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Jennifer and Julie, identical twins from Felixstowe, are spending the day in London. We meet with a view of an iridescent Thames in the mid-afternoon – a scaffolded Big Ben opposite, the London Eye to the east – but the romantic novelties of a day up in town end there. Because we are on the fourth floor of St Thomas’ Hospital, and Jennifer and Julie have been up since 04.50. Their day started with a fast, and proceeded to include stool samples, the fitting of a glucose monitor, a Dexa to establish their unique body compositions, supervised meals and half-hourly blood tests.

The twins (pictured, Julie on left) are just two in a 500-and-counting sample taking part in Predict, an ambitious study by Dr Tim Spector, professor of genetic epidemiology at King’s College London and author of The Diet Myth, which sets out to understand the intricacies of individual nutrition. Spector thinks we’ve had the wrong end of the stick about diet for too long, trying to apply a one-size-fits-all solution for health and weight loss.

“We’re oversimplifying nutrition into calories and macronutrients but food isn’t like that,” says Spector, going on to explain that not only is all food composed of thousands of chemicals (ie a piece of toast is not simply a carbohydrate with “x” number of calories) but each of us has a unique microbiome (gut bacteria) and particular metabolic response to what we eat. Broadly, some people respond better to fats, others to carbohydrates – so an umbrella approach to cutting out whole food groups is too crude. “Calorie-counting and cutting out macro nutrients like fat, carbs or dairy have failed,” says Spector. “We’ve got fatter, not thinner, because we haven’t accounted for individual variability.”

Nowadays we are surrounded by nutritional advice from a vast array of sources that is often bafflingly contradictory. The received wisdom of the government and doctors has been interrupted by the opinions of nutritionists, fitness buffs, food and wellness industries, bloggers, holistic therapists and, well, anyone with internet access and food anxiety.

Spector’s goal with Predict is to have a piece of research so comprehensive that he can develop an algorithm and app to advise anybody on what foods are best suited to their constitution. While his sample isn’t limited to identical twins, they are especially useful in demonstrating that genes are only part of the story of metabolism. Two genetically identical people can have vastly different responses to the same foods because of differences in their gut microbes, which are affected by all sorts of factors, from medication and pollution to pet ownership. With Predict, Spector has measured the twins’ different responses to meals and linked these back to the profile of their respective microbiomes.

Jennifer and Julie are already using a version of the app. For the next fortnight they will be weighing and photographing their meals and recording the exercise they each take daily which, along with blood-sugar readings taken by the glucose monitor (resembling a bottle top fitted into their arms), gets sent back to the clinical research facility. Among other things they will be instructed to eat some rather unappetising muffins of varying fat and sugar content provided by Spector’s team.

Until the app is made available to everyone, Spector advises that we all double our fibre and eat as many diverse plants as possible for a healthy microbiome. That, he says, is the only one-size-fits-all advice there is.

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