Overestimation Alert: Study Suggests CGMs Misrepresent Blood Sugar Impact of Foods in Healthy Adults

A recent study from the University of Bath has raised critical concerns about the accuracy of Continuous Glucose Monitors (CGMs) when used by healthy, nondiabetic individuals. The research suggests that these devices may consistently overestimate the impact of food on blood sugar, potentially leading users to adopt unnecessary or poor dietary restrictions.

CGMs: A Double-Edged Sword for the Non-Diabetic

Continuous Glucose Monitors, widely adopted as an indispensable tool for managing diabetes, are increasingly being used by healthy adults interested in optimizing their diet and tracking their personal responses to food. However, the study, published in The American Journal of Clinical Nutrition, indicates that for individuals with good glucose control, CGMs may be miscalibrated, leading to erroneous interpretations of food’s glycemic effect.

Javier Gonzalez, PhD, a professor at the University of Bath and a study author, emphasized the distinction in accuracy: “CGMs are fantastic tools for people with diabetes because even if a measurement isn’t perfectly accurate, it’s still better than not having a measurement at all. However, for someone with good glucose control, they can be misleading based on their current performance.”

Gonzalez warned that over-reliance on CGM data in healthy individuals “could lead to unnecessary food restrictions or poor dietary choices.”

The Core Discrepancy: Interstitial vs. Capillary Glucose

The root of the inaccuracy lies in where the CGM takes its measurement. Unlike traditional capillary blood tests (finger pricks), which measure glucose directly in the blood, CGMs measure glucose in the interstitial fluid (the fluid surrounding the cells). This difference can introduce measurement errors due to factors such as time delays, blood flow, and the process of glucose moving between body compartments.

Study Findings: Misclassifying Glycemic Response

To evaluate this discrepancy, researchers conducted an open-label, randomized crossover study involving 15 healthy individuals. Participants consumed seven different carbohydrate challenges—ranging from pure glucose to whole fruit and commercial smoothies—while their glycemia was simultaneously recorded using CGMs and traditional capillary blood samples for 120 minutes.

The results demonstrated a systematic overestimation by the CGMs:

Higher Readings: CGMs consistently measured fasting and postprandial (after-meal) glucose approximately 0.9 nmol/L higher than the capillary tests.
Misclassification of Food: The CGMs incorrectly assessed the Glycemic Index (GI) of certain foods. For example, a fruit smoothie was reported as having a medium-GI when capillary tests showed it was actually low-GI. Similarly, whole fruits were misclassified as having a higher GI than their true value.
Overstated Time Above Threshold: The devices dramatically overstated the time spent above a typical blood sugar threshold by nearly 400%, which could cause unwarranted anxiety and lead to drastic, unneeded changes in a user’s diet.

Conclusion: Reconsidering CGM Use for Healthy Eating

The findings strongly suggest that CGMs, in their current form, are not reliable for accurately determining the Glycemic Index of foods or for guiding dietary decisions in healthy, nondiabetic populations.

Dr. Gonzalez advised that for an accurate assessment of blood sugar responses, “traditional methods are still the way to go.” The research team plans to use these findings to better identify the sources of error and improve CGM performance for future non-diabetic applications.