Concurrent Validity of a Continuous Glucose-Monitoring System at Rest and During and Following a High-Intensity Interval Training Session

Purpose: To assess the concurrent validity of a continuous blood-glucose-monitoring system (CGM) postbreakfast, preexercise, exercise, and postexercise, while assessing the impact of 2 different breakfasts on the observed level of validity. Methods: Eight nondiabetic recreational athletes (age = 30.8 [9.5] y; height = 173.6 [6.6] cm; body mass = 70.3 [8.1] kg) took part in the study. Blood glucose concentration was monitored every 10 minutes using both a CGM (FreeStyle Libre, Abbott, France) and fingerprick blood glucose measurements (FreeStyle Optimum) over 4 different periods (postbreakfast, preexercise, exercise, and postexercise). Two different breakfasts (carbohydrates [CHO] and protein oriented) over 2 days (2 x 2 d in total) were used. Statistical analyses included the Bland-Altman method, standardized mean bias (expressed in standardized units), median absolute relative difference, and the Clarke error grid analysis. Results: Overall, mean bias was trivial to small at postbreakfast (effect size +/- 90% confidence limits: -0.12 +/- 0.08), preexercise (-0.08 +/- 0.08), and postexercise (0.25 +/- 0.14), while moderate during exercise (0.66 +/- 0.09). A higher median absolute relative difference was observed during exercise (13.6% vs 7%-9.5% for the other conditions). While there was no effect of the breakfast type on the median absolute relative difference results, error grid analysis revealed a higher value in zone D (ie, clinically unsafe zone) during exercise for CHO (10.5%) compared with protein (1.6%). Conclusion: The CGM device examined in this study can only be validly used at rest, after both a CHO and protein-rich breakfast. Using CGM to monitor blood glucose concentration during exercise is not recommended. Moreover, the accuracy decreased when CHO were consumed before exercise.

Automated summary

This study aimed to assess the validity of a continuous blood-glucose-monitoring system (CGM) at different time points and under different breakfast conditions. The results showed that the CGM device was valid during rest, after breakfast, and without exercise, but not recommended for use during exercise. Moreover, the accuracy decreased when consuming a carbohydrate-rich breakfast.