Are You Under-fueling? What a Continuous Glucose Monitor Can Tell You

I'd like to start this post by sharing my background in using continuous glucose monitors. Over the past five years, I've worked for a company specializing in these devices, spending hundreds of hours analyzing glucose data from thousands of individuals. Additionally, I have experience working with people who have pre-diabetes and diabetes, providing me with a comprehensive understanding of glucose and insulin, as well as the physiology of normal and altered glucose metabolism and metabolic diseases. Moreover, I have extensive experience with endurance athletes, having worked with hundreds of them using continuous glucose monitors. I'm not mentioning this to brag, but to highlight my unique qualifications for discussing the nuances of using continuous glucose monitors (CGMs) for athletes.

Continuous glucose monitors have been available for some time. They were initially approved by the FDA around 1999, with the first device designed for clinicians to use with patients, rather than being a commercial product. By the mid-2000s, the first personal real-time CGMs appeared. However, these devices were primarily used by individuals with diabetes and were not available over the counter. This required users to obtain a doctor's prescription, and the devices were costly. Additionally, most CGMs needed daily finger-stick calibration. It is only recently that CGM devices have become more widely available for commercial use, with over-the-counter options now accessible.

A CGM is a device that is most commonly placed on the upper arm around the triceps or the fatty tissue in the back of the arm, but some can be placed in the abdomen. An applicator is used to put the CGM in the arm. It is a small filament sensor that is inserted in the interstitial space (below the cells of the skin). It stays in the arm for up to 15 days and measures glucose continuously, usually every 5-15 minutes and charts this out on a graph using an app that you can have on your phone. The sensor uses an enzyme called glucose oxidase and this provides information to the sensor that correlates with glucose concentrations.

With the ease of access to continuous glucose monitors for nearly everyone, their usage has significantly increased. This technology can be especially beneficial in preventing diabetes. It is estimated that 1 in 3 adults has pre-diabetes, and many may eventually develop diabetes. Often, individuals are unaware they have pre-diabetes. A CGM can serve as an initial signal of issues with glucose regulation or insulin resistance. If this tool is so effective for those with diabetes and pre-diabetes, is it truly beneficial for athletes or individuals without any signs of a metabolic disorder?

Recently, athletes have shown increased interest in using CGM, a trend popularized by elite athletes and professional cyclists who have been spotted with these devices. CGM provides athletes with a continuous overview of glucose changes throughout the day. But does it truly aid athletes in improving their fueling strategies or offer valuable insights into aspects like recovery or stress?

The CGM allows for the observation of:

• Glucose responses to meals

• Glucose changes during exercise

• Overnight and fasting glucose patterns

• How much the glucose levels are fluctuating throughout the day

• Dips in blood sugar and low glucose events, also known as hypoglycemia

The CGM might provide additional information for athletes, including:

• Effects of training load and recovery

• Fueling insights during exercise

• Insights regarding how stress and poor sleep impact glucose levels

• Insights into fueling before or after exercise

• Energy levels and feelings of fatigue that may occur throughout the day

The use of CGMs among athletes is on the rise, but due to the difficulty in obtaining a CGM until recently, there is limited research on athletes. I want to explore what current research indicates and share some of my own perspectives. In some cases, I disagree with the research conclusions. However, as technology advances and more athletes adopt CGMs, more insights may emerge on how athletes can use these devices to optimize their fueling strategies. For example, we currently lack knowledge on the appropriate glucose range for athletes. We know that for individuals with diabetes, the goal is to maintain glucose levels between 70 and 180 mg/dl, and for those without diabetes, the range might be narrower, around 70-140 mg/dl. However, the acceptable range for athletes is unknown and may vary individually or change based on the athlete's objectives. Additionally, we lack a clear understanding of exercise-induced glucose spikes and when a significant rise during exercise becomes a concern for athletes. Another important metric could be glucose variability or standard deviation. For non-athletes, keeping this below 30 is ideal for people with diabetes, and more optimally below 20, but what is a realistic target for athletes? We currently do not have enough data to determine this.

The following are conclusions drawn from the research and my personal thoughts:

1. CGMs can provide useful insights into glucose trends, but are not precise real-time measurements.

I remember watching the Tour de France last year and learning that CGMs are banned from the Tour and all UCI-sanctioned races. I do understand that the use of CGMs could be considered a performance advantage, but given my knowledge of the data and how the CGM is measuring glucose, I am not sure it would provide a lot of real-time useful information regarding fueling. This is because CGMs measure interstitial glucose, not blood glucose. Because of this physiological difference, readings can lag behind blood glucose by 5–45 minutes, especially during exercise or after carbohydrate intake when glucose levels might be changing rapidly. This means that when the athlete notices what is happening on the CGM, they might be too late to make any meaningful adjustments. In my opinion, it is best to use the CGM as a learning tool. It is important to look at the data after the race or the training session and try to figure out what factors contributed to the changes in glucose and how to make some fueling adjustments for the next session or race. It is not the best tool for real-time monitoring and adjustments.

In addition, it is important to remember that CGM is not perfect. Research comparing CGM data to laboratory blood measurements shows that:

• CGMs are fairly accurate at rest

• Accuracy declines during exercise and rapid glucose changes

This means CGM values may not reflect real-time metabolic conditions during training, particularly when glucose is rising or falling quickly. We want to use the data as another tool to help us with fueling, but like many tools we use, it should be used in addition to other inputs such as symptoms.

2. CGMS should not be used as a "fuel gauge" for athletes

The research concluded that although CGMs are sometimes marketed as tools to determine when athletes need carbohydrates, the current evidence does not support this use.

CGMs measure circulating glucose, but endurance performance is primarily influenced by muscle glycogen availability, which CGMs cannot measure.

Because of this limitation, the research suggests that CGM data alone cannot reliably determine:

• Whether an athlete is adequately fueled

• When carbohydrates are required during exercise

• Whether an athlete is experiencing true carbohydrate depletion.

I agree that the CGM data alone cannot reliably determine if an athlete is fueling adequately. However, the CGM can be a tool that, when used with other tools such as an athlete's symptoms or subjective feeling during a workout and even rate of perceived exertion, can be used as a "fuel gauge." Also, let's face it, we don't really have a better tool for measuring glycogen. The tools available are either extremely invasive (muscle biopsy) or not easy to access and are expensive, including ultrasound-based technology. There is certain information we can get regarding what is going on with glycogen with the use of the CGM. For instance, if an athlete's glucose levels are dropping during exercise, it might be an indication that they are starting to run low on glycogen. When you combine this information with the subjective feeling of "bonking," you can get a pretty good idea that glycogen was likely starting to become depleted.

3. CGM data can be difficult to interpret in athletes

Glucose responses during exercise are influenced by many factors, including:

• Exercise intensity

• Training status

• Meal composition

• Hormonal responses

• Stress

• Hydration status

Because of these variables, a specific glucose value does not directly indicate performance readiness or fueling adequacy. Without proper context or expert interpretation, athletes may misinterpret glucose fluctuations.

I completely agree with this perspective, which is why I recommend that any athlete lacking expertise in glucose and CGM data consult with a specialist for assistance in interpreting the information. Without expert guidance, athletes might draw conclusions that could harm their performance or, in extreme cases, lead to disordered eating or inadequate fueling.

4. CGMs may have some useful applications in athlete monitoring.

Despite their limitations, CGMs may still provide valuable insights in certain contexts.

Potential applications include:

Understanding individual glucose responses

CGMs can help athletes observe how glucose responds to:

• Different foods

• Pre-exercise meals

• Training sessions

• Post-exercise meals

This may help inform personalized fueling strategies.

CGM data can Identifying exercise-related hypoglycemia. CGMs may detect situations where glucose drops during or after exercise, including:

• Exercise-related hypoglycemia

• Rebound hypoglycemia after pre-exercise carbohydrate intake.

However, true hypoglycemia is relatively uncommon in well-fueled athletes.

CGM data may help in monitoring broader metabolic patterns and identify patterns related to:

• Low energy availability

• Chronic under-fueling

• Large glucose fluctuations across the day or larger swings in glucose that can contribute to the feeling of fatigue or low energy during the day or while training

• Lower glucose or dips in glucose values overnight or during an activity might be a sign of under-fueling and/or inadequate glycogen replenishment

These insights may be more useful when interpreted alongside other performance and nutrition data and while working with an experienced professional.

5. There is currently no evidence that CGMs improve athletic performance

Currently, research does not show a definitive connection between CGM data and enhanced athletic performance in athletes who do not have diabetes. I contend that simply wearing a CGM on your arm will not enhance your athletic performance. However, the potential impact lies in how you utilize the data collected. It's the insights gained from the data and how you apply them that can be beneficial.

There are some things to keep in mind when using the CGM, specifically:

• As I mentioned before, there is no established optimal glucose range for athletes

• Stable glucose does not necessarily indicate optimal performance, and it is important for the athlete to use other information/data in addition to the CGM

• Fluctuating glucose levels do not necessarily mean impaired performance. It is possible for the glucose values to rise and fall a bit during exercise, and that can be completely normal

More research may be helpful to determine whether CGM-guided strategies can meaningfully impact training or race outcomes. However, I have worked with enough athletes to know that the CGM can make an impact on training and race outcomes when used strategically and with a skilled professional.

6. There are practical limitations that can affect CGM use in sport

Several logistical challenges may limit the widespread use of CGMs in athletes:

• Sensor accuracy can decrease during exercise

• Devices may detach during training- the CGM is usually secured in place by a patch with a sticky adhesive, but things like sweat and arm movement can cause the CGM to come out of the arm.

• Data syncing and calibration issues

• Device cost- it is getting less expensive to use a CGM, but the cost is still around $80-100 for two CGMs that might last 30 days.

• Potential competition restrictions such as with cycling so it is important that elite athletes know these restrictions and not use the CGM during competition in these cases.

These factors further complicate the use of CGMs as routine performance tools.

Overall Conclusion

Continuous glucose monitors offer valuable physiological insight into glucose trends, but their current role in sport should be viewed as exploratory rather than prescriptive.

While CGMs may help athletes better understand their individual metabolic responses, they should not be relied upon as standalone tools for guiding fueling decisions or optimizing performance.

Effective interpretation of CGM data requires:

• Context from training and nutrition data

• Understanding of glucose physiology

• Careful consideration of the device’s limitations.

• Ideally, the help of a skilled professional to interpret the data

Further research is needed before CGMs can be confidently integrated into evidence-based performance strategies for athletes without diabetes. However, they can be a really helpful tool even if more research is needed to provide more information on optimal ranges for athletes.

If you're interested in using a CGM, several companies provide CGMs along with apps to help interpret the data. Currently, there are no CGM companies or apps specifically designed for athletes. Most available CGM apps are more focused on optimizing glucose data rather than adequately fueling for sports. You can buy a CGM over the counter and use the accompanying app, like those with Stelo sensors, but this alone may not provide much useful information. I recommend finding a company that also offers the option to work with a nutritionist, dietitian, or other healthcare professionals experienced in using CGMs and interpreting the data.

If you're interested in collaborating with me, I'm currently accepting new clients. You can reach out through my private practice, and if you're using a CGM via Stelo, I can access the data as a healthcare professional through the Dexcom Clarity app. If you're considering using a CGM and want an app that provides data insights, I recommend Nutrisense. Please note, I work for Nutrisense, so I may have some bias. However, by using my link below to get started, you can add your insurance details to check if the nutrition consultations are covered. Many insurance plans do cover nutrition. You'll still need to pay for the sensors and the app, but if the nutrition consultations are covered, you can collaborate with me, and I can assist in interpreting the data.

Below is my referral link to work with me at Nutrisense:

 https://members.nutrisense.io/video-calls?coachId=8959

If you are also interested in using CGM through the Nutrisense app, you can go to this link.

Click here to watch to the podcast all about the use of CGMs with endurance athletes.

Please stay tuned for another post related to CGM data in which I share visuals and insights from my data and data from other athletes I have worked with!

Resources for research conclusions stated in article:

1. Bauhaus H, Erdogan P, Braun H, Thevis M. Continuous Glucose Monitoring (CGM) in Sports-A Comparison between a CGM Device and Lab-Based Glucose Analyser under Resting and Exercising Conditions in Athletes. Int J Environ Res Public Health. 2023 Jul 25;20(15):6440. doi: 10.3390/ijerph20156440. PMID: 37568982; PMCID: PMC10418731. https://pmc.ncbi.nlm.nih.gov/articles/PMC10418731/

2. Bowler, A. M., Whitfield, J., Marshall, L., Coffey, V. G., Burke, L. M., & Cox, G. R. (2023). The Use of Continuous Glucose Monitors in Sport: Possible Applications and Considerations. International Journal of Sport Nutrition and Exercise Metabolism, 33(2), 121-132. Retrieved Apr 2, 2026, from https://doi.org/10.1123/ijsnem.2022-0139

3. Flockhart M, Larsen FJ. Continuous Glucose Monitoring in Endurance Athletes: Interpretation and Relevance of Measurements for Improving Performance and Health. Sports Med. 2024 Feb;54(2):247-255. doi: 10.1007/s40279-023-01910-4. Epub 2023 Sep 2. PMID: 37658967; PMCID: PMC10933193. https://pubmed.ncbi.nlm.nih.gov/37658967/

4. Helleputte, S., Podlogar, T. & Gonzalez, J. Application potential of continuous glucose monitoring (CGM) in elite endurance athletes without diabetes: What do physiology and current evidence tell us?. Perform. Nutr. 1, 13 (2025). https://doi.org/10.1186/s44410-025-00013-7

5. Sports Science Institute. Continuous Glucose Monitoring Use in Athletes Without Diabetes. https://www.gssiweb.org/sports-science-exchange/article/continuous-glucose-monitoring-use-in-athletes-without-diabetes