Continuous Glucose Monitors (CGMs) are becoming popular among health-conscious individuals, but new research suggests they may not be as accurate as many believe—especially for those without diabetes.
A study from the University of Bath, published in The American Journal of Clinical Nutrition, found that CGMs often overestimate blood sugar levels, which could lead to unnecessary dietary restrictions.
CGMs vs. Traditional Finger-Prick Tests
Originally designed to help people with diabetes manage their blood sugar, CGMs track glucose levels by measuring glucose in the fluid surrounding cells, rather than directly in the blood. While these devices are helpful for those with diabetes, researchers questioned whether they provide reliable data for healthy individuals.
To test this, the researchers compared readings from the Abbott Freestyle Libre 2 CGM—one of the most commonly used CGMs—to the traditional finger-prick test, considered the gold standard for measuring blood sugar. The study involved healthy participants with normal blood sugar regulation, consuming different fruit-based foods, including whole fruit and smoothies.
Key Findings: CGMs Overestimate Blood Sugar Spikes
The study found that CGMs consistently reported higher blood sugar levels compared to finger-prick tests, leading to several important findings:
- Overestimated Glycemic Index (GI): When participants drank a smoothie, the CGM overestimated its glycemic index (GI) by 30%, reporting a GI of 69 (medium) compared to the finger-prick result of 53 (low).
- Misclassification of Whole Fruit: CGMs classified whole fruit as medium or high-GI, while traditional tests showed they were actually low-GI. This could lead users to mistakenly avoid fruit, fearing unnecessary blood sugar spikes.
- Exaggeration of High Blood Sugar Events: CGMs overestimated the time participants spent above recommended blood sugar levels by nearly 400%, which could cause unnecessary concern for people with well-regulated glucose levels.
- Debunking the Smoothie Myth: The study also disproved a common belief that blending fruit into a smoothie raises its GI. Whether eaten whole or blended, fruits like apples, bananas, mangoes, and oranges remained low-GI, meaning they provide steady energy without causing sharp blood sugar spikes.
Implications for Health-Conscious CGM Users
Professor Javier Gonzalez, who led the study, emphasized that while CGMs are valuable for people with diabetes, they may be misleading for those without blood sugar issues.
“For healthy individuals, relying on CGMs could lead to unnecessary food restrictions or poor dietary choices,” he explained. “Traditional finger-prick tests remain the most accurate way to assess blood sugar.”
He also pointed out that CGMs’ inaccuracies likely stem from how they measure glucose. “Unlike a blood test, CGMs measure glucose in the interstitial fluid (the fluid surrounding cells), which can introduce discrepancies due to time delays, blood flow, and how glucose moves between different areas of the body.”
The Future of CGMs and Nutritional Advice
The study highlights the need for improvements in CGM accuracy, especially as more non-diabetic individuals use them for tracking food responses. Future research will focus on identifying the sources of these errors and refining CGM technology to better serve all users.
Meanwhile, nutrition experts, including Helen Whitby from Innocent Drinks, support the study’s findings. “Smoothies and whole fruits provide steady energy and essential nutrients without sharp blood sugar spikes,” she said. “This research confirms that blending fruit doesn’t reduce its health benefits, which is great news for people looking to eat well.”
For now, the study suggests that while CGMs remain a useful tool for diabetes management, they may not be reliable for the general population. Health-conscious individuals should be cautious when interpreting CGM data and continue relying on balanced, science-backed dietary advice.
The research findings can be found in the American Journal of Clinical Nutrition.
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