What are Continuous Glucose Monitors?
A continuous glucose monitoring system (CGMs) is a single-use, disposable, minimally invasive device applied to the body (usually the back of the arm, sometimes the stomach) which monitors glucose levels in the interstitial fluid. This technology has been used in people with diabetes for a long time but is becoming increasingly popular in the health, longevity, and performance spaces.
Why the Interstitial Fluid?
The interstitial fluid approximates the blood glucose levels quite well at rest, though it is less reflective in states of significant change. In people with diabetes, numerous accuracy studies have been conducted on various CGM devices.
Most studies show a modest reduction in sensor accuracy when glucose levels change rapidly because of rapid changes in the interstitial fluid that can be caused, for example, by exercise or feeding. However, it is important to underline that CGMs are considered more than accurate enough to support decision-making even in the clinical setting (1-4). That said, there is a suggestion that in a population of people without diabetes, measuring glucose from the interstitial fluid is more accurate (5).
One thing to note here is that glucose values, as shown on CGMs, are the net result of delivery and utilization: delivery from the blood to the interstitial fluid and uptake into the muscle for utilization. As a result of the two-factor nature of this number, increases can result from increased delivery or decreased utilization and vice versa. Similarly, if a situation arose where there were perfectly matched increases in both, the net result would be unchanged from the baseline (this is rarely, if ever, seen)
Why Would an Athlete Use a Continuous Glucose Monitor?
Maintaining stable glucose concentrations has also been described as one of the potential determinants of endurance performance (6,7). Athletes have gravitated towards CGMs to understand their fueling better and avoid ‘bonking’ or running out of energy. The goal for these athletes is generally greater confidence in their fueling strategy.
CGMs allow us to swiftly see the impact of a certain nutritional product or its timing on our glucose levels. CGMs provide insight into circulating glucose availability in the body, which reflects the interplay of both liver glycogen mobilization (muscle glycogen acts locally and is thus not visualized on CGMs) and exogenous carbohydrate intake (any carbohydrates ingested).
What Do Continuous Glucose Monitors Tell You?
There are many metrics that CGMs can output, which include the live visibility of the glucose level itself. They can also give insight into the rate of change of glucose, which may be more related to how an athlete feels, but definitely carries important information if trying to avoid ‘bonking’ because while the glucose level is relevant, the direction that it is moving is almost as important, if not more so, in many situations.
CGMs classically output some statistics that are relevant for individuals with diabetes like ‘variability’ (a measure of how much fluctuation there has been in glucose levels) and ‘time in range’ (a measure of duration in percentage within a certain glucose range, often 70-140mg/dL or 70-180mg/dL). Additionally, some companies have started to target individuals without diabetes and the performance market and have developed other metrics, such as fueling scores.
Given the propensity for under, rather than overfueling in endurance athletes, there may be some questions as to whether CGM has potential for use and even if it could be detrimental in this aspect. Whenever bringing attention to something — in this case glucose — there is the likelihood that you start to emphasize it more. Indeed, many athletes using CGM report that they have noticed they are underfueling.
As with any coaching process, feedback is the key driver to change and improvement. This may be the most helpful aspect of CGM through the visibility it provides in an objective and non-judgmental way. Without this visibility, the athlete can be unsure exactly what is happening, and change can lack the biofeedback required to expedite the process in a timely manner.
How Do You Use a CGM in Training?
Use in training really depends on the phase of the season you are in and the goals for using a CGM. Offseason or “base building” phases of training likely lend themselves to experimentation with different types of fuel, be that form (solids versus gels versus liquids) or source (comparing brands of gels, for example). This phase of the season may also lend itself to using CGMs in the evaluation of fat adaptation, for example, extended durations of low-intensity training within minimal carbohydrate intake and utilizing CGM to evaluate the body’s ability to sustain glucose levels at these intensities.
Alternatively, as the year progresses, there is a role for CGM in determining whether sessions are being appropriately fueled and/or dialing in race day nutrition strategies.
How do You Evaluate CGM output?
While it is difficult to know the best way to evaluate what ‘ideal’ glucose is in individuals without diabetes, given the relative recency of uptake of this technology in these populations, there are some generally agreed-upon aspects. These aspects predominantly have their origins in exercise physiology, with some influence from understanding the physiology of those with diabetes.
With research in its infancy in the space of CGM use in non-diabetic populations, it is likely that this will continue, and practitioners will be helping inform the scientists for quite a while before the tables turn.
Remember that many factors impact glucose, the most significant three being exercise, food, and stress, so understanding and interpreting glucose can be challenging. Glucose generally remains largely unchanged during lower exercise intensities (below the first threshold). Beyond this intensity, there can be increases in glucose and generally, as metabolic stress accumulates, glucose will also increase. In short, sprint-type activities with long rest periods it is unlikely to see much change in glucose either. This is most likely due to the metabolic pathways involved as intensity drives glucose oxidation and long rest breaks preventing significant metabolic stress.
Metrics currently used to understand and evaluate glucose during exercise include glucose levels and variability. The glucose levels and their trends give a good insight into circulating glucose availability — the combination of mobilized liver glycogen and carbohydrate intake. This is particularly relevant when evaluating the efficacy of fueling strategies.
Declining glucose levels, low glucose, or periods with unexplained but sizable reductions in glucose probably reflect suboptimal glucose availability. These all play some role in variability, with the goal being less variable, more stable glucose. Whilst glucose is very individual and heavily contingent on the intensity of exercise, these principles look to be a good place to start from when evaluating glucose in general and in training.
How Can You Use CGM In Racing?
It should first be noted that CGM use in UCI-sanctioned racing is banned, but it can be used in training and non-UCI races. CGM also remains available for use in other sports, such as triathlon and running.
The first, and perhaps simplest, answer to this is that you can use your glucose data in real-time to help adjust fueling decisions. This is available for some CGMs like Dexcom on Garmin devices.
In these cases, you may use your current glucose level in addition to the trend of your glucose to adjust your fueling in real-time, based on experience from training on where you feel and perform best from a glucose perspective at a given intensity. Keep in mind that in a race, you may have slightly higher glucose levels due to the stress/excitement of racing.
Beyond real-time fueling adjustment, there may still be value in live visibility of glucose. Similarly to using other metrics in training but not necessarily taking as much note of them in racing, you may use glucose likewise. That is, working hard to dial in your feeling with your glucose levels may aid in better intuition in the medium to long term. Ultimately, the goal is not to be reliant on data but optimized thanks to extensive use of data to a point where you don’t necessarily need the data to execute well.
What if You Don’t Have Live Glucose Visibility?
In much the same way you evaluate your heart rate, pace or power post-race, you can learn from your glucose data post-race. One challenge with glucose data is that context is of the utmost importance, in contrast to something like power, which may have meaning regardless of whether you are riding on the flat or uphill. Without context from things like external work (pace or power), internal work (heart rate) and nutrition intake, it is exceedingly difficult, if not impossible, to analyze glucose data in a meaningful way.
To this point, Supersapiens is the only company offering the ability to overlay data in a meaningful way with all of your other workout metrics via their dashboard (and the ability to grant access to others to view this, for example, a coach or nutritionist). (editor’s note: Supersapiens has been formally shut down.) However, this can be achieved with other CGMs by manually exporting data to CSV and graphing it.
Factors you would evaluate are largely similar to what you would assess in training: variability, glucose trend, and periods of lower glucose. Similarly, the context of what was needed in a race and how glucose responded is very relevant here. Were there periods where glucose was low and subjective experience was correspondingly low in energy? Were there periods where bursts of activity did not see a concomitant change in glucose?
Are There Other Ways CGM May Help?
There is some promising research showing the benefits of CGM in the behavioral change space, specifically concerning diet quality (8).
Similarly, there is some early indication from athletes and some companies that overnight glucose levels may be a canary in the coal mine for a mismatch between training load and caloric intake. It’s too early to be certain, but this may actually be the most powerful use case for this technology if it eventuates to hold true across a broader population.
References
- Murray-Bachmann R, Leung TM, Myers AK, et al. Reliability of continuous glucose monitoring system in the inpatient setting. J Clin Transl Endocrinol. 2021;25:100262.
- Guillot FH, Jacobs PG, Wilson LM, et al. Accuracy of the Dexcom G6 Glucose Sensor during Aerobic, Resistance, and Interval Exercise in Adults with Type 1 Diabetes. Biosensors (Basel). 2020;10(10):138.
- Moser O, Mader J, Tschakert G, et al. Accuracy of Continuous Glucose Monitoring (CGM) during Continuous and High-Intensity Interval Exercise in Patients with Type 1 Diabetes Mellitus. Nutrients. 2016;8(8):489.
- Yardley JE, Sigal RJ, Kenny GP, Riddell MC, Lovblom LE, Perkins BA. Point Accuracy of Interstitial Continuous Glucose Monitoring During Exercise in Type 1 Diabetes. Diabetes Technol Ther. 2013;15(1):46–9.
- Klonoff DC, Nguyen KT, Xu NY, Gutierrez A, Espinoza JC, Vidmar AP. Use of Continuous Glucose Monitors by People Without Diabetes: An Idea Whose Time Has Come? Journal of Diabetes Science and Technology. 2022;0(0). doi:10.1177/19322968221110830
- Coyle EF, Coggan AR, Hemmert MK, Ivy JL. Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate. J Appl Physiol. 1986;61(1):165–72.
- Coggan AR, Coyle EF. Carbohydrate ingestion during prolonged exercise: effects on metabolism and performance. Exerc Sport Sci Rev. 1991;19:1–40.
- Ehrhardt N, Al Zaghal E. Continuous Glucose Monitoring As a Behavior Modification Tool. Clin Diabetes. 2020 Apr;38(2):126-131. doi: 10.2337/cd19-0037. PMID: 32327884; PMCID: PMC7164990.
