Continuous Glucose Monitoring Gives Wildly Different Readings For Same Meal

General Discussion
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Continuous glucose monitors (CGM) are electronic devices that measure the concentration of blood sugar, or blood glucose, and provide the value to its wearer.

Although continuous glucose monitors were initially designed to assist in the clinical management of both insulin-dependent and noninsulin-dependent diabetes, there is now interest in the application of real-time glucose monitoring in the biohacking and athletic communities.

Here marks the first study I’ve read in 2025.

A new paper published in the American Journal of Clinical Nutrition looked at how reliable continuous glucose monitors were in individuals without diabetes.

Briefly, this data was obtained while subjects were admitted to a research facility. All of their meals were prepared for them and while they were instructed to eat as much (or as little) as they wanted, the researchers measured how much food was eaten at each meal.

For this particular study, there were over 1000 direct comparisons in blood sugar measurement after a meal where the Calorie difference and pre-meal blood sugars were very similar.

The variation in blood sugar readings from duplicate meals was about the same as the difference in blood sugar readings between different meals! The authors said of this:

Surprisingly, our study found that the reliability of postprandial CGM responses to many duplicate multicomponent meals was poor and that the within-subject variability to duplicate meals was roughly as large as the variability across different meals.

We talked about this concern in our podcast on continuous blood glucose monitoring. The current crop of monitors are not very reliable for individuals without diabetes, where normal blood sugar changes are smaller than hyperglycemia sometimes seen in individuals with diabetes.

Reliability issues aside, there’s little evidence suggesting that health improves when people make lifestyle changes based on continuous glucose monitoring data. For now, I don’t think there is a legitimate use case for CGM in individuals without diabetes or metabolic syndrome.

2 Likes
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Yeah, I saw that study too. Seems like CGMs aren’t all that useful for healthy people. The variability is just too high to make any real conclusions about your diet based on the readings. Waste of money, IMO.

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Agreed. I also think they may lead to some disordered eating habits in some individuals.

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I think the assumption that variance between CGM readings of duplicate meals is a measure for CGM reliability is absolutely wrong. There are two many confounding variables that impact the blood sugar response like previous meals, sleep, stress etc.
Said said I don’t claim CGMs are super reliable. It is just nothing you can conclude from these data.

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It’s a tool after all. And a tool in the hand if a fool… It may not belong in the hands of overly perfectionist people with obsession to control everything…

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When testing the reliability of CGM via duplicate meals, the setting is the same re: time, fasting status, and so on. I disagree that nothing can be concluded from this data. Instead, we can conclude that CGM do not provide accurate or precise data on blood glucose in individuals without significant insulin resistance, which is a bugaboo when it comes to management in this population.

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Ok, they apparently controlled some relevant variables but certainly not all, which would be hardly possible at all. The question is if blood glucose, independently of how it is measured, is a helpful parameter to manage anything useful at all in healthy individuals? What comes to my mind is potentially avoiding hypoglycemia in multi-hour endurance sports. Anything else?

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I do not think it can be used to avoid hypoglycemia in sport, nor do I think it is useful in healthy individuals for any specific purpose.

Regarding CGM measurements, these were inpatient metabolic ward studies. I think it’s reasonable to say pretty much all relevant variables were controlled.

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