History tells us that true innovators and elite performers will assess the utility of strategies, interventions and technology themselves, with the science to follow, often a few years behind. That said, sometimes these individuals are the scientists or indeed the time difference between adoption and scientific publication is reduced. It’s no doubt with these factors in mind, that the latest paper on CGM use in athletes was produced.
Of course we read it while it was still fresh out of the oven and have distilled it down for you into the key takeaways, because we get the “time crunched athlete” thing. We also get that you probably want ‘our take’, so we’ve given that too…we’ve got you.
The paper is nicely organized into different sections around key concepts; if it ain’t broke…
1) Glucose: A Circulating Fuel
Glucose homeostasis (maintenance of balance) is crucial and the body works hard to prioritize this mechanism.
The key players in this are insulin (drives glucose into cells), glucagon (works antagonistically to insulin - driving up glucose levels through release of glycogen aka glycogenolysis and production of new glucose aka gluconeogenesis) and muscle contraction which also removes glucose from the blood (we covered that here).
More recently lactate (NOT lactic acid - it’s not an acid, and it’s definitely not a waste product that causes the burning sensation) has been shown to be a key metabolic regulator and fuel source.
“Consequently, skeletal muscle tissue can affect blood glucose homeostasis not only by glucose import but also by lactate export.”
Our Take: Lactate dynamics are a key variable in understand glucose dynamics - we can’t wait for continuous lactate monitors
2) Endurance Athletes Spare Glucose During Exercise
“Skeletal muscle glucose uptake can increase up to 50-fold during exercise compared with rest”
Glucose control during exercise is different to what it is at rest, but it’s a continuum as is usually the case in physiology. As one transitions from rest where insulin and glucagon play primary roles, muscle contraction starts to play a more significant role. As muscle contraction takes over as the dominant glucose lowering mechanism, insulin levels drop.
During higher intensity exercise, glucagon release (to increase glucose levels) is stimulated by catecholamines (stress hormones, like adrenaline) - this looks to be more pronounced in endurance trained individuals. This glucose increase is from the liver (glycogenolysis and gluconeogenesis - see above for definitions above) and is due to the rate of glucose release from the liver being in excess of the rate of glucose uptake (we covered some of these dynamics here). The difference between these is exacerbated by intermittent activity (e.g. interval training) because uptake (being driven by muscle contraction) changes rapidly with changes in activity whereas glucose release from the liver is slower to change.
Additional differences between endurance trained and non-endurance trained individuals include increased fat oxidation rates in endurance trained individuals which adds to the prevailing picture of elevated glucose concentrations in high intensity activities in elite athletes.
Our Take: The authors mention they have seen this picture (high glucose in well trained individuals during high intensity activity) and as a company that has seen some of the world’s best athletes’ glucose levels, we wholeheartedly agree that:
“elite athletes have a glucoregulatory response to exercise that differs from the healthy recreationally active subjects. Conclusively, hyperglycemia can be expected during high-intensity exercise in endurance athletes.”
3) Carbohydrate Supplementation Protects Blood Glucose Homeostasis During Exercise
Whilst endurance trained individuals are adapted to better maintain glucose levels, hypoglycemia still occurs and can impact performance, in fact some of the early research on this was as early as the 1920s (as we detailed here). Carbohydrates themselves have been shown to improve performance in a number of ways (some without even needing to swallow the carbohydrates), and appropriate dosing should be undertaken.
Our Take: Whatever your dietary paradigm, and whatever paradigm you subscribe to with respect to the role of carbohydrates in endurance sport (maintaining blood glucose itself or being a preferential fuel source) there is a role for better understanding your glucose dynamics, and how your body responds to different fueling strategies. This is where CGM can play a role.
4) Hypoglycemia and Hyperglycemia in Endurance Athletes
Athletes commonly experience both hypo- and hyperglycemia (despite the aforementioned homeostatic control, we are learning that the range of glucose is much broader than traditionally thought) . Some of the hyperglycemia (high glucose) can result from exercise intensity itself. As a result of this and post exercise carbohydrate replacement requirements, athletes can often have less stable glucose than non-athletes.
There is some evidence that glucose tolerance is reduced post exercise, even though it is generally thought that exercise improves insulin sensitivity (which would improve glucose tolerance). Some recent evidence suggests that increased proportions of type-I (aka slow-twitch) muscle fibers improves insulin sensitivity and increased proportions of these muscle fibers is a result of endurance training itself.
There does not appear to be evidence that hyperglycemia in endurance athletes is a risk factor for later life metabolic disease.
Our Take: Any situation should be considered in its net output, as such, the transient reduced glucose tolerance and reduced glucose stability in athletes likely represents either a variant of normal or a situation where the net result is positive (because of the preceding exercise). This may account for what may be seen as deleterious effects and lower rates of metabolic disease in aging athletes.
5) Low Energy Availability can Reduce Blood Glucose Levels
There is early evidence that glucose levels may serve as an early proxy for low energy availability (LEA) and thus perhaps RED-S (relative energy deficiency in sport). We have previously discussed this here.
Our Take: We have noted and seen some relationship between both training load and its interactions with fueling such that it looks as low overnight glucose may be an interesting early signal in a LEA type of picture. It is certainly something some athletes using Supersapiens note, whereby their overnight glucose being lower and more stable suggests to them to keep an eye on fueling.
6) Hypoglycemia Might Disturb Sleep
Having glucose drop below certain levels may precipitate an awakening from sleep for individuals. Of course, this could impair recovery (which we discussed here). Yet to be shown specifically in research in athletes, though, is whether the frequent overnight low glucose observed in this population may contribute to poor sleep.
Our Take: Whilst we have had users tell us that they notice some drops can wake them up it may be more useful to consider if this was spurious. If so, it is still unlikely that glucose which is lower than normal or more frequently dropping than normal is going to be helpful to an athlete. Perhaps the best signal of all in glucose is the change from the individual's normal. The group data from Supersapiens users does shed some interesting light on sleep and glucose.
7) Surveilling Interstitial and Blood Glucose During Exercise and Recovery
There are some differences between blood and interstitial glucose (we’ve covered this a few times including here) and as noted by the authors:
“during exercise, changes in interstitial glucose seem to occur more rapidly than in blood”.
At times there can be differences between glucose responses to identical meals, speaking to the complexity of glucose and a use case for glucose visibility (as we covered here). There can also be some minor differences in readings with differing sensor placement (which we discussed here)
The signal that is glucose, may be a valuable one for athletes to track with respect to the recovery and refueling picture, or as the authors said:
“CGMs hold promise as a tool for monitoring glucose variability, energy balance, and recovery status in endurance athletes”
“However, a failure to increase glucose during high-intensity exercise has been associated with maladaptations to the training load during overreaching, possibly through a reduced catecholamine response during exercise. Postexercise glucose measurements have therefore the potential to be used to detect early indices of overtraining.”
Our Take: Blood and interstitial measures of glucose should neither be confused nor equated, they may at times be similar but the two measures are distinct. The changes in interstitial glucose have been suggested to be more reflective of what is occuring at the muscle level and may well serve as a useful biometric as part of a holistic athlete monitoring system when it comes to recovery, performance and health.
Leaving you with some of the authors’ quotes may be the best way to summarize this paper:
“These findings challenge our traditional assumptions about glucose control and suggest that interstitial and blood glucose levels may be an overlooked parameter in optimizing athletic performance”
“These insights may help athletes tailor their training and nutrition plans to meet their individual needs, ultimately leading to improved performance and better health outcomes.”
Our Take: We love to see it!
Reference:
- Flockhart, M., Larsen, F.J. Continuous Glucose Monitoring in Endurance Athletes: Interpretation and Relevance of Measurements for Improving Performance and Health. Sports Med (2023). https://doi.org/10.1007/s40279-023-01910-4
- Jeukendrup, Asker E; Chambers, Edward S. Oral carbohydrate sensing and exercise performance. Current Opinion in Clinical Nutrition and Metabolic Care 13(4):p 447-451, July 2010. | DOI: 10.1097/MCO.0b013e328339de83
