Application potential of continuous glucose monitoring (CGM) in elite endurance athletes without diabetes: What do physiology and current evidence tell us?

Continuous glucose monitoring (CGM) has become of increasing interest in the world of endurance sports. This wearable technology interstitial glucose concentrations has been proposed to offer valuable insights into blood glucose metabolism; however, its true application potential in athletes without diabetes remains under debate. This paper discusses current evidence on the possible relevance of CGM use in elite endurance athletes without diabetes, regarding fuelling strategies, exercise-associated hypoglycaemia, monitoring of training load and energy balance, sleep, and nutritional guidance outside sports. (1) Validity issues exist around the use of CGM (interstitial versus blood glucose), and challenges further arise during exercise with a decreasing accuracy due to the time lag between blood and interstitial glucose; (2) CGM does not measure muscle glycogen concentrations or carbohydrate (CHO) flux, thereby limiting its efficacy as a ‘fuel sensor’ or to guide fuelling strategies. Also importantly, despite existing claims, evidence causally linking specific glucose concentrations (outside the hypoglycaemic range) or glucose stability to performance, remains absent; (3) While frank hypoglycaemia can cause premature fatigue and CGM might theoretically predict low glucose concentrations during prolonged exercise, its role in avoiding so-called ‘bonking’ seems limited by the late onset and low likelihood of hypoglycaemia in well-fuelled athletes, as well as doubts whether its use would actually change fuelling practices; (4) CGM might help identify athletes susceptible to rebound hypoglycaemia by analysing glucose responses in relation to pre-exercise CHO intake and timing, however performance implications of such transient hypoglycaemia are still unclear and symptoms should also be evaluated; (5) Whether CGM data could provide early indications of overtraining or negative energy balance, or be used to improve recovery or sleep – through analysis of nocturnal glucose trends – is uncertain since current evidence is scarce and inconclusive; (6) Application of CGM outside sports to optimise nutrition can currently not be recommended, and must be approached cautiously to avoid misinterpretation. We emphasise the need for informed interpretation to prevent unnecessary dietary adjustments or impairment of recovery. These include the (physiological) time delay in glucose measurement and accuracy concerns during (high-intensity) exercise, the fact that CGM is not a sensor for overall CHO availability, and risks of overinterpretation or worrying. Research is needed to determine whether optimal glucose ranges with regard to performance exist in athletes without diabetes, and to investigate the integration of multi-metabolite sensors for real-time monitoring of additional substrates. The application of CGM in healthy, elite endurance athlete practice is currently limited because (1) Validity and accuracy issues limit its adoption even if there is a physiological rationale; and/or (2) Physiological rationale is often missing or flawed; and/or (3) There is a lack of causal evidence for several of the proposed benefits. More controlled, real-life studies are needed to shed light on how CGM may play a role in this population and thoroughly investigate its potential applicability in elite sports contexts.