Continuous Glucose Monitoring (CGM) is a technology that tracks the body's glucose levels in real time, providing valuable insights for athletes. It offers many benefits compared to traditional blood glucose testing. It involves a small sensor placed under the skin that measures interstitial glucose levels continuously, transmitting data to a device or smartphone. Its usefulness as a training tool and for performance enhancement of athletes is still being debated, and research is ongoing.

endurance athletesContinuous Glucose Monitoring is useful for endurance athletes

What Are Optimal Glucose Levels for Athletes?

Understanding optimal glucose ranges is essential for athletic performance. Research indicates different targets based on exercise intensity and duration:

Pre-Exercise

Target: 126-180 mg/dL (7.0-10.0 mmol/L). This provides adequate glucose availability without risk of hypoglycemia during activity.

During Moderate Exercise

Target: 90-120 mg/dL (5.0-6.7 mmol/L). Optimal for sustained energy during Zone 2-3 training, long runs, and endurance efforts.

During High-Intensity Exercise

Target: 130-180 mg/dL (7.2-10.0 mmol/L). Higher glucose supports maximum power output during threshold work, intervals, and competition.

Post-Exercise Recovery

Target: Return to 80-100 mg/dL (4.4-5.6 mmol/L) within 2-3 hours. Sustained elevation may indicate stress or poor recovery.

How to Use CGM Data for Athletic Performance

Continuous glucose monitoring provides athletes with actionable data to optimize training and competition. Here's how to interpret key metrics:

Time in Range (TiR)

Time in Range measures the percentage of time your glucose stays within the target range of 70-180 mg/dL. Athletes should aim for at least 70% time in range, with less than 3% time below 70 mg/dL (hypoglycemia). High time in range indicates good glucose control and effective fueling strategies.

Coefficient of Variation (CV)

The CV measures glucose variability by dividing standard deviation by mean glucose. Athletes should target a CV below 36%, indicating stable glucose control. Higher variability may suggest poor fueling timing, inadequate carbohydrate intake, or excessive stress. Endurance athletes typically show higher glucose variability (CV around 21%) compared to healthy controls (CV around 16%) due to training demands.

Glucose Management Indicator (GMI)

The GMI provides an estimated A1C based on average CGM glucose. For athletes without diabetes, this metric helps track long-term glucose control and can indicate metabolic health. A GMI below 5.7% is considered normal.

Target Population: CGM is useful for athletes in endurance sports and team sports, particularly diabetic athletes who need to manage glucose during exercise.

Test Purpose: CGM helps athletes optimize their performance by identifying glucose fluctuations, preventing energy crashes, and enabling them to fine-tune nutrition strategies. Endurance athletes, in particular, benefit from maintaining stable glucose levels to sustain energy output. It also aids in recovery by ensuring adequate fuels are ingested post-exercise.

Equipment Required: CGM sensor, transmitter (sends data to a receiver or smartphone), compatible smart device to receive signals, adhesive patches to secure the sensor during activity, and calibration tools if required.

Procedure: Insert the sensor under the skin; the typical location is on the arm or abdomen. Connect the transmitter to send glucose readings wirelessly. Sync the device with a smartphone or receiver. Monitor real-time glucose trends before, during, and after exercise. The sensor lifespan is typically 7-14 days.

Results: Continuous glucose readings are displayed in real-time and saved to the device. You can review graphs showing glucose fluctuations and compare them to activity levels. You can set alerts to notify you when glucose levels are too high or too low.

Interpretation of Results: Stable glucose levels indicate optimal fueling, while rapid drops suggest energy depletion or poor carbohydrate intake. Spikes may indicate overconsumption or a stress response.

Sport-Specific CGM Applications

Endurance Sports

Marathon runners, cyclists, and triathletes can use CGM to prevent "bonking" by monitoring glucose depletion during long efforts. Research shows endurance athletes should target 30-90g carbohydrates per hour depending on intensity, with CGM providing real-time feedback on fueling effectiveness.

Team Sports

Soccer, basketball, and hockey players experience intermittent high-intensity efforts that cause glucose fluctuations. CGM helps identify individual responses to match demands and optimize half-time nutrition strategies.

Strength Training

Weightlifters and CrossFit athletes may see glucose spikes during intense efforts due to adrenaline release. CGM can help optimize pre-workout nutrition and post-training recovery meals.

Combat Sports

Wrestlers, boxers, and MMA fighters managing weight cuts can use CGM to monitor glucose stability during caloric restriction and ensure adequate energy for competition.

Advantages: Does not require regular taking of blood samples. CGM provides personalized sports nutrition and performance monitoring. It is beneficial for diabetic athletes managing their glucose levels during exercise. Helps assess the impact of different foods and training intensities.

Disadvantages: It can be expensive. Some CGMs require calibration with a blood glucose meter. May cause pain, discomfort, and skin irritation. Data accuracy varies by device and individual. Interpretation of results may be complicated to decipher. Note: The International Cycling Union (UCI) prohibited the use of CGM technology during competition in 2021.

Carbohydrate Recommendations Based on CGM Data

CGM data can guide carbohydrate intake strategies for athletes:

Duration Carb Intake CGM Target
<45 minutes Not required 90-140 mg/dL
45-75 minutes 30-60g/hour 100-160 mg/dL
1-2.5 hours 30-60g/hour 110-180 mg/dL
>2.5 hours 60-90g/hour 120-180 mg/dL
Ultra (>6 hours) 90g+/hour 100-160 mg/dL

Frequently Asked Questions

What is a good glucose level during exercise?

For most athletes, optimal glucose during exercise ranges from 90-180 mg/dL (5.0-10.0 mmol/L). During moderate intensity, aim for 90-120 mg/dL. For high-intensity efforts, 130-180 mg/dL provides adequate fuel without performance impairment. Below 70 mg/dL risks hypoglycemia and performance decline.

What is Time in Range (TiR) and why does it matter for athletes?

Time in Range measures the percentage of time glucose stays within target (typically 70-180 mg/dL). Athletes should aim for 70% or more time in range, with less than 3% time below 70 mg/dL. High TiR correlates with better energy stability, recovery, and reduced risk of performance-limiting glucose extremes.

How is glucose variability (CV) calculated?

Coefficient of Variation (CV) is calculated by dividing standard deviation by mean glucose, then multiplying by 100. For example, if your average glucose is 100 mg/dL with a standard deviation of 30 mg/dL, your CV is 30%. Athletes should target CV below 36% for stable glucose control.

Should non-diabetic athletes use CGM?

Non-diabetic athletes can benefit from CGM to optimize fueling strategies, identify individual glucose responses to foods and training, prevent energy crashes during competition, and monitor recovery. However, research on performance benefits in non-diabetic athletes is still emerging, and cost may be a consideration.

Why do glucose levels rise during high-intensity exercise?

High-intensity exercise triggers adrenaline (epinephrine) release, which stimulates the liver to release stored glucose (glycogenolysis). This is a normal physiological response to provide fuel for intense efforts. Glucose may rise 20-50 mg/dL above baseline during sprints or intervals before returning to normal post-exercise.

How accurate is CGM compared to blood glucose testing?

Modern CGM devices have Mean Absolute Relative Difference (MARD) of 8-14%, meaning they're generally within 8-14% of actual blood glucose values. During rapid glucose changes (like exercise), there may be a 5-15 minute lag between interstitial glucose (what CGM measures) and blood glucose. For critical decisions, confirm with a blood glucose meter.

Can CGM detect overtraining?

Research suggests CGM may help detect overtraining by identifying abnormal glucose patterns. Signs include: failure to increase glucose during high-intensity exercise, elevated overnight glucose, and impaired glucose responses to meals. A blunted catecholamine response during overreaching may manifest as abnormal glucose patterns.

References

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  2. Riddell MC, et al. (2024). "Continuous Glucose Monitoring and the Athlete with Type 1 Diabetes." Sports Science Exchange. Gatorade Sports Science Institute.
  3. Flockhart M, Larsen FJ. (2023). "Continuous Glucose Monitoring in Endurance Athletes." Sports Medicine. 54:35-47.
  4. Holzer R, et al. (2022). "The Use of Continuous Glucose Monitors in Sport." International Journal of Sport Nutrition and Exercise Metabolism. 33(2):121-132.
  5. Thomas DT, et al. (2016). "Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance." Journal of the Academy of Nutrition and Dietetics.
  6. Jeukendrup A. (2014). "A Step Towards Personalized Sports Nutrition: Carbohydrate Intake During Exercise." Sports Medicine. 44(Suppl 1):25-33.
  7. Stellingwerff T, Cox GR. (2014). "Systematic review: Carbohydrate supplementation on exercise performance or capacity of varying durations." Applied Physiology, Nutrition, and Metabolism.
  8. Danne T, et al. (2017). "International Consensus on Use of Continuous Glucose Monitoring." Diabetes Care. 40(12):1631-1640.

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