Triathletes are very much in to collecting data and metrics—and, for the most part, with good reason. But it’s one thing to collect data and another to actually utilize it in a useful way. How do you know if the latest technology is all it claims to be or just another gadget that generates a lot of useless numbers?

Recently, you may have seen or heard about the latest piece of data collection: continuous glucose monitoring (CGM). This is also known as real-time blood glucose monitoring. You may have seen high-profile triathletes posting on social media about one of the new glucose monitoring devices on the market or seen the sponsorship entry into the endurance space by those companies. The two main devices are made by Levels and Supersapiens. These real-time glucose monitoring devices take the form of a small disc that attaches to your arm and transmits data to an app on your phone.

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Before I get in to the science that either supports or refutes the use of these devices, we need to first understand the basic physiology of glucose control in the body and then we can discuss whether or not monitoring it makes any sense for non-diabetics.

Consuming food is a requirement for survival. The nutrients that we take in when we eat serve many purposes but one of the principal ones is to provide metabolic fuel for our cells. Fuel comes in many forms but the preferred form for all of our cells is glucose and that is obtained through the consumption of carbohydrates. Most of our cells can make use of alternative fuels like free fatty acids or ketones, but these are much less efficient fuel sources and so glucose is always preferred. Some cells, specifically those in highly metabolic active tissues such as the heart, nerves, kidneys, and white blood cells, will only make use of glucose except in rare instances.

The body has developed a means of ensuring that when there is a shortage of glucose, those cells that can avail themselves of alternative fuels will not use what glucose is available. Conversely, when glucose is plentiful, this same system ensures that all cells get access to the preferred fuel.

The means by which this is tightly controlled inside our bodies is through the secretion of insulin when glucose is plentiful and counter-regulatory hormones like glucagon when it is not. Insulin is required for cells to be able to access glucose in the blood, except for those cells that are mandatory users of glucose—ie. the heart, kidney, brain etc. Without insulin, only those specific tissues can use glucose, the others must use alternative fuels.

Consider what happens then after you have just eaten. In this setting the glucose level in your blood is high. So the pancreas secretes insulin, and all of your cells get to take up the available glucose and use it as their fuel. When glucose levels are low, only those cells that don’t require insulin get to use the glucose available in your body—ie. the heart, kidneys, brain—and the insulin-dependent cells, like your muscles, are then forced to use less efficient alternative fuels. In this setting, the counter-regulatory hormone glucagon is secreted and then various processes are started to actually create glucose via the breakdown of glycogen and fats and protein—all to try and restore the normostasis of glucose levels.

This system is highly complex and controlled within your body. Healthy people keep their glucose levels within a pretty narrow range of concentrations, even under a very wide variety of environmental conditions. Even in periods of prolonged starvation, glucose levels will be maintained at the low end of this range in order to fuel those glucose dependent cells.

People with diabetes have lost the ability to produce insulin. (Type 2 diabetics actually produce insulin, but have just become resistant to the hormone’s effects.) As a result, glucose cannot get in to their cells and continues to rise higher and higher, causing all manner of problems. For a long time, diabetics would check their glucose levels periodically and then inject themselves with insulin depending on the level of sugar in their bloodstream. But this was not a real-time way to manage the disease and, inevitably, there would be periods were blood glucose would run high and affect their long-term health.

This is where continuous glucose monitoring comes in. Over the past couple of decades, CGM coupled with an insulin pump has allowed patients with diabetes to much more precisely control their blood glucose in real time and to decrease many of the long-term effects of the disease.

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Now, CGM has come to endurance sports. The premise is that knowing your glucose levels on an ongoing basis will allow you to better control your metabolism and fueling—and ultimately improve your performance.

As for actual evidence, there are only a couple of studies out there on CGM with relation to athletes. The first was out of Japan and looked at ultra runners—but only studied seven of them. It essentially found that, unsurprisingly, all the runners’ glucose levels stayed within the normal range throughout the test. There was a suggestion that higher glucose levels correlated with higher running speeds in some of the participants, but because the participants were running on varying terrain the authors could not be sure.

A second study on CGM from 2017 was better designed but also small. This study on cyclists showed that again glucose levels fluctuated over time and related to food intake, but for the most part stayed within the normal range and in almost all cases did not correlate with performance.

Clearly, it is still early in the research on athletic performance and CGM—but for now there is simply not a whole lot to go on and nothing to suggest the technology is going to be anything more than a novelty.

Dr. Lisa Kosmiski, an endocrinologist at Denver Health Medical Center who works extensively with diabetics, was surprised these devices were being considered for athletes. To paraphrase her take: Why in the world would anyone need to know their real-time glucose levels (if they’re not diabetic) and what would they expect the data to tell them? They can’t change how their bodies react to glucose anyway.

For a technology or a metric to be useful, an athlete must understand what it is telling them and then be able to act in response to the data. In other words, they have to be able to change behavior based on the metric in order to improve performance. I am unconvinced at this time that CGM rises to this level, except for diabetics who then respond with insulin injections. Our bodies handle glucose in a predetermined manner that cannot be changed, and it is not certain how knowing our glucose levels, then, is either relevant or useful.

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