coach. I am an enthusiast of technology applied to sports and athletes' well-being, and it is in this context that I present my reflections on the use of continuous glucose monitors (CGMs) by non-diabetic endurance athletes.

A Valuable Tool for Diabetics and Athletes?

CGMs have revolutionized many lives, especially those of diabetic patients and athletes living with the condition. Their ability to constantly monitor blood glucose provides a detailed overview of blood sugar levels, allowing for precise adjustments in diet, medication, and training routines.

cgm reading

However, with the increasing marketing by some brands, the use of CGMs by non-diabetic endurance athletes has raised questions about their effectiveness and practicality for this audience.

Firstly, what is a CGM and what does it do?

It is a device that measures blood glucose levels every few minutes, 24 hours a day, 7 days a week. This provides a much more complete picture of glucose levels than traditional finger-prick tests, which only provide a reading at a specific moment.

It is typically applied (with its own applicator) to the upper arms and from this virtually painless application, a filament like a very thin needle then records glucose levels in the interstitial fluid for about 14 days (the duration of each sensor) which transmits the data to the phone in dedicated apps and in some cases even pairs with GPS devices like Garmin (watches and bike computers).

how cgm works

The acquisition of these devices represents a significant investment.

Many athletes have used this device to monitor blood sugar levels to determine when they should and need to eat, particularly during sports activity.

At the level of professional competitive cycling, the UCI has banned its use in competition, of course.

And now for the common mortals, the amateur cyclist, in events like ultra-marathons, ultracycling, or gran fondos, does this device offer quality and justify the investment?


If we stopped here, you would have only wasted your time  probably😊

I think it's important first to understand a bit about how the process of carbohydrate decomposition works in our body.


When we ingest carbohydrates, our body initiates a series of complex and fascinating processes to transform them into usable energy. This epic journey unfolds in several stages, each playing a crucial role in converting carbohydrates into fuel for our cells.

1. Digestion: The Swallowing of Carbohydrates, starting in the mouth, stomach, small intestine, and ending with absorption - Glucose;

2. Transport: Fuel for the Cells. Here insulin is key for glucose to enter the cells;

3. Regulation: Maintaining Glucose Balance: Insulin, Glucagon, Adrenaline, and Cortisol. These are the hormones that work together in regulation:

Insulin: Decreases blood glucose, storing it in the liver and muscles or converting it into fat.

Glucagon: Increases blood glucose, releasing glycogen from the liver or converting amino acids into glucose.

Adrenaline and Cortisol: In stress or "fight or flight" situations, they increase blood glucose to provide energy quickly.

insulin & glucagon

4. Excretion: Eliminating Byproducts, through the kidneys, excess blood glucose is filtered and excreted in the urine.


It is also worth mentioning the role of Metabolism: Transforming Carbohydrates into Energy. In muscle cells, glucose can follow two paths:

Glycolytic Pathway: The glycolytic pathway is the quickest route to convert glucose into energy, providing ATP (the cellular energy molecule) for immediate muscle contractions.

Anaerobic Pathway: During intense exercise, when oxygen is limited, glucose is converted into lactic acid, and byproducts such as hydrogen ions and pyruvate, resulting in muscular acidosis.


Well, in general, you now understand the process whenever we ingest carbohydrates.

Just to complement, before answering the big question, whether CGMs serve us or not, I explain a little how Insulin and glucagon work whenever we practice sports.

Insulin functions like a magic key, opening the doors of muscle cells, allowing glucose to enter and be used as energy. Imagine that each muscle cell is like a house, and insulin is the key that opens the door for sugar (glucose) to enter and feed the house.

Glucagon, on the other hand, acts as a messenger signaling the liver to release stored glucose, in the form of glycogen, if blood levels are low. Imagine that glucagon is like a phone that calls the liver asking to send more sugar into the blood.

In general terms, Insulin captures glucose, and Glucagon "releases" it.


But when we train, the response of one and the other depends on the intensity applied in the exercise, but let's imagine we had a training session of 3 or more hours. Minutes after we start, there is a decrease in insulin secretion, due to the increase in Glucagon, because our body, leaving a state of homeostasis, receptors indicate that we need blood sugar "like never before" and there Glucagon, as explained, makes the call and releases glucose/glycogen stored in the liver into the bloodstream, thus ensuring a state of normoglycemia.

And in this, we are emptying our reserves, first the hepatic ones and then at the musculoskeletal level.

sugar response to exercise

We store on average about 100-150g in the liver and 400-500g in the musculoskeletal structure. Therefore, as you can see, replenishment is important because this balance is quickly spent. But calm down, we also have other reserves and do not only depend on glycogen, this article here (again)helps to understand that.

It depends on the intensity and other genetic factors, how we respond to the absorption of carbohydrates, also regarding the amount stored and how efficient we are at metabolizing adipose stores (fat) 😊


Having said that, I will give my answer. Do CGMs serve endurance athletes?

yes & no

Yes and No. 😊

YES, because it can very well, in cases where there is a need to reformulate the athlete's diet and help him find the food that best suits him, as an athlete and equally support a healthy lifestyle, and help to meet a model of metabolic efficiency- know more about this here.

It can help us to know ourselves better. How we react to certain foods in our diet, to states of anxiety and stress, to understand if coffee interferes with our blood sugar levels, etc.

It can open a Pandora's box and with that, we obtain much more information and thus be able to reformulate our diet and our habits. But for this, we do not need to use a CGM for life. Of course not! I think 2 months is enough for us to record all the data and responses we get through our diet and states.

glycemic index foods


The NO, I reserved for a specific aspect. The use of CGMs to control our Carbohydrate intake during sports practice.


Here is where, for me, it is wrong to do so. From the point of view of possible imprecision/timing of the response in the concrete environment of long and intense training; in removing analytical capacity from the athlete and giving away the autonomy and knowledge of his own body and knowing by himself when and how much he has to ingest sugars.

It's just one more piece of data to tie us to the GPS or mobile phone display and we lose more and more of our ability to control ourselves and hand over that function to a device - which could very likely fail and be late.

So use it off the bike some few months to get to know yourself and how your bloodstream react to sugars and, on the bike or on the trails, don't use it to decide whether you should eat or not. Adjust your sugar intake according to time and intensity, or according to the specifics of the session and what it aims to develop.

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