Zoe, the British start-up that built its success on weight loss aids

Members of the community can be identified by the yellow patch they wear on the back of their arms, concealing a glucose monitor. Followers of the British nutrition system Zoe each paid 299 pounds (345 euros), with a monthly subscription of 24.99 pounds, to access a personalized eating plan.

This provides them with a kit consisting of a glucose monitor, which they must wear for two weeks, a blood test, and a container for collecting a stool sample, according to the instructions on the website of the company founded by Tim Spector, a professor of genetic epidemiology at King's College London known for his books and Netflix documentaries on nutrition. They must also eat two blue biscuits—one high in sugar, the other high in fat—four hours apart.

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