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How can CGM be used?

CGM is a tool that can give us insights into “blood glucose” in real time! Although we must understand the limitations and be wary of not over interpreting or wrongly interpreting any data that is obtained, the technology can also be really useful for athletes. In this blog we discuss how CGM can best be used!


mysportscience use of CGM in sport

There are a number of very clear and straightforward applications for your CGM. Most of these are related to individual glucose responses to things like foods, meals and snacks and to how your glucose responds during training, sleep and competition. When using CGM, it will become pretty clear that average advice and recommendations are just that: average. The advice we receive is derived from studies in relatively small groups of individuals, sometimes athletes, and is based on “average” responses. Of course, very few people are “average” and almost no one is “average” all the time! So CGM will provide insights that are highly individual and can therefore be used as a learning tool for yourself (we call this “precision care” or “personalized care”). The more and the longer it is used and the more systematically it is applied the more useful insights will be obtained. (Avoid drawing conclusions immediately, just observe and learn. Certainly, don’t start changing your diet after observing a peak in glucose).


CGM will provide insights that are highly individual and can therefore be used as a learning tool for yourself.

Perhaps the most obvious application is avoiding low blood glucose during exercise (called hypoglycaemia). By definition, hypoglyceamia is a sensor reading <70 mg/dL for 15 consecutive minutes. But even a sensor glucose <70 mg/dL for just a few minutes, if it is an accurate measurement, can have effects on sports performance.


Below are 4 possible uses of CGM in sport...


CGM Use 1: Avoid hypoglycaemia during exercise

By monitoring glucose and glucose trends, you can get a better handle on when (and what) to eat or drink in training or in competition. Based on extensive studies in laboratory settings, we tend to see glucose levels begin to start to drop about 15-30 min before the hypoglycaemia occurs. So, watching glucose levels start to drop on a CGM allows for hypoglycaemia prevention since you can begin to ramp up your fuelling….


Hypoglycaemia is usually (not always) associated with lack of power, dizziness, nausea and a number of other undesirable symptoms, particularly when glucose levels drop well below 70 mg/dL.


We suggest that if a downward trend in glucose is noted during prolonged exercise, particularly when the glucose dips below ~80-85 mg/dL (4.5 mmol/L), it should serve as an urgent reminder to eat or drink some carbohydrate containing fuels. Of course, if you have just consumed carbohydrate, this is not necessary to consume more at that moment, you can just wait for that carbohydrate to be absorbed into circulation. As noted in one of our previous blogs, the interstitial fluid glucose often lags behind the blood glucose level and so you might need to wait for several minutes before you see a deflection or rise in you CGM glucose.


When glucose dips below ~80-85 mg/dL (4.5 mol/L), it should serve as an urgent reminder to eat or drink some carbohydrate containing fuels.

Note: Check to see if CGM is permitted for any competitive race or event you enter, since some regulatory agencies have deemed CGM as an unfair advance for athletes such as professional cyclists. It must also be said that when proper fuelling guidelines are followed, hypoglycaemia should not happen in the first place, but each athlete may have their own nutrition protocol that could be fine-tuned with CGM. Sometimes even professional athletes forget to drink and eat in training or competition and CGM could serve as a useful warning signal.


CGM Use 2: Optimise pre-exercise nutrition routines

Rebound hypoglycaemia (sometimes called reactive hypoglycaemia) is the drop in glucose that often occurs during exercise when carbohydrate is consumed 15-75 min before exercise (as we discussed in a previous blog). The reasons hypoglycaemia occurs is that the ingested carbohydrate (or protein) dramatically increases insulin secretion from the pancreas when at rest, but then once exercise starts, insulin sensitivity increases rapidly, and the insulin that has already been secreted can’t be cleared quickly enough. High rates of blood flow during exercise also appear to result in more glucose uptake than the muscles actually need, the liver has difficulty in releasing glucose into circulation when insulin levels are elevated and glucose levels drop precipitously in some, but not all, individuals. In other words, exercise soon after a carbohydrate rich meal tends to result in a relative

hyperinsulinemia in a setting of increased contraction induced uptake by the working muscle. Having a CGM will help you determine if you are susceptible to rebound hypoglycemia and help you modify your carbohydrate intake regimen pre-exercise.


CGM will help you determine if you are susceptible to rebound hypoglycemia and help you modify your carbohydrate intake regimen pre-exercise.

Susceptible athletes can avoid hypoglycaemia in a number of different ways:

  1. Initiate simple carbohydrate feeding only once the exercise begins

  2. Eat complex carbohydrates well before exercise (3 hours or more) instead of shorter before exercise

  3. Change the size of the meal to a larger, more complex carbohydrate-rich meal (e.g. whole wheat pasta or whole grain rice)

  4. Change the composition of the meal to a meal with low glycemic index carbohydrates (e.g. beans, whole grains and unprocessed carbohydrates), more fat (or protein), and more fibre

Note: Some of these strategies may not be desirable in some competition situations where supply of fuel is important early on.


All of the above are tools in your toolbox to manipulate and control blood glucose. Your CGM is the additional tool to see how well your strategy works.


Practice your pre-competition meal

It is recommended to practice your pre-competition meal; use the exact composition and timing of intake you would use before competition. It’s best to use CGM data to help refine your strategies. Initially, this can be in a training situation where you try to simulate the entire race day. Then, start the exercise and study the glucose response from your own CGM tracings, without reacting too much to the ups and down of the glucose on your CGM during the training session. If mild or more advanced hypoglycaemia develops, call it a CGM value <85 mg/dL if you want, this would be a reason to tweak the meal and/or the timing of the meal and possibly aim for earlier in-race fuelling with simple carbohydrates (i.e., gel, fluids). If hypoglycaemia does not develop, and you feel good with your CGM values, this would be confirmation that your nutrition plan is probably adequate, at least for hypoglycaemia prevention. Repeat the routine and measurements a few times (also in real competition if possible) and confirm that the fuelling strategy is working, but that you are not under-fuelling in general for performance.


Once you have a routine that works, stick to it. If you had to change your meal or the timing, just keep repeating this process until no hypoglycaemia develops and you feel well from a gastrointestinal perspective. Sometimes, you need to train your gut too for a new fuelling strategy.


CGM Use 3: Learn how your blood glucose responds to various foods

When an athlete starts to use CGM for the first time, a common observation is that responses are not according to what they have read or thought (i.e., the “textbooks are wrong!” Or the “device is inaccurate”). Also, when comparing your data to someone else’s, it will be clear that CGM responses are highly individual.


Most advice we get is based on averages, but at an individual level, everyone responds differently. With CGM, these individual differences become measurable and visible. You might find some foods push your glucose very high, while other foods and/or beverages keep your glucose flat. This may or may not correspond to what we know from various research sources and textbooks. Now you have a tool that helps you to see how you respond to certain meals and foods and you will learn a lot over time. Please note that there is no immediate action attached to these measurements: just learning!


For example, you might notice that your glucose spikes with a certain food item (like a breakfast cereal or oatmeal). You might want to check first how reproducible the response is and you also may want to test subcomponents of the meal or snack. If you have instant oats with skim milk, the CGM glucose might rise much faster then if you change to a higher fat milk with the oatmeal, use steel cut oats with no added sugar or if you add a bit more protein to the cereal, like a pile of walnuts or add a protein powder... Even having the same food at a different time of day or after an exercise session rather than before can make a huge difference.


CGM Use 4: Evaluate your glucose during sleep

Of course, CGM measures 24h a day including when you are sleep. This means we are obtaining glucose data during sleep, providing insights we had never had access to before. There are very few studies that have published “normative” glucose values during sleep in healthy athletes, and therefore it is impossible to give clear advice based on this data. It is likely that this is an area that will develop rapidly in the years to come.


Anecdotally, one of the early observations is that when fuelling during the day is not optimal during hard training days, glucose levels tend to drop overnight (called nocturnal hypoglycaemia). This may interrupt sleep cycles and impair your capacity to recover. Low glucose may wake you up and you will probably wake up hungry. Some people don’t wake up in the night when this occurs, but it still may be impacting your rest/recovery cycle. Using CGM-guided bedtime snacks may help improve sleep and recovery.


It is important to understand how to use a technology, but it is also important to understand what mistakes should be avoided. There are several mistakes that we have seen athletes make, who used CGM for the first time. Here are the top 3:


CGM Myth 1: Avoid all foods that cause a "spike in glucose"

Some athletes will see a large swing in glucose after eating certain foods. We must remember that this is a normal physiological response to that food and sometimes it might be some other factor that brings the glucose up and then back down. Your body is doing what it is supposed to do. In general, most of your glucose values should be between 70 and 120 mg/dL (3.9- 6.7 mmol/L) (1), but from time-to-time values will be lower or higher because of certain foods, training, and competition. There is likely no need to avoid a particular food altogether because of your CGM results, unless you repeatedly see something unusual (like reactive hypoglycaemia). Maybe there is a need to avoid it at certain times, but there is likely no need to remove it from the diet altogether. From a health perspective, we must always keep in mind that there are many factors at play and a transient increase in glucose is totally normal. It’s not just circulating glucose that is important, but also the micronutrients and macronutrients that support growth, repair and recovery. Avoiding certain foods will likely mean you are replacing it with something else anyway and that replacement may or may not be a “healthier” one for your overall health. By removing one food from your diet and replacing it with another, you may unknowingly promote an undesirable change in another important health risk factor (for example, a rise in LDL cholesterol).


CGM Myth 2: My CGM is my fuel sensor (and a higher glucose is better!)

As discussed in a previous blog, glucose concentration is not a reflection of how much fuel is being used or even when your fuel is running low. Higher glucose is not necessarily better, and lower glucose is only a problem if the values are in the hypoglycaemia range (i.e., <70 mg/dL) and staying in that range for more than a few minutes or if it tends to be associated with symptoms of low glucose (like fatigue, headache, shakiness, hunger, irritability or nausea). Normally, the body can defend against hypoglycaemia on its own, by increasing various glucose rising hormones, but some of that defence mechanism can give you mild symptoms (e.g., adrenaline increases heart rate, causes sweating and shakiness).


CGM Myth 3: You MUST avoid glucose fluctuations and/or high glucose values

It is often said that we need to avoid fluctuations in glucose. This is because in the general population (as a population we are very physically inactive) as well as in people living with diabetes, such fluctuations and high values are seen as risk factors for poor glucose control (or the development of diabetes). To quantify swings in glucose over a given period, some CGM systems report some sort of glycaemic variability index such as glucose coefficient of variability or a glucose stability score. Athletes, and especially endurance athletes, are generally highly insulin sensitive and the glucose responses observed are usually a very normal physiological response. Athletes may eat more foods than the average person and utilize more glucose as fuel. They can also be under competition stress which often increases glucose levels. But they also have good insulin sensitivity so their glucose will quickly return to normal. Therefore, their glucose may rise and fall more often. Athletes will also train hard, and this also means that glucose can sometimes be pushed to higher (and lower) values. This will add to the daily variation, but there is no reason to believe that this has any negative effects. At least, so far no one has shown this.


Average responses over longer periods are probably much more meaningful from health perspective. In other words, if glucose is high on average, this is likely a risk factor for pre-diabetes, or type 2 diabetes over time, or it could be a signal that some hormone are chronically elevated like cortisol or thyroid hormone. In individuals living with diabetes, high average glucose levels and high glucose variability have both been linked to cardiovascular disease morbidity and mortality (2), but there is still much debate about the long-term risk for brief and transient spikes in glucose level that last only a few minutes in people who do not have diabetes. In one interesting study, however, centenarians (people living to 100 years of age or more) in China were found to have lower average glucose values as measured by CGM and less overall glucose variability as compared to non-centenarians (3), but that relationship may not necessarily be cause and effect. In underweight individuals who are not athletic, glucose variability appears to be slightly elevated, and that elevation in glucose variability may be linked to irregular eating habits (4). However, in normal weight individuals, and certainly in endurance athletes, there is far less data available to determine if higher mean glucose levels for a brief period of time, or more glucose variability, will impact health and/or life expectancy!


Summary

CGM is a relatively new technology, at least in sport. As is the case with any new technologies, it takes time to understand the best ways to use the technology. CGM has the potential to be very useful for athletes in and around sport competition, but many of these lessons may be simply reaffirming what you are already doing well. In this blog we stressed 4 possible applications:

  1. Avoid hypoglycaemia during exercise

  2. Optimising pre-competition nutrition routines

  3. Learn how your blood glucose responds to various foods

  4. Lear how your body regulates glucose during sleep and stress

Very recently a review paper on this top was published and for more detail the reader is referred to this (5). There are other useful blogs on Supersapiens, such as Should glucose spikes be avoided? as well as a separate blog on Glucose spikes and crashes.



Reference

  1. Juvenile Diabetes Research Foundation Continuous Glucose Monitoring Study Group; Fox LA, Beck RW, Xing D. Variation of interstitial glucose measurements assessed by continuous glucose monitors in healthy, nondiabetic individuals. Diabetes Care. 2010 Jun;33(6):1297-9. doi: 10.2337/dc09-1971. Epub 2010 Mar 9. PMID: 20215454; PMCID: PMC2875442.

  2. Li S, Nemeth I, Donnelly L, Hapca S, Zhou K, Pearson ER. Visit-to-Visit HbA1c Variability Is Associated With Cardiovascular Disease and Microvascular Complications in Patients With Newly Diagnosed Type 2 Diabetes. Diabetes Care. 2020 Feb;43(2):426-432. doi: 10.2337/dc19-0823. Epub 2019 Nov 14. PMID: 31727686.

  3. Ji SH, Dong C, Chen R, Shen CC, Xiao J, Gu YJ, Gao JL. Effects of Variability in Glycemic Indices on Longevity in Chinese Centenarians. Front Nutr. 2022 Jul 8;9:955101. doi: 10.3389/fnut.2022.955101. PMID: 35879983; PMCID: PMC9307500.

  4. Kashiwagi K, Inaishi J, Kinoshita S, Wada Y, Hanashiro S, Shiga K, Kitazawa M, Tsutsumi S, Yamakawa H, Irie J, Kishimoto T. Assessment of glycemic variability and lifestyle behaviors in healthy nondiabetic individuals according to the categories of body mass index. PLoS One. 2023 Oct 4;18(10):e0291923. doi: 10.1371/journal.pone.0291923. PMID: 37792730; PMCID: PMC10550127.

  5. Flockhart M, Larsen FJ. Continuous Glucose Monitoring in Endurance Athletes: Interpretation and Relevance of Measurements for Improving Performance and Health. Sports Med. 2023 Sep 2. doi: 10.1007/s40279-023-01910-4. Epub ahead of print. PMID: 37658967.


This blog post was co-written with Professor Mike Riddell, PhD from the Muscle Health Research Center, York University, Toronto, Canada.


Disclaimer: Asker Jeukendrup is a consultant to Supersapiens. Michael Riddell serves as a scientific advisor to Supersapiens and as a consultant to Dexcom Inc, another CGM device company.

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