Continuous Glucose Monitoring in Sport

Continuous Glucose Monitoring in Sport

Written by: Lachlan Mitchell

Sports Dietitians Australia hosted a lunchtime webinar presented by Amy-Lee Bowler (PhD candidate, Bond University) on the topic of Continuous Glucose Monitors in sport. In her presentation, Amy-Lee discussed the function of continuous glucose monitors, the capacity for continuous glucose monitors to provide information on daily fuel needs for athletes, and whether continuous glucose monitors can provide information about an athlete’s energy availability status. Below is a summary of Amy-Lee’s presentation.

Continuous glucose monitors (CGM) are routinely used in the diabetic setting to  monitor blood glucose. The monitor is placed on the skin surface, typically on the back of the upper arm, where a small needle is inserted into the subcutaneous tissue. This measures the glucose concentration in the interstitial fluid providing an indirect measure of blood glucose. Recently, there has been an emergence of CGM use in athletes to try to gain an edge over their fellow competitors.

Two types of monitors are currently available. The first type, strictly named a ‘continuous’ glucose monitor, is common in the research setting and hides results from the participant, storing and transmitting glucose data to a computer at the conclusion of the testing period. These monitors do not require a syncing period during use, however, do require finger-prick calibration four times per day. The second type of glucose monitor is labelled as a ‘Flash’ glucose monitor, as it requires the participant to flash their phone with a linked mobile app over the monitor to view glucose data in real-time via blue tooth. This monitor must be synced to the app every 8 hours, with the monitor only able to store the most recent 8 hours of glucose data. A limitation of this monitor is that it requires a 24-hour calibration period before values reflect blood glucose accurately. CGM will take glucose readings every 5-15 minutes, dependent on the model used, and can be used for 1-2 weeks before being replaced.

Three key measurements derived from CGM use are:

• MARD – mean absolute relative difference. The MARD refers to the accuracy of the CGM based on direct blood glucose assessment. In athletic populations, CGM have demonstrated a smaller MARD value (that is, increased accuracy) compared to diabetic population data.
• MAGE – mean amplitude of glucose excursion. The MAGE refers to the glucose variability across the day, describing major fluctuations in whole body glycaemia.
• MODD – mean of daily differences. The MODD describes the between-day variability in blood glucose, whereby comparison is made across days at the same time of day.

Measures of variability are important as increased glucose variability is a marker of disruption of glucose control. This is evidenced in diabetic populations where large fluctuations are associated with greater diabetes complications. Observational data has shown the variability within (MAGE) and across days (MODD) in blood glucose to be similar between athletes and healthy non-athletes.

Can continuous glucose monitors provide information on daily fuel needs?

A small number of studies have explored the ability of CGM to provide information of fuel availability during exercise. These studies monitored blood glucose during endurance events (2 to 5-days in duration), showing there were differences in blood glucose concentrations and variability between athletes across the duration of events (1-3). Coupled with carbohydrate intake data, it is suggested that CGM have the potential to provide insights into, and assist with, the distribution of carbohydrates during an event, as well as the post-event recovery requirements.

Can CGM provide information about an athlete’s energy availability status?

The potential for CGM to provide insight into energy availability status stems from the blood glucose response to insufficient energy intake. Previous low energy availability intervention studies have demonstrated acute reductions in fasting blood glucose due to imbalances in energy intake and exercise energy expenditure. These reductions occur in under five days of low energy availability. It has also been shown that leptin and ghrelin concentrations, two blood glucose-regulating hormones, are also disturbed by low energy availability. Furthermore, a severe energy deficit intervention elicited through a 2-day military training camp produced reductions in interstitial glucose and increased time in a hypoglycaemic state as determined through continuous glucose monitoring (4). In total, this evidence suggests there may be potential to detect acute low energy availability through continuous glucose monitoring. From a practical perspective, real-time measures in the form of CGM could potentially be used to avoid long-term consequences of low energy availability. At this stage though, no study has directly looked at low energy availability and CGM use. Unpublished pilot data examining race walkers suggests there may be changes in glucose variability during acute low energy availability.

Limitations to the use of CGM

Several limitations must be considered before exploring the use of CGM. Firstly, the potential for loss of data is significant. The monitor can be dislodged during exercise or other activities and the monitor must be scanned every eight hours to save data. When factoring in the 24-hour calibration time, a lost monitor can be quite disruptive, not to mention expensive, given monitors typically cost $100 AUD. Although the monitor can provide real-time feedback to athletes, they should be working in collaboration with a sports dietitian. Most athletes have limited understanding of the data, and there is potential for athletes to obsess over values and respond inappropriately to glucose readings. As such, ensuring a nutrition professional is on hand to assist in interpreting results is important.

In conclusion, CGM provides a unique view of blood glucose. Ultimately though it is about how the data is used. There is potential for CGM to become an additional tool in the sports dietitian’s tool kit, to be used in conjunction with dietary intake and training load data, with the potential to identify athletes at risk of under fuelling sooner than currently can be done. With this in mind, and considering the costs, continuous glucose monitoring may best be utilised during periods of high training volume where risk of under fuelling may increase.

References:

1. Francois M. E., Cosgrove, S. D., Walker, N. M., Lucas, S. J. E., Black, K. E. (2018). Physiological responses to a five-day adventure race: Continuous blood glucose, hemodynamics and metabolites the 2012 GODZone field-study. Journal of Exercise Science & Fitness, 16(3), 78-82. https://doi.org/10.1016/j.jesf.2018.07.002
2. Ishihara, K., Uchiyama, N., Kizaki, S., Mori, E., Nonaka, T., Oneda, H. (2020). Application of Continuous Glucose Monitoring for Assessment of Individual Carbohydrate Requirement during Ultramarathon Race. Nutrients, 12, 1121. https://doi.org/10.3390/nu12041121
3. Sengoku, Y., Nakamura, K., Ogata, H., Nabekura, Y., Nagasaka, S., & Tokuyama, K. (2015). Continuous Glucose Monitoring During a 100-km Race: A Case Study in an Elite Ultramarathon Runner, 3. International Journal of Sports Physiology and Performance, 10(1), 124-127. https://doi.org/10.1123/ijspp.2013-0493
4. Smith, T. J., Wilson, M. A., Karl, J. P., Austin, K., Bukhari, A., Pasiakos, S. M., O’Connor, K. L., & Lieberman, H. R. (2016). Interstitial glucose concentrations and hypoglycemia during 2 days of caloric deficit and sustained exercise: a double-blind, placebo-controlled trial. Journal of Applied Physiology, 121(5), 1208–1216. https://doi.org/10.1152/japplphysiol.00432.2016