Continuous Glucose Monitoring for Athletes: Trend or Game Changer?

A continuous glucose monitor (CGM) is a powerful and revolutionary tool that provides a dynamic, real-time view of your body's glucose levels. Unlike a single-point-in-time snapshot from a traditional fingerstick test, a CGM delivers minute-by-minute data, painting a complete picture of how your diet, exercise, and lifestyle choices impact your blood sugar throughout the day and night.

In recent years, the use of CGMs has surged among athletes and fitness enthusiasts of all levels. Why? To gain a competitive edge by optimizing their training, nutrition, and recovery strategies. Essentially, CGMs have become the ultimate tool for "biohacking"—the practice of using technology to enhance your body’s performance and overall well-being.

As a registered dietitian (RD), I believe this trend is highly relevant for both athletes and nutrition professionals. The insightful data provided by a CGM can help fine-tune nutritional strategies, such as determining the optimal timing and type of carbohydrates for exercise. This information empowers athletes to maximize their fueling and recovery, ultimately leading to better performance.

What is CGM and How Does It Work?

Historically, monitoring blood sugar required a time-consuming and often painful fingerstick test. This practice is a critical daily routine for individuals with diabetes, particularly those with type 1, who must carefully manage their glucose levels to prevent serious complications like heart failure, kidney disease, and nerve damage.

Enter CGM technology. A CGM is a wearable sensor, typically attached to the back of the upper arm, that continuously measures glucose levels from the interstitial fluid just beneath the skin. This real-time data is transmitted to a smartphone app, allowing the user to easily track their glucose trends and make timely adjustments to their medication, diet, or activity. The continuous nature of this feedback makes it a far more convenient and effective tool for proactive health management.

Until recently, CGMs were primarily available by prescription for individuals with diabetes. However, a growing number of over-the-counter (OTC) options are now accessible to non-diabetic athletes. It's important to note that these OTC devices may have some key differences, such as a lack of high or low glucose alerts and a less frequent data collection rate compared to their medical-grade counterparts.

Why Athletes Are Turning to CGM

In the world of sports, where a fraction of a second can determine victory, optimizing every aspect of training and recovery is paramount. CGMs offer a unique advantage by providing granular insights into how an athlete’s body responds to different foods, exercise types, and even stress. This data helps create a highly customized fueling plan, particularly concerning carbohydrate intake, to maintain stable energy levels and prevent the dreaded "bonk" or energy crash during a race or workout.

Beyond fueling, continuous glucose monitoring can also reveal patterns that signal overtraining, inadequate recovery, or even high levels of physiological stress. The ability to proactively identify and address these issues provides athletes with a significant competitive edge, allowing them to tailor their nutrition to their individual, real-time responses. This biohacking approach appeals to both elite competitors and dedicated amateurs seeking to unlock their full potential.

CGM and Sports Nutrition: A Registered Dietitian's Insight

While CGMs are a powerful tool, they are not a replacement for fundamental, evidence-based sports nutrition principles. Rather, they serve as a complement, helping to refine and enhance a solid nutritional foundation. The core principles of fueling for performance—ensuring adequate carbohydrate availability before exercise and proper replenishment afterward—must always come first. The CGM then helps an athlete observe how their body responds to these strategies, allowing for smarter, more effective adjustments.

It is crucial to understand that glucose levels naturally fluctuate. Spikes and elevated readings are not inherently "bad." Using CGM data without proper context or professional guidance can lead to unnecessary confusion, stress, and frustration. For example, a significant glucose spike after a pre-workout meal is a normal and often desirable response, as it indicates fuel is being made available for the muscles. A nutrition professional, like an RD, is essential for translating this data into actionable, context-aware insights, helping athletes avoid misinterpretation and make sound decisions to support their performance and recovery.

Nutrient Timing Through the Lens of CGM

CGM technology provides valuable, real-time insight into how quickly food is digested and utilized, making it an invaluable tool for optimizing nutrient timing. This can inform decisions on when to fuel before exercise, how to adjust during a session, and what to consume for sustained energy.

• Pre-Workout Fueling: While general recommendations suggest fueling with a carbohydrate-rich snack before exercise, a CGM can help an athlete pinpoint their ideal timing. It can show if eating 60 minutes or just 30 minutes before a session leads to more stable energy levels. If blood glucose is trending low before a workout, the data can confirm the need for a pre-session snack to meet energy demands.

• During Long Sessions: For endurance events or long training sessions, a CGM can help an athlete understand when and how many carbs are needed to sustain energy. If blood glucose drops below the optimal range (e.g., below 70 mg/dL) and is accompanied by fatigue, the device provides a clear signal to refuel. This helps the athlete develop a plan to prevent energy dips and ensures consistent performance.

• Individualized Response: It's important to remember that everybody responds differently to different foods, portion sizes, and training modalities. A CGM can reveal which specific types of carbohydrates work best for an individual and how their body utilizes glucose during different types of exercise, such as strength training versus endurance activities. This is particularly beneficial for "hybrid athletes" who engage in multiple disciplines.

Pre-Workout Fueling Strategies Refined by CGM

Carbohydrates are the primary and most efficient fuel source for the body, especially during moderate- to high-intensity exercise. The body's ability to efficiently break down and utilize carbs is directly correlated with an athlete's power output and ability to sustain effort.

CGMs are not meant to reinvent these fundamental principles but rather to fine-tune them. Since our bodies respond uniquely to different foods and quantities, a CGM can help an athlete:

• Optimize Timing: Determine the ideal window for pre-workout carb consumption to ensure glucose is readily available when exercise begins.

• Refine Portions and Food Types: Pinpoint the right amount and type of carbs (e.g., fast-digesting vs. slower-digesting) that provide stable energy without causing an unwanted crash. For instance, if a pre-workout snack leads to a quick spike followed by a dip, the CGM data might suggest a different food choice or portion size.

• Adapt for Training Modalities: Help athletes develop specific protocols for different types of training, as strength and endurance workouts have different metabolic demands.

Post-Workout Recovery and Glucose Trends

The goal of post-workout nutrition remains the same: replenish glycogen stores and repair muscle tissue. CGMs can provide a window into how effectively the body is accomplishing this.

• Evaluate Glucose Trends: By observing post-exercise glucose dips or spikes, an athlete can gain insight into their body's recovery process. A rapid, sustained decline in glucose might signal effective glycogen repletion, while a prolonged dip could suggest inadequate fueling.

• Optimize Recovery Nutrition: A CGM can highlight whether the timing and composition of a recovery meal (the classic combination of protein and carbohydrates) are effective. If glucose levels take too long to stabilize, it may indicate a need to adjust the meal's timing or macronutrient composition to speed up recovery.

• Identify Over-training or Under-fueling: By tracking glucose trends over time, a CGM can help identify patterns consistent with over-training or chronic under-fueling. This data can inform decisions to incorporate more anti-inflammatory foods, increase rest days, or focus on other recovery protocols like improved sleep.

Benefits and Limitations of CGM for Athletes

Benefits:

• Personalized Feedback: Provides a unique, individualized look at how your body responds to specific foods, exercise, and lifestyle factors.

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• Enhanced Recovery Protocols: Helps identify patterns that may indicate overtraining and can guide decisions to optimize rest and recovery.

• Proactive Health Management: Reveals "hidden" insights, such as unexpected hypoglycemia, and promotes a more mindful approach to eating and nutrition.

• Unlocks Potential: Provides data that can help an athlete break through plateaus and optimize performance.

Limitations:

• Over-reliance on Data: The greatest tool an athlete possesses is their own body. Over-relying on data can lead to losing touch with internal cues like hunger, fullness, and fatigue.

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• Misinterpretation: Data without context can be misleading and lead to unnecessary stress, food fear, or restrictive behaviors. Glucose fluctuations are normal and expected, especially around exercise.

• Cost and Accessibility: CGMs are not widely affordable or accessible for all athletes. The devices must be replaced regularly, and a monthly membership or subscription is often required, making it a significant financial investment.

Final Thoughts

A CGM is a powerful tool when used appropriately and with expert guidance. It is not designed to replace the fundamental principles of sports nutrition but to enhance them. Just like with supplements, a CGM cannot fix a flawed nutritional strategy. The basics—fueling with carbs before and after a workout—should already be in place. A CGM is most helpful when an athlete is looking to "level up" their game or address a perceived performance gap.

While we can't completely control our bodies' responses, having a CGM in your toolkit can help you become a master of your own physiology. By understanding your blood sugar fluctuations, you might just find that you have more "good days" than "bad ones," allowing you to show up as the best version of yourself, day in and day out.

If you're an athlete interested in incorporating a CGM into your training, I highly recommend consulting with a sports registered dietitian or a nutrition coach (like the ones here at Working Against Gravity!). They can help you determine if a CGM is right for you and, most importantly, how to translate the data into practical, performance-supporting actions.