Continuous glucose monitors (CGMs), which are sometimes also known as blood sugar monitors, can be key for people with diabetes. 

They help those with the condition to keep an eye on their blood sugar levels, potentially preventing them from getting dangerously high or low.

But in a site entry, the University of Bath (whose scientists were involved in a recent paper published in The American Journal of Clinical Nutrition) said: “Originally designed to help people living with diabetes... these devices are now being used by the health-conscious to track how different foods affect their glucose levels.” 

And according to the recent research, it may be doing healthy people more harm than good. 

What did the study find?

The researchers said that CGMs can often overestimate how much sugar is in a person’s blood. 

When participants drank a smoothie, the CGM overestimated the glucose it gave participants’ blood by 30% compared to a blood prick test.

Whole fruit, like eating an apple, was classified by the CGM as a medium-glycemic index (GI, or blood-sugar ‘spiking’) or high-GI food, whereas blood prick tests found they were actually low-GI. 

CGMs also inaccurately said that participants’ blood sugars stayed elevated for nearly 400% above the threshold recommended by Diabetes UK, which could lead to unnecessary and even unsafe diet changes and stress.

“CGMs may be inaccurate because they measure glucose in the fluid surrounding your cells, not directly in your blood,” Professor Javier Gonzalez from the University of Bath, who was involved in the study said. 

“This can lead to discrepancies due to factors like time delays, blood flow, and how glucose moves between different parts of the body.”

So... are CGMs useless?

Dr Gonzalez said they’re “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”.

But “for healthy individuals, relying on CGMs could lead to unnecessary food restrictions or poor dietary choices”.

Writing for the expert-led site Science Media Centre, where professionals react to large or viral studies, Dr Adam Collins, associate professor of nutrition at the University of Surrey, said: “The use and interpretation of CGM devices in healthy individuals should be undertaken with caution.”

“They certainly shouldn’t be used as the sole basis for dietary changes, restrictions, or extreme lifestyle changes.”

Dr Nicola Guess, academic dietitian at the University of Oxford, said: “The investigators found that CGMs overestimate glucose to a clinically-relevant degree in healthy people without prediabetes or diabetes. This means that people who have normal glucose may be led to believe they have prediabetes.

“This is one of a growing number of studies which demonstrate how cautious we need to be when interpreting CGM data from a person without diabetes.

“In this study, the CGM estimated the “time out of range” as 4-fold higher compared to the capillary value.  Other studies have found CGMs can also underestimate glucose (for example during exercise) and are particularly inaccurate when glucose concentration changes rapidly.

“Taken together, I would advise people without diabetes using CGMs to interpret the output with a pinch of salt.”