CGM Accuracy During Exercise: What Diabetics Should Know

What Was Studied and Why It Matters

For people with type 1 diabetes, exercise is a double-edged sword. It's great for overall health, but it can cause blood sugar to swing unpredictably — making accurate glucose monitoring during workouts absolutely critical. Researchers wanted to know: how well do two of the most popular continuous glucose monitors (CGMs) — the Dexcom G6 and the FreeStyle Libre 2 — actually perform while you're exercising?

This 2024 study, published in Diabetes Technology & Therapeutics, compared both devices during two common types of workouts: steady-state aerobic exercise (like jogging or cycling at a moderate pace) and high-intensity interval training (HIIT), where you alternate short bursts of intense effort with rest periods.

How the Study Worked

Twenty-six men with type 1 diabetes wore both CGM devices simultaneously while completing aerobic and HIIT sessions. Every 10 minutes, researchers also drew blood to measure actual glucose levels using a highly accurate lab instrument called the YSI 2500 — considered the gold standard for glucose measurement. Readings were taken starting 20 minutes before exercise and continued 20 minutes after finishing.

What the Researchers Found

• Both CGMs were less accurate during exercise compared to lab blood glucose measurements — this was true for aerobic exercise and HIIT.
• Both devices performed better during HIIT than during steady aerobic exercise. The Dexcom G6's average error was about 14% during HIIT but jumped to nearly 32% during aerobic training — a significant difference.
• FreeStyle Libre 2 outperformed Dexcom G6 during aerobic exercise, but Dexcom G6 was more accurate during HIIT.
• Using the FDA's accepted accuracy standard (the 40/40 criteria), both devices still passed, scoring above 93% across all conditions and above 99% during HIIT — meaning they remain clinically acceptable tools even during exercise.

What This Means for Real Patients

If you rely on a CGM during workouts — and many people with type 1 diabetes do — this research offers some practical guidance:

• Don't rely solely on your CGM during moderate aerobic exercise. Readings may be off by a meaningful margin, especially with the Dexcom G6. Consider confirming with a fingerstick if something feels off.
• CGMs tend to be more reliable during HIIT-style workouts, so if accuracy during exercise is a priority, your workout type may matter.
• CGM readings lag behind real blood sugar changes during exercise because sensors measure fluid under the skin, not blood directly — and exercise speeds up circulation in ways that can widen that gap.

Having reliable supplies on hand is just as important as understanding your device's limits. MDS Diabetes carries CGM supplies, sensors, and testing accessories to help you stay prepared for every workout and every day.

Bottom Line

Both the Dexcom G6 and FreeStyle Libre 2 remain useful tools during exercise, but neither is perfectly accurate — especially during steady cardio. The Dexcom G6 struggles more during aerobic workouts, while both devices perform better during high-intensity interval training. If you exercise regularly with type 1 diabetes, talk to your care team about how to best interpret your CGM readings during different types of activity, and always trust how your body feels alongside your device's numbers.