CGM in Cycling – Practical Applications in Training

By Jiri Kaloc

Continuous glucose monitoring boasts a lot of benefits, many of which depend on how well you use the information it gives you in practice. Let’s take a closer look at two examples, using CGM pre-training and during training.

Use CGM to optimise your pre-ride nutrition

Your last meal before you head out for a ride has a big impact on how you feel and perform. Cyclists should practice pre-ride nutrition in training to avoid something called rebound hypoglycaemia. CGMs give you exact numbers to evaluate which foods, portion size, and timing reduce the risk.

What is rebound hypoglycaemia? It is a big drop in blood-glucose levels (less than 85 mg/dL on your CGM) which can happen after eating carbs 15-75 minutes before exercise. That’s because eating carbs causes your body to produce insulin. Insulin helps decrease blood-sugar levels back to normal. The problem is that when you start exercising, the increased demand for sugar and blood flow in the muscles decreases you blood-sugar levels. Add the elevated insulin that decreases it further and you can find yourself in a hypoglycaemic state.

CGM will help you find out whether you’re susceptible to rebound hypoglycaemia. If you are, here are a few tips on what to change to avoid low blood sugar.

  • Don’t eat anything in the last 60 minutes before your ride and only start fuelling on the bike after your legs are spinning.
  • Instead of a sugary snack just before you hop on the bike, eat a meal with complex carbs (rice, pasta, bread, etc.) about three hours pre-ride.
  • Experiment with the size and composition of your last pre-ride meal. Choosing low-glycaemic-index carbs (beans, whole grains, etc.), adding extra fat, protein, and fibre slows the release of glucose into your blood stream.

Experiment with all three of the suggestions above and use a CGM to see which strategy helps you avoid rebound hypoglycaemia best.

Pasta Bowl
Eat a meal with complex carbs before your ride. © Profimedia

Use CGM to avoid hypoglycaemia during training

The risk of hypoglycaemia only increases as you ride for longer at a high intensity. The good thing is that based on lab studies we know that glucose levels begin to drop about 15-30 min before the hypoglycaemia occurs. CGM allows you to keep an eye on your glucose levels in real time and anticipate “bonking” while you still have time to do something about it.

Without a CGM, physical symptoms of hypoglycaemia such as a lack of power, dizziness, or nausea could be the first signs you notice. Unfortunately, these often appear too late, when your glucose levels drop below 70 mg/dL and you may need to slow down significantly or stop altogether to recover.

If you see a downward trend in glucose during a long ride, especially if it dips below 85 mg/dL, consider it an urgent warning to eat or drink something rich in easy-to-digest carbs. Just note that you may need to wait a few minutes to see the effects of your fuelling. Interstitial-fluid-glucose levels (that’s where GCMs do their measurements) often change a few minutes after blood-glucose levels.

Should you use a CGM when racing?

It’s clear that a CGM would give you an advantage in a race because with real-time glucose data you would see exactly how well-fuelled you are, so you could time your nutrition to have enough in the tank for the most important part of the race. Unfortunately, CGMs are currently banned by the UCI, so most pro races don’t allow them. As an amateur, you can check with the organisers of smaller local races to see their policy on using CGMs.