The pros and cons of continuous glucose monitors for cyclists

Athletes and coaches love data. But discerning which data are useful and which devices provide value for athletes can be confusing. Along with other wearable tech, the appeal of continuous glucose monitors (CGMs) has gone up and down in popularity with endurance athletes. CGMs monitor interstitial glucose concentrations in (close to) real time. Because the measure is not directly taken from blood, there is a time delay in the appearance of glucose in the interstitial, or between-cell, fluid. Even so, some athletes are intrigued by the insight into how their bodies respond to energy and carbohydrate intake, exercise, and the combination. The real question is how do we use these insights to inform fueling behaviors? 

Normal regulation of blood glucose

When we eat carbohydrates outside of exercise, the normal homeostatic response of the body is to release insulin in response to the higher blood glucose to maintain blood glucose concentration within the normal range. The amount and type of carbohydrates, as well as combining carbohydrates with fat and protein, can influence the glycemic response and therefore the insulin response. Generally, higher-fiber complex carbohydrates and the addition of fat and protein decrease the rate of and magnitude of glycemic response. Additionally, more endurance training improves insulin sensitivity, so when blood glucose is increased, the body may need less insulin in response. Further, the first phase of recovery following a workout involves insulin independent glucose uptake into the muscle, which means that even though our blood glucose increases after post-exercise carbohydrates, we don't necessarily require insulin to manage this. Non-diabetics should not be concerned with fluctuations in glucose in response to carbohydrate intake, as this is a normal, healthy, physiological process. Thus, for a non-diabetic athlete, the usefulness of a CGM in everyday life is fairly limited. 

During exercise

Fuel oxidation during endurance exercise is somewhat complex. Fat and carbohydrate are the main fuel sources, with stored carbohydrate (liver and muscle glycogen) being limited in capacity, necessitating the intake of exogenous carbohydrate to support endurance performance. Because our bodies maintain blood glucose within a tight range via homeostatic regulation, the liver will release glucose by breaking down glycogen and amino acids as necessary to maintain blood glucose. During exercise, it's not unusual to see blood glucose drop to near hypoglycemic levels, ~70 mg/dl or slightly below. It's also not unusual to see it increase at the start of exercise and during intense exercise. 

While this response is relatively understood about blood glucose during exercise, we still don't know exactly how much glycogen we have in our liver and muscle at any given time, and muscle glycogen is a main contributor of glucose for fuel oxidation during exercise. Additionally, when we eat or drink carbohydrates during exercise, blood glucose concentration doesn't completely reflect rates of blood glucose appearance from the liver or GI tract and disappearance or uptake into tissues. Also, we don't completely understand if and the degree to which blood glucose itself could influence an athlete's RPE, pace, power output, or performance. Because fatigue is so complex and multifactorial, simply maintaining high blood glucose does not mean an athlete won't experience fatigue due to other factor(s). Combine this with the 5- to 10-minute time delay of glucose concentration in the subcutaneous interstitial fluid where CGMs measure, the usefulness of CGMs for nondiabetics is again somewhat limited during exercise. Recent evidence indicates that CGM sensors may overestimate glucose compared to venous measures, but they may still be reliable at predicting a blood glucose pattern.

What if I still want to use one?

If you have the opportunity to use one or you can't resist investing in one, CGMs might provide some insight into your fueling needs and strategy. For example, CGM data may help you improve your choice of types and quantities of carbohydrates and timing carbohydrate intake. However, because glucose levels do not necessarily translate into performance outcomes, it's still questionable whether a CGM could actually help with your during-race fueling strategy. Finally, keep in mind that the location of the device can also induce some variability in the measure during exercise.

The bottom line

There is still a lack of evidence that CGM usage can improve athlete performance. Could it possibly help improve behavior? For some people who prefer to learn using wearable tech, maybe. Following established, evidence-based nutrition guidelines from a reputable sports dietitian nutritionist and learning your own individual response to food types, quantities, and timing are also extremely valuable, however. For non-diabetic athletes, over-focusing on small, nonphysiologically relevant swings in interstitial glucose can be distracting and potentially lead to unhealthy fueling behaviors. Finally, CGM has not been validated for assessment of adequate energy availability, nor is it reflective of glycogen storage or substrate availability and fuel oxidation. In my opinion, CGM usage is oversimplifying a highly complex interaction of systems within the body. Athletes have higher carbohydrate requirements and should focus on meeting those, timing them optimally, and eating nutritious sources of carbohydrates to support health and performance.

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