Is Zoe scientific?

Blood glucose 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, the costs of which start at around £300. In the Zoe programme, participants log their food intake and wear a continuous glucose monitor (CGM) for two weeks to measure their 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.

However, 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, and that high blood sugar is a symptom, not directly a cause, of diabetes. In people without diabetes, other researchers have said, “the evidence base is nothing” for understanding what the swings in blood sugar mean.

However, Zoe points to some evidence 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, although most data is still from diabetic patients. 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.”

While some of Zoe’s critics agree that CGMs could potentially be a helpful tool for some people to motivate them and change their diet, Dr Guess is concerned that patients who use Zoe’s products are cutting out foods that are good for their health, because they seem to spike their blood sugar. That in itself can lead to health problems, and people who avoid carbohydrates can get a temporarily “exaggerated glucose response” the next time that 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.

Dr Guess thinks that while a lot of Zoe’s advice, including around eating more whole foods and fewer processed foods, is sensible, she believes this message is ‘not compelling enough’ to sell a £300 product.

Zoe has carried out a study to understand changes caused by the programme, but it has not yet been published.

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