Continuous glucose monitoring (or CGM) is a body-worn “wearable” device that measures and displays interstitial sugar (glucose) levels. This technology has been used as a glucose management tool by individuals living with diabetes for decades but is now becoming available for people without diabetes, including endurance athletes, to help fine tune their nutrition and training activities. In a series of blogs Dr Mike Riddell and I will explore what CGM exactly measures, how accurate it is and how it can be used in sport. In this first blog we will describe what CGM is and how it evolved.
What is CGM?
CGM is a technology that allows for the continuous measurement of glucose in the interstitial space that surrounds the body’s cells. There are several devices on the market, but the most common sensor used by athletes today sits on the upper arm and provides minute by minute data on glucose levels over a 2-week wear time that can be viewed in real time on a watch, receiver, or smartphone.
If the phone, watch or receiver device is not near the sensor when you are wearing it (within a meter or so), the sensor can still store data but only one value every 15 min. Software allows for deep data analytics, such as average glucose level, percent of time spent above or below a certain glucose range, how certain foods impact the user’s glucose levels and more. These calculations are done both in real-time, during the wear time, and after the end of wear time (called retrospective analyses). We will discuss some of the practical aspects of CGM in a separate blog.
How does CGM work?
The CGM consists of a body worn sensor, which is a thin filament that penetrates the skin to sit in the interstitial space just below the skin and a transmitter attached to the filament that sticks on top of the skin. The insertion of the sensor consists of a sterile microneedle guide that punctures the skin for a brief second, then retracts, allowing for the sensor to sample fluid from the interstitial space. The measurement of glucose in the interstitial space largely reflects the glucose level in the bloodstream. The sensor is constantly analysing circulating glucose levels and send the results to a smartphone or watch via Bluetooth connection. Software then displays the value and helps with the interpretation of the results.
The measurement of glucose in the interstitial space largely reflects the glucose level in the bloodstream.
Is CGM new?
CGM is not new. But the technology has been improving dramatically over the last few years. It was first used in the field of diabetes in the early 2000’s but the devices were much larger, could only be worn for 3 days and the data was only retrospective (you couldn’t see the glucose data in real time on a watch, phone, or receiver) (1). The technology has developed such that the CGM devices are much smaller, near painless and much more user friendly with more advanced analytical features. The devices are also much more accurate today, with the technology allowing for decision-support and automated insulin delivery for people living with diabetes who use insulin pumps. Now CGM is ready for more widespread use, as the costs, accuracy and access improve. In a separate blog we will discuss the accuracy of the technology in more detail.
Is blood glucose important?
The measurement of circulating glucose has been a critical element of both medicine and the sport sciences. In the average sized adult, only about 4 grams of glucose floats around in the bloodstream, playing a critical role in health and performance (2). Having too low or too high a glucose level in circulation can be a symptom of disease (like diabetes or reactive hypoglycemia) and it’s well established that glucose levels need to be maintained in a fairly tight range for both health and performance.
As we will see, in this series of blogs, the regulation of circulating glucose is complex, depends on many factors and we must distinguish between the acute changes in blood glucose and chronically elevated levels of blood glucose.
Major advances in CGM technology
Because glucose levels can change rapidly and can vary across the day depending on a number of factors, scientists and clinicians have been interested in using CGM for decades. And technologies have continued to improve since the late 1990s (1). The earliest devices looked like a Walkman and had a long cable attached from the sensor place under the skin to the recording device. It was painful to insert, not waterproof and gave only 3-days of retrospective glucose data. It also required calibration from a finger stick and blood glucose meter. Newer devices got progressively smaller, more portable, waterproof, easier to insert, more comfortable to wear, and much more accurate. In 2003, because of the improvements in accuracy, wearability and improvements in overall glucose control, the Federal Drug Administration (FDA) approved the first CGM-integrated insulin pump for people living with type 1 diabetes. Since then, the technology continues to improve and now the use of CGM is becoming more common in athletes and others interested in monitoring and improving their glucose “control”. In Europe, and soon in other countries perhaps, CGM will emerge as an easily assessable wearable tool used by athletes who wish to gain more insight into their dietary needs, training loads and recovery strategies.
As we discussed in this blog about technology, we should always be aware of the limitations of technologies as well, so we can come to the best possible conclusions and interventions. In the next few blogs we will discuss:
Is CGM accurate?
Is CGM a fuel sensor for athletes?
How can CGM be used by athletes?
Practical aspects of CGM
Didyuk O, Econom N, Guardia A, Livingston K, Klueh U. Continuous Glucose Monitoring Devices: Past, Present, and Future Focus on the History and Evolution of Technological Innovation. J Diabetes Sci Technol. 2021 May;15(3):676-683.
Wasserman DH. Four grams of glucose. Am J Physiol Endocrinol Metab. 2009 Jan;296(1):E11-21. doi: 10.1152/ajpendo.90563.2008.
Disclaimer: Asker Jeukendrup is a consultant to Supersapiens. Michael Riddell serves as a scientific advisor to Supersapiens and as a consultant to Dexcom Inc, another CGM device company.