Image result for There’s No Single Diet That’s Best for Everyone, Study FindsIt seems like every day a new diet is declared the healthiest — paleo, ketogenic, Atkins, to name a few — while government agencies regularly release their own recommended dietary guidelines. But there may not be an ideal one-size-fits-all diet, according to a new study.

Researchers instructed 1,100 adults from the U.S. and the UK ⁠— including 240 pairs of identical twins ⁠— to eat the same set meals for two weeks and kept track of their fat, insulin and sugar levels. The meals included breakfast muffins, glucose drinks and sandwiches. The researchers also measured the participants’ gut microbiome and recorded sleep and exercise habits.

The researchers, primarily from King’s College London and Massachusetts General Hospital, found that none of the participants reacted to the diet in the same way, even if they were twins with almost the same DNA. One person’s blood sugar increasing in response to a particular food did not mean their fellow participants would do the same. A participant could even have different reactions after eating the same food at different times of the day, Business Insider reported.

“Our recommendations, medically and public-health wise, have just been assuming that if people follow the standard plan, they’ll lose weight,” co-lead researcher Tim Spector, a professor of genetic epidemiology at King’s College London, told Time. “Really, that thinking has now been exposed as completely flawed.”

In addition to the pre-selected meals, the participants were given glucose monitors that embedded in their skin, other sensors that took blood samples, and wristbands that monitored their activity and sleep levels.

Using data these devices collected, the researchers concluded that broad dietary guidelines were not the best indicators for how someone might react to a particular food. Instead, more accurate predictions could be made based on each participant’s prior readings, Business Insider reported.

“We should be personalizing diets and not just trying to squeeze everyone into the same shoe size,” Spector said. “For most people, we can make basic recommendations about how they respond to carbs in general, or fatty foods.”

The study has not yet been published in a peer-reviewed journal, but Spector presented some of the results at the American Society for Nutrition conference earlier this week. The research was funded by ZOE, a company Spector founded with the intent to provide at-home nutrition tests and personalized diets to consumers. ZOE aims to use the test results to create an app that contains personalized databases of food reactions to help people fine-tune their own diets.

Jennie Brand-Miller, a professor of human nutrition at the University of Sydney, who was not involved with the study, told the New York Times that the “one-size-fits-all nutrition guideline is antiquated,” as they use data from questionnaires and people are typically bad at recalling what they ate over the last year.

Still, researchers are yet to find definitive evidence that personalized recommendations are indeed better at improving someone’s health than broader diets, the New York Times reported. To this end, Spector and his team are already recruiting participants for a larger version of the original study.

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