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The Risks Associated with CGMs During High-Intensity Exercise

Oct 19, 2019
Editor: Steve Freed, R.PH., CDE

Author: Adam Chalela, B.S. Doctor of Pharmacy Candidate USF College of Pharmacy

When using CGMs during high-intensity interval training (HIIT), patients with type 1 diabetes may need to change their monitoring regimen. 

Normally, patients with type 1 diabetes do not produce adequate amounts of insulin to achieve healthy glucose homeostasis, thus requiring supplementation with insulin injections throughout the day. Continuous glucose monitors are heavily utilized within this population to track just how much insulin is needed with each administration. A large focus in the recent American Diabetes Association consensus report is towards diabetic nutrition therapy and physical exercise as an add on to existing pharmacotherapy or for medication sparing effects.


One form of physical exercise that is growing in popularity is called high-intensity interval training (HIIT). In HIIT, subjects quickly rotate between short periods of high-intensity physical exercise and lower-intensity physical exercise to mimic a cool down period or rest break without ceasing physical activity. Multiple studies have demonstrated the cardiovascular benefits of HIIT exercise over the conventional continuous training methods (e.g. jogging on a treadmill for 15 minutes straight without rest).

A recent study enrolled 17 patients with type 1 diabetes who continuously monitored their blood glucose with Dexcom continuous readers to determine the real time accuracy of the CGMs during high-intensity interval training. The HIIT regimen was completed a total of four times while participants were fasted over the course of the four week study period. Blood glucose readings from the continuous meter were compared to readings from traditional monitoring as reference. Traditional monitoring of blood glucose was done numerous times before the exercise began, during the HIIT interval training and after it was completed. The HIIT regimen was kept constant for each of the four workouts over the study period and consisted of both cardiovascular exercise and high repetition, lightweight resistance exercise.

The primary efficacy endpoint of this study was the difference in time between half-maximal glucose rise between traditional blood glucose monitoring and continuous monitoring with the Dexcom meter. The secondary endpoint included absolute relative differences of glucose readings as a continuous variable

Results from this study showed a decrease in accuracy from the continuous Dexcom meter not only during the HIIT exercise but also up to one hour after its completion. The largest inconsistency was up to 35.3 mg/dL when compared to traditionally monitored values, occurring during exercise, and the magnitude of bias decreased over time. Dexcom readings were consistent with traditional monitoring values before and for the rest of the follow-up period after the first hour. The overall glycemic effect of HIIT on patients with type 1 diabetes was hyperglycemia with an average increase of 90+32.4 mg/dL that peaked at just over an hour after the start of HIIT over a total of 64 total sessions.

Results from this study suggest that patients with diabetes might expect net increases in blood glucose values during and shortly after exercise. Specifically, for patients with type 1 diabetes who regularly exercise rigorously, an increase in blood glucose should be expected and accordingly planned for. A patient who is particularly sensitive to glucose shifts may be at a higher risk for workout-related injury than one that is not. Emphasis on working out with a partner should be made to patients with type 1 diabetes to increase overall safety. Some instances of hyperglycemic episodes may even warrant larger than normal doses of insulin if a meal is planned to be eaten before a high intensity exercise.

Also, patients with type 1 diabetes who work out should not rely solely on a continuous blood glucose monitor for accurate readings to make any decisions in treatment plan alterations, such as total daily carbohydrate intake or specific insulin doses. If patients with type 1 diabetes wish to accurately monitor their blood glucose during and after a workout, they should traditionally monitor their blood glucose.

Practice Pearls

  • Blood glucose values raise the most right after the first hour after starting HIIT.
  • Continuous blood glucose monitors can be delayed by 35 minutes and provide values up to 35.3 mg/dL different than traditional monitoring.
  • Providers should recommend to patients with type 1 diabetes to monitor blood glucose after exercise by conventional means when considering their effects on treatment plans.

Reference for “The Risks Associated with CGMs During High-Intensity Exercise”:

Li A, Riddell MC, Potashner D, et al. Time lag and accuracy of continuous glucose monitoring during high-intensity interval training in adults with type 1 diabetes. Diabetes Technology & Therapeutics 2019; [Epub ahead of print]. DOI: 10.1089/dia.2018.0387

Adam Chalela, B.S. Doctor of Pharmacy Candidate USF College of Pharmacy