Study Reveals Continuous Glucose Monitors May Overestimate Blood Sugar Levels, Researchers Caution

Continuous Glucose Monitors (CGMs) have surged in popularity in recent years, especially among the health-conscious community seeking to optimize their diets and manage their blood sugar levels. Originally devised as a tool for diabetes management, these innovative devices have found their way into the hands of fitness enthusiasts, biohackers, and anyone keen to better understand their body’s response to food. However, emerging peer-reviewed research from the University of Bath raises questions about the accuracy of these devices, suggesting that they may not provide the reliable data many users assume. This research, published in The American Journal of Clinical Nutrition, indicates that CGMs could significantly overstate blood glucose levels in healthy adults, leading to misguided dietary decisions.

The study conducted at the University of Bath’s Centre for Nutrition, Exercise and Metabolism, funded by innocent drinks, aimed to explore the precision of CGMs when measuring blood glucose responses to various fruit-based products. The research involved healthy volunteers, all non-diabetic and within a healthy body mass index (BMI) range. Participants were assessed using both a CGM device—specifically, the Abbott Freestyle Libre 2, widely available on the NHS—and the gold standard finger-prick blood test. By adopting two distinct measurement methods, the study sought to ascertain the extent of discrepancies between the CGM readings and traditional blood sugar metrics.

Notably, the findings were startling. The research revealed consistent overestimations by the CGM, with the device recording significantly higher blood sugar levels compared to the finger-prick method. This disparity became particularly pronounced when participants consumed smoothies. In this instance, the Abbott Freestyle Libre 2 indicated a glycaemic index (GI) of 69—a medium level—vastly overstating the actual results, which showed a much lower GI of just 53. These inaccuracies extend beyond smoothies; the study demonstrated that whole fruits were often misclassified as medium or high-GI foods by the CGMs, when in reality, they were classified as low-GI through more traditional testing.

Such misclassifications could have substantial implications for users, who might erroneously conclude that consuming fruits poses a risk of harmful blood sugar spikes. The potential consequences of these inaccuracies are particularly alarming, as CGMs were also found to exaggerate the duration that participants spent above the recommended blood sugar level thresholds. Reports indicated that CGMs could overestimate this duration by nearly 400%. Such significant inaccuracies risk inducing unnecessary anxiety in individuals who are, in fact, maintaining well-controlled blood glucose levels.

The study’s insightful conclusions challenge the long-held belief that the process of blending fruits into smoothies inherently raises their GI. Contrary to popular opinion, the research demonstrated that whether fruits are ingested whole or in blended form, their glycaemic index remains low. This is a critical finding, as it dispels the myth that smoothies are detrimental to blood sugar control and reassures consumers that these healthy options maintain their nutritional benefits when processed.

Professor Javier Gonzalez from the University of Bath, an expert in nutrition and health, emphasized the utility of CGMs for individuals with diabetes. Gonzalez stated that, despite the inaccuracies of CGMs, the ability to acquire any measurement is better than having none at all for those managing diabetic conditions. However, he raised caution for otherwise healthy individuals, asserting that reliance on such devices could foster unnecessary dietary restrictions or result in less healthy food choices. He advocated for traditional methods to assess blood sugar accurately and highlighted the importance of identifying the roots of the inaccuracies found within CGMs to enhance their future performance.

Professor Gonzalez pinpointed various factors contributing to the observed inaccuracies in CGMs. He noted that these devices measure glucose in the interstitial fluid surrounding cells, rather than directly in the bloodstream. This fundamental difference can lead to discrepancies in readings due to several variables, including time delays and blood flow. Additionally, the way glucose is distributed in the body can influence these measurements, highlighting the complexity of achieving precise readings through CGM devices.

To contextualize the potential impact of these findings, the study involved 15 healthy volunteers who ingested an innocent drinks fruit smoothie made from a blend of mangoes, passion fruit, and apples. This methodology allowed researchers to assess the blood sugar responses meticulously while contrasting CGM outputs with established, traditional testing methods. The insights gleaned from this research raise important considerations for any user looking to CGMs as a reliable gauge of dietary impact on blood sugar levels.

With CGMs increasingly marketed towards non-diabetics seeking to improve their health, understanding their limitations is crucial. Consumers must be educated about the potential inaccuracies and reassured of the benefits of traditional testing options. Organizations and nutritionists must facilitate this education to ensure that users can make informed decisions based on accurate data rather than misleading information from CGM devices.

In conclusion, while CGMs provide significant benefits for diabetes management, their application among healthy individuals requires scrutiny. The overestimations highlighted in the University of Bath’s research could lead to unnecessary dietary anxieties and restrictive habits among users who may not need to alter their consumption of fruits and related products. The findings serve as a reminder that while technology can offer insights into our health, it is vital to approach such data critically and complement it with traditional methods when accuracy is paramount.

Subject of Research: Accuracy of Continuous Glucose Monitors in Healthy Individuals
Article Title: Continuous glucose monitor overestimates glycemia, with the magnitude of bias varying by postprandial test and individual – A randomized crossover trial
News Publication Date: 26-Feb-2025
Web References: https://www.bbc.co.uk/news/articles/c1ddk1ddme5o, https://www.bath.ac.uk/research-centres/centre-for-nutrition-exercise-and-metabolism/, https://www.innocentdrinks.co.uk/home
References: DOI: 10.1016/j.ajcnut.2025.02.024
Image Credits: University of Bath

Keywords: Continuous Glucose Monitors, Blood Sugar Level, Glycaemic Index, Diabetes Management, Fruit Consumption, Nutritional Science, University of Bath, Health Technology, Dietary Choices, Accuracy of Measurements, Interstitial Fluid, Blood Flow Variability.

Tags: Abbott Freestyle Libre 2 evaluationblood glucose measurement methodsCGMs in healthy adultsclinical nutrition insightsContinuous Glucose Monitors accuracydietary impact of glucose monitoringfitness enthusiasts and CGMsglucose monitoring technology concernsimplications for non-diabetic usersoverestimation of blood sugar levelspeer-reviewed nutrition studiesUniversity of Bath research findings