CGM in Cycling – Myths

By Jiri Kaloc

Continuous glucose monitoring has a lot of benefits and there are practical ways to use it in training as a cyclist. But there are also a lot of myths about these devices and about monitoring glucose in general. Let’s take a closer look at the most common ones to help you understand how not to use CGMs.

Myth #1: Higher glucose is better for performance

Cyclists are told to eat a lot of carbs on the bike, 60-90 grams per hour is a lot to consume! It’s only natural that these same cyclists would enjoy seeing high glucose numbers on their CGMs. It may seem like higher glucose levels are a sign of good fuelling and performance. Unfortunately, blood glucose levels alone don’t tell you how much glucose is currently being used by muscles or how much is coming in from food, and that’s much more relevant to your performance.

While riding hard, your CGM can be showing the same numbers as when you’re sitting on the couch. The difference is that on the bike, there is a lot of glucose coming into your bloodstream from food or your glycogen stores, and a lot of it is also quickly leaving the bloodstream as your leg muscles burn it. The CGM doesn’t measure this flow of glucose to and from your bloodstream, it only senses the rises and falls of the overall glucose level. So, for the purposes of exercise, there’s no optimal glucose level you should be trying to hit for the best performance. The only thing you should worry about is avoiding hypoglycaemia (when glucose levels drop below 70 mg/dL). We talked more about that in the previous article.

Myth #2: Food that spikes glucose should be avoided

When you have a CGM on your arm and start seeing the impact of different meals on your blood glucose, it’s only natural to feel like large spikes are bad for you. Blood sugar spikes are often talked about in the context of diabetes, which only fuels the fear of them. It’s important to keep in mind that for healthy people, and physically active people especially, blood sugar spikes after meals are completely normal.

If you occasionally see values outside of the normal range of 70 to 120 mg/dL, it’s not a reason to worry in itself. Only if you keep seeing a pattern with a specific food, then it’s something to take note of. But always keep in mind the scenario during which it happens. There are many factors that influence increases and drops in glucose levels, so it could be that you just have to change the timing of when you eat certain foods. Completely cutting out foods that are otherwise healthy just because you see a glucose spike may not be the right call, especially if the replacement food is less nutritious.

Myth #3: Glucose fluctuations should be avoided

Just like with glucose spikes, seeing glucose fluctuations on your CGM can seem like there’s something wrong. For sedentary people or people with diabetes, large fluctuations or elevated glucose levels can in fact be a warning sign that their glucose control is impaired. Some CGMs even report variability in glucose levels to patients with diabetes for this exact reason. But things are different when it comes to endurance athletes such as cyclists.

Cyclists are generally highly insulin sensitive thanks to regular exercise. Cyclists who ride often and hard can temporarily experience high glucose levels because their muscles create a large demand for glucose. They also eat a lot more food than sedentary people so their glucose levels simply have reasons to change more often. But these fluctuations typically happen for a good reason and aren’t a sign of something going wrong.