Wearable blood sugar monitors hailed by celebrities like Davina McCall as a tool for controlling appetite are effectively useless for people without diabetes - and is unlikely to help with weight loss, experts have warned. 

The devices, worn as a small circular sticker on the arm, provide detailed information on blood sugar levels after eating food via a small probe in the skin which beams information to paired smartphone.

While they have been used by diabetics for years, the devices are now increasingly being sold to healthy people as way to track foods that can cause them to experience 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.

Identifying the foods that have this effect - and avoiding them - is therefore said to reduce the risk of obesity and its related diseases. 

But a new study, where experts tested the devices on 30 non-diabetic volunteers, has cast doubt on the accuracy of such monitors for people with healthy blood sugar control. 

Analysis of their results suggests a healthy person using the devices would need to eat the same meal up to 67 times to get an accurate picture of its effect on blood sugar.

In the new experiment, participants were given a carefully crafted menu for two weeks in a row. The meals in the first week were an identical match to those in the second. 

This enabled scientists, from the National Institute of Diabetes and Digestive and Kidney Diseases in the US, to compare the accuracy of blood sugar monitors for the same exact meal and for the same person, but at two different points in time.

The results showed the readings were 'highly variable' between the weeks, and were also widely different to results from standard blood tests.

Results, published in The American Journal of Clinical Nutrition, were so inconsistent the authors determined the monitors, also called continuous glucose monitors (CGM), were essentially ineffective for personalised nutrition. 

'CGM responses need to be reliable to be useful,' they wrote.

They added: 'Our study found that the reliability of post-prandial after eating) CGM responses to many duplicate multicomponent meals was poor.'

The researchers said their results came during a growing trend of healthy adults using the devices as part of getting 'personalised diet advice'. 

Nicola Guess, an academic dietitian and researcher at the University of Oxford who was not involved in the study, said the research demonstrated the devices were 'absolutely useless' for healthy people. 

'Proponents of CGMs claim they help people identify what foods they cannot tolerate - ie if they eat a banana and they get "a spike" and their partner doesn't, it means they can't tolerate the banana but their partner can. 

'But what this study shows is that you and your partner might need to each eat that food 67 times to be able to tell whether you genuinely respond differently to it.'

She added that there is limited evidence to support the notion that identifying foods that 'spike' blood sugar has any health benefit.

'It's silly that people are being prompted to "monitor" their glucose with CGMs because so far as we know, as long as it sits in the normal range, it's not a problem,' she said. 

Blood sugar levels vary by individual but are typically between 4 and 6 mmol before meals, and up to 8mmol up to two hours after eating.

This isn't the first time the use of CGMs in healthy people has been questioned.

Earlier this year, experts warned the devices could potentially cause people to develop anxiety and eating disorders.

Additionally, while experts say the evidence for the benefits of CGMs in diabetics was clear, evidence of them being useful to healthy people was far more limited.  

ZOE has claimed on its website that blood sugar spikes can, over time, lead to an increased risk of heart disease, and offers a diet to help minimise them.

Davina has featured in a number of promotional videos using ZOE's CGM.

In one post, on her Instagram, she described the device as a 'trick up my sleeve to help soften my sugar spikes and foods that I know trigger me'.

However, several experts have questioned the research evidence behind ZOE's claims, as well as other similar programmes.