Should Athletes Monitor Their Blood Glucose Levels?

Athletes have a tendency to become a bit obsessed with data.  Determined to be the healthiest and fittest versions of ourselves, we track metrics like miles logged, daily step count, sleep, heart rate, and the like. 

Knowledge is power after all and thanks to advancements in technology we increasingly have access to all sorts of information about our health and fitness. Just glancing at my smart watch I could tell you how many hours of restful sleep I got last night (not nearly enough apparently), my respiration rate (no idea how my watch has that figured out), my training status (patting myself on the back for being “productive” today), my resting heart rate (all indications point to the fact that I am still alive), total steps for the day (sitting at my desk all morning hasn’t done me any favors), and that’s just to name a few. 

For many athletes, these stats can be a tool to not only optimize their health, but also performance. As if all those metrics weren’t enough, athletes are just starting to tap into a new tool; continuous glucose monitoring (CGM). 

To be clear, CGM devices aren’t new. Individuals with diabetes have relied on them for years as a way to manage their blood sugar instead of taking regular readings from finger pricks. 

While diabetics use CGM devices to avoid the health complications from blood sugar getting too high (hyperglycemia) or too low (hypoglycemia), a few startup companies are looking to offer these devices for individuals who don’t have an actual medical need. These companies claim CGM devices can also be used to increase energy, reduce inflammation, promote recovery, and enhance athletic performance. Sounds good in theory, but can CGM devices really live up to the hype?

What CGM Devices Do

CGM devices measure an individual’s glucose, more commonly referred to as sugar. As a health indicator, monitoring one’s blood glucose can be very eye-opening. Anytime you consume a source of carbohydrates, those carbohydrates are digested then absorbed into the blood stream as glucose, causing a rise in blood glucose levels. 

The body likes to maintain rather tight control on blood glucose levels, so a big spike in blood glucose from a large pasta dinner or candy bar elicits a response from the body to rein this in, primarily through the activity of insulin. Insulin functions almost like a key by binding to cells and allowing them to take glucose in, thereby lowering how much is circulating in the blood. 

In an ideal world when blood glucose levels become elevated the pancreas pumps insulin into the bloodstream and glucose falls back into a normal range pretty quickly. However, the average American diet is far from ideal. Refined grains and added sugars, which comprise much of the standard American diet, cause a more significant rise in blood glucose than our bodies were designed to handle. That’s bad news for our health because prolonged elevations in blood glucose and large post-meal spikes increase the risk for type 2 diabetes, cardiovascular disease, stroke, obesity, and cancer. 

That’s where CGM devices supposedly come in. By inserting a tiny sensor in the upper arm or abdomen, CGM devices transmit continuous glucose readings to a device or app on the user’s phone without the need for constant finger pricks throughout the day. The sensor is covered by a patch and is typically replaced every 7 to 14 days. As opposed to finger pricks, CGM devices don’t actually measure blood glucose, but instead measure glucose levels in the interstitial fluid. Fortunately, the levels of glucose in blood and interstitial fluid are very close so the numbers can still be interpreted the same. 

Exercising, even virtually, with a partner can improve performance, a 2012 study found. Stock/Adobe

Regardless, the question is what’s the benefit from knowing  your glucose levels if  you do not have type 1 or type 2 diabetes?  It really comes down to the individual. Imagine you just polished off a large creemee and got a reading from your CGM device that your glucose levels are off the chart. Would that really deter you from indulging in such a summertime staple in the future? 

That’s where we come back to the adage that knowledge is power. Knowledge is only powerful if we use it. Perhaps realizing how certain foods effect glucose levels would prompt some individuals to consume a healthier diet.  However, to date there are no published studies showing CGM is correlated with any dietary changes or positive health outcomes for those without diabetes. The jury is still out, but it seems for now using CGM to improve one’s health is no magic bullet and only beneficial for those already motivated to adopt a healthier diet and lifestyle. 

How Athletes are Using CGM

Even if CGM isn’t necessarily the ticket to optimal health, there is increasing interest among athletes in using the device to help optimize performance. Any athlete who has “bonked” before can understand the serious toll low blood glucose can have. That feeling of legs turning into cement blocks and the inability to form a coherent sentence comes down to a lack of fuel going to the muscles and nervous system. 

Just like a car when it runs out of gas, when your body runs low on glucose no amount of grit or determination will allow you to continue on. This is because during high-intensity exercise our muscles rely heavily on glycogen as a fuel source. Glycogen, the storage form of carbohydrate in the body, is broken down into glucose to provide a quick and accessible fuel source to power muscles. 

Unfortunately, the amount of glycogen our bodies can store is limited to enough to power roughly 90 to 120 minutes of exercise. Hence the recommendation to consume exogenous forms of carbohydrates in the form of gels, sports drinks, gummies, and the like during prolonged activity. In theory this is where CGM could help. By essentially acting as the body’s fuel gauge, athletes could use CGM to determine when and how many carbohydrates to consume in order to avoid a dip in glucose that could hinder performance.  

However, there is a big discrepancy between the theoretical and real-world application. The effect glucose levels have on athletic performance is complicated. Although glycogen serves as the predominant fuel during high-intensity exercise it is not the only fuel being used. Fat is also a fuel source for working muscles. 

The degree to which fat versus glycogen is used is determined by a number of factors, the most significant of which is exercise intensity. Go for a leisurely walk and glycogen hardly factors in as fat provides nearly all the necessary energy to put one foot in front of the other. Take that walk up a notch to an easy trot and the body starts to rely a bit more on glycogen, but only to a small degree. As exercise intensity gradually increases, the body shifts to relying more on glycogen and less on fat. Break into an all-out sprint and almost instantly glycogen becomes the go-to fuel source with fat just lending an assist. While that may not seem so complicated it is far from the whole story.

Another factor is how long you are exercising. Blood glucose, which as discussed earlier is kept in a pretty tight range by the body, is largely unaffected during short-duration activity. That’s because even during intense exercise blood glucose is mainly used by the nervous system as opposed to muscles. 

However, as glycogen levels near depletion, the body has no choice but to turn to glucose in the blood for a fuel source. When this shift occurs, muscles can continue working as long as blood glucose levels remain high enough to meet energy needs. 

This is where a CGM device could potentially come into play. It seems simple enough. When users notice blood glucose levels start to drop, they knock back a sports drink or take in a gel before the dreaded “bonk” hits. Once again, theoretically this works, but practically speaking it’s not that simple. Throughout a bout of exercise, it is perfectly normal for blood glucose to rise and fall without impacting performance. Thus, an athlete may incorrectly assume they need to consume carbohydrates before glycogen stores are even close to depleted or conversely not realize they’re running on empty before it’s too late. 

Just as knowledge is powerful only if we use it, knowledge is similarly only powerful if we know how to use it. When it comes to enhancing athletic performance, it seems the how is still yet to be determined. 

CGM devices may not be new technology, but the science of using them to improve health and optimize athletic performance is. At least for now the recommendations to consume carbohydrates prior to endurance exercise and to consume exogenous carbohydrates during exercise lasting 90 minutes or more remains unchanged. As more research is done on the application of CGM for both health and performance we may be able to better use this as a tool for both in the future. 

However, another glance at my watch reminds me that we have no shortage of data available to us already.

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