Wearable blood sugar monitors beloved by celebs like Davina McCall are essentially unreliable for non-diabetics, experts have warned

Experts found the devices, promoted by diet companies, vastly overestimated blood sugar readings.

Experts have warned that wearable blood sugar monitors, which count stars like Davina McCall and Dragons' Den star Steven Bartlett among their fans, could be doing more harm than good.

Academics examining the devices, which are being promoted by diet companies like ZOE, found they massively overestimated the size and duration of blood sugar spikes.

This, they said, could lead to healthy people "misdiagnosing" themselves with problems like prediabetes and becoming unnecessarily anxious.

The false readings could also lead to users believing healthy foods like fruit were causing a worrying blood sugar spike and shunning them as a result, experts warned.

The technology, which is embedded in several health and diet programmes, sees users wear a small circular sticker on their arm which provides detailed information on blood sugar levels after eating food via a small probe in the skin which beams information to a paired smartphone.

While the devices were developed for and have been used by people with diabetes - a condition which causes persistently high blood sugar - for years.

But the technology is increasingly being sold to healthy people as a way to find foods that can cause them a worrying "blood spike".

Health brands like Professor Tim Spector's ZOE, which sells its own monitor, say avoiding these spikes can help reduce cravings, limiting the intake of excess calories.

This, they add, reduces the risk of obesity and its related diseases.

But experts said the new study is yet more evidence the devices are unreliable when worn by people who do not have diabetes.

To test them, University of Bath experts took a small group of healthy non-diabetic volunteers, nine women and seven men, who each undertook seven tests.

During each visit participants ate either whole fruit, blended fruit, smoothies or a pure glucose product which acted as a control.

The 15 volunteers wore a CGM and had medical grade finger prick blood tests taken every 15 minutes for two hours.

Experts then compared the blood sugar reading provided by the CGM to the actual levels recorded by the blood tests.

They found the CGMs they tested, the Abbot Freestyle Libre 2, bumped up blood sugar readings by 30 per cent compared to their real level.

But the biggest error was in blood sugar spike duration. Scientists found the CGMs overestimated by 400 per cent the length of time a person's blood sugar was raised.

CGMs are said to be unable to measure glucose in blood directly but rather in the fluid surrounding cells, which could be influencing the results

He added that problems with CGMs' accuracy had been reported in previous research, adding that the devices appeared to struggle to keep pace with rapid changes in blood sugar.

Writing in The American Journal of Clinical Nutrition, the authors concluded that CGMs are "unlikely" to be a valid way to determine if a food causes high blood sugar or not in people without diabetes.

Professor Javier Gonzalez, an author of the study and an expert in nutrition and metabolism, said: "For healthy individuals, relying on CGMs could lead to unnecessary food restrictions or poor dietary choices. If you want to assess your blood sugar accurately, traditional methods are still the way to go."

Professor Gonzalez said how much glucose was in this fluid could be influenced by several external factors.

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

Dr Adam Collins, an expert in nutrition at the University of Surrey and who was not involved in the study, said the finding echoed observations his team had made in a similar experiment.

"We are in the middle of a similar validation study investigating the accuracy and reliability of CGMS and have observed that discrepancies can be as high as 1-1.5mmol/L," he said.

He said this "misinterpretation and misdiagnosis" among users was leading some to make dietary and lifestyle changes in response to blood sugar readings that were essentially "normal variations".

Dr Collins added research his team was working on also showed CGMs worn by the same person on different body parts could show different results.

"For example, differences between having the device fitted on your dominant or non-dominant arm," he said.

Several firms, including the ZOE programme - founded by diet guru Professor Tim Spector - offer the high-tech gadgets, called continuous glucose monitors (CGMs), which track a customer's blood sugar levels via a stick-on patch on their arm

Nicola Guess, an academic dietitian and researcher at the University of Oxford who was not involved in the study, said the results suggested healthy people using CGMs could be led to believe they had a worrying health condition.

"The investigators found that CGMs overestimate glucose to a clinically-relevant degree in healthy people without prediabetes or diabetes," she said.

"This means that people who have normal glucose may be led to believe they have prediabetes.

"I would advise people