Continuous Blood Glucose (CGM) use in Athletes. How can Glucose Data Enhance Performance?
By Amelia Baker, RDN (Registered Dietitian Nutritionist), CDCES, (Certified Diabetes Care and Education Specialist), CSSD
and Briana Bruinooge RDN, CSSD (Certified Specialist in Sports Dietetics)
Continuous Glucose Monitors (CGMs), have been FDA approved for those with diabetes since 2000, but only recently became a technology of interest for athletes hoping to gain an edge from optimizing glucose levels.
How can tracking glucose data optimize fueling strategies and performance? Is glucose monitoring a tool you should incorporate into your training? In this post we explore CGM tracking for the athlete, and provide anecdotal information based on our experience wearing a CGM.
What is a CGM and how does it work?
A CGM is a tool that has two components, a transmitter and receiver. Some people use a smartphone as a receiver for the device. The transmitter/sensor component is a small piece of plastic about the size of a quarter that’s typically worn on the back of the upper arm. It’s placed on the body using a spring, in order to insert a silicon cannula which houses a filament that passively measures the glucose concentration of interstitial fluid (fluid between cells) under the skin. This device is worn for 10- 14 days, and glucose is tracked once every 5 minutes, to supply 256 glucose readings in a 24 hour period.
What’s normal blood sugar for a person who doesn’t have diabetes or prediabetes?
Table 1: Goal Glucose Ranges for People without Diabetes
Glucose values constantly change throughout the day and can be affected by many outside factors. These factors include:
amount of carbohydrate and type of carbohydrate in a meal
amount of fiber, fat and protein in a meal
stress
growth and development
lack of sleep
alcohol consumption
menstrual cycle or menopause
allergies
exercise intensity type and duration
fitness
low energy availability (LEA)
having type 1 or type 2 diabetes
‘Normal’ blood glucose is a spectrum, and can range from under 70 mg/dL to over 180 mg/dL depending on a number of external and internal variables.
Figure 1: “Continuous glucose monitoring (CGM) can be used by endurance athletes to track glucose levels during exercise and recovery. With knowledge of the typical glucose responses to exercise and diet in an athlete, deviations from expected glucose responses can be detected and be used to interpret levels of stress, energy availability and health. However, it should be noted that further research is needed to determine the value of CGM as a tool to better understand an athlete’s nutritional needs and to surveil the quality of recovery”
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10933193/
I’m wearing a CGM and my glucose is high, should I be worried?
Always consult with your doctor or a Registered Dietitian Nutritionist (RDN) if you have concerns about high glucose values. The hemoglobin A1C lab will be more helpful in identifying glucose control for a doctor to identify risk for diabetes, or to diagnose diabetes. For most athletes, short periods of hyperglycemia (glucose over 180 mg/dL) is no cause for concern.
Exercise can (and usually does) increase glucose values! This is because cortisol (a hormone released during exercise), promotes the breakdown of glycogen in the liver and muscles to bring more glucose into the bloodstream/interstitial fluid. It is important not to equate short term glucose responses with long term outcomes. It is well established that exercise is good for the individual, and can reduce long-term risk of prediabetes and Type 2 diabetes. Think about glucose spikes as you would cortisol levels with exercise and also blood pressure. In the short term, these are all elevated with exercise, but cause no long term harm.
Figure 2: 24 hr segment from continuous glucose monitoring (CGM) in a male elite cyclist
https://pubmed.ncbi.nlm.nih.gov/24584695/
Above is a sample CGM upload of an elite cyclist vs a healthy control on a workout day. As you can see, 2 hours after mealtimes, the glucose value of both individuals returned under 8 mM (140 mg/dL), and pre-meal glucose values were <6.1nM (110 mg/dL). What’s important to note is that in between these times, glucose values can increase quickly in relation to exercise, stress, and digestion. The combined effect of carbohydrate ingestion and exercise on glucose values after a hard effort caused in this elite cyclist a glucose value of 12 nM, (216 mg/dL). For athletes concerned about hyperglycemia (glucose values higher than 180 mg/dL) it should be noted that there is no evidence that having occasional hyperglycemia might have negative effects on endurance athletes’ health in the long term. Instead, aging athletes appear to be protected against developing metabolic diseases and master athletes have been shown to “have lower fasting blood glucose levels than the general population”.
I’m wearing a CGM and my glucose is LOW <60, should I be worried?
In the first 24-hours of wearing a CGM, most people will have false lows, sometimes as low as in the 50-60 mg/dL range. Check your device’s instructions. An example of this from my experience with the G7, is that the first day, my phone was giving me alerts that my blood sugar was too low, but it was just incorrectly reading my blood sugar levels. An example of why using a CGM could potentially cause unnecessary stress if read improperly.
Other lifestyle factors and glucose values
How does alcohol affect glucose values?
The sugar and alcohol content of the beverage, as well as whether the person being measured has food in their system will affect the numbers. Alcohol can decrease blood glucose in the short term. If the alcoholic beverage contains carbohydrates (beer, mixed drinks, wine), the hypoglycemic effect may be countered by the quick-digesting carbohydrate component of the drink.
How does caffeine affect glucose values?
Caffeine could affect BG by stimulating the release of cortisol and adrenaline. Most people will have no change in glucose values with caffeine ingestion <400mg per day (about two 8 oz coffees per day). However, if there is any effect at all on glucose, typically it would be a rise in BG from caffeine rather than a decrease in blood glucose. Some people with diabetes will need to bolus insulin for black coffee. Caffeine is not only found in coffee, but also soft drinks, teas, and energy drinks. Some studies have shown that drinking coffee (caffeinated or decaffeinated) may reduce the risk of developing Type 2 diabetes (mayo clinic).
How do the macronutrients fat and protein affect glucose values?
Fat and protein are different from caffeine, in the way they can affect glucose values. If you were to have a meal without carbohydrate, but with protein, a portion of the protein may be used by the body to create glucose (a process called gluconeogenesis). Fat and protein in the presence of carbohydrate in a meal will slow digestion to cause a delayed increase in blood glucose. If you're seeing a blood glucose rise overnight, this could be due to delayed digestion, or it could also be the effects of hormones on your blood glucose concentration.
Does hydration affect blood glucose, and how?
Hydration status can affect glucose values. Dehydration reduces the water in the bloodstream and interstitial fluid (which the CGM measures). With reduced fluid comes increased concentration of glucose, and therefore higher numbers.
Why does too little sleep cause an increase in blood glucose?
Lack of sleep can increase cortisol in the body, which can weaken sensitivity to insulin.
Why does stress increase blood glucose?
Cortisol and possibly adrenaline, which are both stress hormones, can affect blood glucose values. Adrenaline and cortisol can elevate BG during exertion, and would likely remain in range for an individual without diabetes. However, each activity can have a different effect on BG and it is possible to see BG rise during and after an intense physical activity >180 mg/dL.
My glucose rises after I fall asleep. Why?
During sleep, there are numerous housekeeping activities your body does. Digestion, sleep and wake hormones, and growth and development hormones can all affect BG concentration.
People with diabetes can experience something called the Dawn Phenomenon or Somogyi effect.
What is the Dawn Phenomenon? Somogyi effect? Why does it happen?
Dawn phenomenon is when your body increases cortisol and growth hormone, which increases your BG in the early morning between 3am and 8am (https://my.clevelandclinic.org/health/diseases/24553-dawn-phenomenon). For someone without diabetes, you may not see this so pronounced, and BGs will remain in range because your well-functioning pancreas will release appropriate insulin to regulate your blood sugar. Dawn phenomenon is part of our body's circadian rhythm. We need circadian rhythm for longevity. For someone with insulin resistance, BG may be out of range from 3-8am due to the dawn phenomenon.
The Somogyi effect also tends to affect only those with Type 1 diabetes. The Somogyi effect is often referred to as ‘rebound hypoglycemia’ because it happens when a low blood sugar (hypoglycemia) episode overnight leads to high blood sugar (hyperglycemia) in the morning due to a surge of hormones.
Does my menstrual cycle affect my glucose values?
Yes. Progesterone and estrogen (sex-related hormones) can augment the body's sensitivity to insulin. For people with diabetes, blood glucose values may trend slightly higher or lower in the week leading up to your cycle. For those without diabetes, values may be affected, but likely not cause glucose values to trend out of range (under 70 or over 140 mg/dL).
Final Thought: You may want to think about the amount of data you will receive once you wear a CGM. The level of detail of every 5 min data points may be anxiety producing for some.
Thoughts as a Dietitian recommending Continuous Blood Glucose Monitors for athletes?
Using a Continuous Blood Glucose (CBG) monitor without professional guidance turned out to be more challenging than I anticipated. Even as a Registered Dietitian Nutritionist (RDN) and Certified Specialist in Sports Dietetics (CSSD), I found myself feeling anxious when my glucose numbers went "out of range," despite knowing that many factors beyond food can affect blood glucose levels.
Over the 10 days of monitoring, I observed surprising fluctuations due to exercise, hormonal changes before my period, seasonal allergies, stress, and delayed digestion from a late-night meal and alcohol consumption. For instance, during what I thought was an "easy" bench press workout at the gym, my blood glucose spiked to over 180 mg/dL, which shocked me. After I went to an indoor rock climbing gym I noticed my BG levels peaked to 182 mg/dL, but eventually came back down to <140 mg/dL. During a soccer match, I saw BG peak at 186 mg/dl, then start to trend back down. I also noticed my glucose levels rising without eating anything, like when it jumped from 80 to 120 mg/dL in just an hour one morning. One night, my BG peaked at 150 and continuously had peaks and drops throughout the night, likely due to delayed digestion from a late-night pb and J and hard cider. These experiences made me worry that I might have pre-diabetes, when in fact, I likely don’t. It was eye-opening to realize how many non-food-related factors—like exercise, hormones, and even possibly seasonal allergies (inflammation)—can affect blood glucose levels, and it reinforced for me the importance of not jumping to conclusions based on CBG data alone.
Luckily, I had Amelia Baker, RDN, CDCES, (Certified Diabetes Care and Education Specialist), CSSD (Certified Specialist in Sports Dietetics) answer all of these burning questions as she has extensive education and experience in this area of dietetics. I felt more calm knowing that my blood sugar looking like mountain peaks throughout the day and night was normal based on my lifestyle. It also showed me how different active individuals can be from the “normal”, which holds true for most things nutrition-related.
I hope this blog answered some of your burning questions. Let us know if you have any more questions related to CGM’s and prediabetes. If you haven’t already, check out some of our other blogs like “Prediabetes and Nutrition for Athletes: Optimizing Performance and Health”.
The Bottom Line: Is a CGM right for me?
This is a tough question. CGMs can give us information that can be very useful in identifying health conditions such as prediabetes, and diabetes. For athletes who do not have diabetes, a CGM becomes less useful. While Continuous Glucose Monitors (CGMs) offer valuable insights into blood glucose fluctuations that can help athletes fine-tune their performance, they may not be the best fit for everyone. For individuals without diabetes, the sheer volume of data can lead to unnecessary stress and anxiety, which in turn could actually raise blood sugar levels. Without the guidance of healthcare professionals, interpreting this data can be overwhelming and potentially counterproductive. Therefore, it's important to carefully consider whether the benefits of using a CGM outweigh the potential drawbacks, especially for those who might not need such detailed monitoring.
Final Thoughts on Continuous Glucose Monitors
In conclusion, while Continuous Glucose Monitors (CGMs) offer a wealth of data that can provide insights into blood glucose fluctuations, their utility for athletes without diabetes may be limited. For those looking to optimize performance or for those at risk for prediabetes or Type 2 diabetes, CGMs can help fine-tune fueling strategies and provide a deeper understanding of how various factors like exercise, diet, and stress impact glucose levels. However, it's important to remember that short-term glucose spikes during exercise are normal and not necessarily harmful. The decision to use a CGM should be based on individual goals and the potential benefits of such detailed monitoring. For many athletes, focusing on balanced nutrition, hydration, and overall health may be more beneficial than the minutes-to-minute tracking of glucose levels.
References:
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Brown, A. (2018, February 13). 42 Factors That Affect Blood Glucose?! A Surprising Update. Diatribe; The Diabtribe Foundation. https://diatribe.org/diabetes-management/42-factors-affect-blood-glucose-surprising-update
Does cutting out caffeine improve blood sugar control? (2024). Mayo Clinic. https://www.mayoclinic.org/diseases-conditions/type-2-diabetes/expert-answers/blood-sugar/faq-20057941#:~:text=For%20most%20healthy%20adults%2C%20caffeine
Flockhart M, Larsen FJ. Continuous Glucose Monitoring in Endurance Athletes: Interpretation and Relevance of Measurements for Improving Performance and Health. Sports Med. 2024 Feb;54(2):247-255. doi: 10.1007/s40279-023-01910-4. Epub 2023 Sep 2. PMID: 37658967; PMCID: PMC10933193.
Goals for Glucose Control. (n.d.). Joslin Diabetes Center. https://www.joslin.org/patient-care/diabetes-education/diabetes-learning-center/goals-glucose-control
Ruiz JR, Fiuza-Luces C, Garatachea N, Lucia A. Reduced mortality in former elite endurance athletes. Int J Sports Physiol Perform. 2014;9(6):1046–9.
