Blood sugar monitors are unnecessary for people without diabetes and could, in extreme cases, fuel eating disorders, leading doctors have warned.
They are part of a personalised diet trend, promoted on social media and spearheaded by companies including ZOE.
But NHS national diabetes advisor Prof Partha Kar said there is no strong evidence the gadgets help people without the condition.
ZOE said research is at an early stage but is "cutting edge".
In people with diabetes, blood sugar - also known as blood glucose - can remain high for several hours after eating. At very high levels this can cause organ damage if it's not monitored and kept in check.
ZOE - previously involved with the Covid symptom-tracking app - is one of the main companies selling the blood sugar monitors to people without the condition.
Now it offers a programme, starting at around £300, which it advertises widely, including across social media platforms.
Participants log their food intake and wear a continuous glucose monitor (CGM) for two weeks to measure blood sugar levels after eating.
In separate tests, they also have their responses to fat, and their gut bacteria tested.
ZOE says all of these tests have helped it identify that even two healthy people can have wildly different responses to the same food - for example one person's blood sugar might spike and dip more after eating carbohydrates than another person's.
It suggests this could guide individual food decisions.
But other researchers argue that what, if anything, those numbers mean - including bigger spikes and dips in blood sugar within the non-diabetic range - is still not properly understood.
Dr Nicola Guess, a dietitian and diabetes researcher at the University of Oxford, said the majority of evidence linking high, and highly varied, blood sugar to health problems is based on glucose levels only seen in people with diabetes or prediabetes.
High blood sugar is a symptom, not directly a cause, of diabetes, she explains.
Type 1 diabetes is when an individual's pancreas stops producing insulin, so regular injections are needed. Type 2 diabetes is more common and occurs when the cells in the body become resistant to insulin and so more is needed to keep blood glucose levels within a normal range. It can usually be controlled through diet, exercise and close monitoring.
In people without diabetes, Prof Kar said "the evidence base is nothing" for understanding what the swings in blood sugar mean.
ZOE points to some evidence - including some very small studies - suggesting that even before it reaches prediabetic or diabetic levels, having higher blood sugar and big variations through the day may be linked to some worse outcomes. Most data is still in diabetic patients, though.
It says it is investigating gut bacteria and starting to see links between gut microbes, diet and health.
Gut microbiome expert and colorectal surgeon at Imperial College London, James Kinross, said while the microbiome was very important, direct-to-consumer testing was "problematic" because "this is such a young science and there are many unanswered questions about how the microbiome influences our health."
ZOE's chief scientist Dr Sarah Berry told the BBC its programme used "decades" of existing nutrition research, and their own studies on links between blood sugar and health.
But she acknowledges that "we don't have all of the evidence".
But given the risks of poor diet we already understand, she said, "it would actually be irresponsible to wait" decades to understand long-term outcomes like heart disease and death.
Dr Ran Crooke, a GP who founded a company providing health services to start-ups, praised the company for trying to gather the evidence, and said not having all the evidence on blood sugar shouldn't be "stifling innovation".
He and others, including some of ZOE's critics, agree CGMs could potentially be a helpful tool for some people to motivate them and change their diet.
However, people have been sounding the alarm over diet-related illness for decades. Yet hundreds of diet programmes have failed to resolve the challenge of getting people to stick to habits when their environment - a modern prevalence of high-sugar foods, for example - and biology seems to be stacked against them. And they aren't without risks themselves.
The company said: "ZOE is scientifically rigorous in its approach, unrivalled by others in the industry in terms of clinical trials, robust research and a dedicated team of scientists and nutrition professionals looking to improve health through useful, evidence-based advice."
But Dr Guess is concerned she is seeing patients who use ZOE's products cutting out foods she believes are good for their health, because they seem to spike their blood sugar.
That in itself can lead to health problems, and is not recommended by the company.
She adds people who avoid carbohydrates will get a temporarily "exaggerated glucose response" the next time they eat them - which she says is "perfectly normal," but that could potentially lead them to think they are unable to tolerate carbs at all.
Prof Kar thinks using CGMs - which are used by people with diabetes - when there's no health reason to do so can drive an obsessive focus on numbers which, in the most extreme cases, "can translate into eating disorders".
According to eating disorder charity Beat, "people with eating disorders often fixate on numbers... as part of their illness, so we'd never recommend that anybody affected uses glucose monitors".
ZOE does attempt to screen out people with a history of eating disorders, and Dr Berry told the BBC the company takes "the wellbeing of our members very seriously," and that customers have access to trained nutrition coaches, who can support them with food anxiety and refer them on if they feel there's a problem.
The company has published research based on the data it has collected from participants to try to find patterns in areas like food choices, hunger and blood test results. But it can't show which aspects are actually causing changes to health and which are coincidences.
ZOE has carried out a study to understand changes caused by the programme, but it has not yet been published.
Critics are concerned this study won't be able to unpick the effects of the programme's different elements, such as personalised diets based on test results versus support and coaching.
Dr Berry argues ZOE's programme is, "a very holistic product that doesn't involve just microbiome testing or just continuous glucose monitoring".
However, Dr Guess believes since these elements remain unproven, without them, it's just a "sciencey-sounding way of having people eat more fruit and veg".
A lot of ZOE's advice, including around eating more whole foods and fewer processed foods, is sensible, she thinks but she believes this message is "not compelling enough" to sell a £300 product.
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