CGM accuracy has been improving, but lags between the glucose they measure and actual blood glucose can be greater when using CGM during exercise.
By Sheri R. Colberg, PhD, FACSM
A topic that comes up frequently nowadays is the use of diabetes technologies with exercise. When I surveyed close to 300 active individuals with diabetes, more than 60 percent used an insulin pump (which is well above the national average), but even more of these exercisers—over 75 percent—wear a continuous glucose monitoring (CGM) device (1). The technology fervor has grown even louder since the FDA recently granted approval in the United States to an implantable, three-month CGM sensor called Eversense (made by Senseonics). Can active people benefit from using these CGM and other devices, especially when active?
Given the challenges associated with exercising with diabetes, using almost any of the latest technologies can be beneficial for managing blood glucose and maximizing the benefits of physical activity. For example, blood glucose meters give immediate feedback on starting levels and single point-in-time exercise responses, whereas CGM devices offer the opportunity for improved decision making in real-time and retrospectively based on glucose values and trends. Insulin pumps offer a more fine-tuned ability to regulate basal insulin levels and bolus doses. Moreover, closed-loop systems (even hybrid ones) integrating pump and CGM use to manage decision-making have the potential to allow users to avoid undesired rises and falls in blood glucose during and following workouts. All forms of technology have inherent drawbacks and limitations, but it’s possible to overcome most of their issues with planning and knowledge.
For this discussion, let’s focus on the use of CGM devices. Their accuracy has been improving over time, but they’re all still limited by their physiological methods: there is a lag of at least six to 20 minutes between the glucose they measure in interstitial spaces (between cells in the skin) and actual blood glucose. This lag can be even greater during times when blood glucose levels are often changing rapidly, such as during exercise (2) because it takes time for glucose to move between these two areas. CGM users may also have problems getting the device to stay in place, similar to issues experienced by pump users with adherence of their infusion sets to the skin (1). Other reported challenges include sensor accuracy (2,3), variability in performance between sensors, breakage of sensor filaments, transportation of the sensor display, and inability to calibrate CGM during exercise (4). (If given the option, use an integrated watch to display values during exercise.) Anecdotally, exercisers have reported a “compression effect” when wearing the CGM sensor under compression shorts (1). This results in a greater lag time, presumably due to a lesser blood flow to skin in compressed areas.
Although technical failures during exercise are possible with the newer versions of these monitors, CGM is still likely to be the wave of the future when it comes to monitoring and managing blood glucose.
However, the cost of using CGM devices is another issue that may limit their widespread adoption. For instance, Dexcom, Medtronic, and Senseonics technologies all cost about $7 to $8 per day to use, but the Eversense implantable one additionally requires a physician office visit to get the sensors implanted and removed every three months (although if approved, the 180-day XL model will reduce these costs by halving the required physician visits). All three of these systems are more expensive than the FreeStyle Libre, which costs about $4 to $5 per day but doesn’t have a wearable transmitter (although some people have come up with a workaround for this using the Ambrosia BluCon device with app for mobile phones); the Libre also lacks real-time alerts.
Insurance reimbursement has gotten more reasonable since CGM devices were first introduced over a decade ago, but it’s still spotty and prejudicial. Currently, only one (Dexcom) model is covered by Medicare for anyone with type 1 diabetes age 65 and over. Although people with type 2 diabetes—especially those using insulin—can also benefit from using CGM, coverage for them has been an even harder sell to insurance companies so far. In any case, the number of model choices has declined recently, and their cost is still quite prohibitive for most people without insurance coverage and even for some with larger insurance copays.
If people don’t have access, can’t afford, or don’t want the hassle of wearing a CGM device, it is still viable for them to engage in frequent self-monitoring using a blood glucose meter to exercise safely and effectively. Remember, just having access to that simple monitoring tool can make all the difference in the world when it comes to managing blood glucose with exercise. Make the most of any tools at your disposal!
- Colberg, S. Diabetic Athlete’s Handbook, 2nd Edition. Champaign, IL: Human Kinetics, 2019 (in press)
- Taleb N, Emami A, Suppere C, Messier V, Legault L, Chiasson JL, Rabasa-Lhoret R, Haidar A: Comparison of two continuous glucose monitoring systems, Dexcom G4 Platinum and Medtronic Paradigm Veo Enlite system, at rest and during exercise. Diabetes Technol Ther. 18(9):561-7, 2016
- Herrington SJ, Gee DL, Dow SD, Monosky KA, Davis E, Pritchett KL: Comparison of glucose monitoring methods during steady-state exercise in women. Nutrients, 4:1282-1292, 2012
- Bally L, Zueger T, Pasi N, Carlos C, Paganini D, Stettler C: Accuracy of continuous glucose monitoring during differing exercise conditions. Diabetes Res Clin Pract, 112:1-5, 2016.
Sheri R. Colberg, PhD, is the author of Diabetes & Keeping Fit for Dummies, as well as Diabetic Athlete’s Handbook—currently being revised and published as a new edition in February 2019. She is Professor Emerita of Exercise Science from Old Dominion University and an internationally recognized diabetes motion expert. The author of 12 books, 27 book chapters, and over 400 articles, she was honored with the 2016 American Diabetes Association Outstanding Educator in Diabetes Award. Contact her via her websites (SheriColberg.com and DiabetesMotion.com).