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Tim founded the UK Twins Registry at Kings College London to unravel the extent to which our genes contribute to a vast range of human conditions and diseases. Over 13,000 pairs of twins have contributed to the research.

The scientist became interested in why identical twins – with identical genetic make-up – often die of different diseases. “What was it about those identical clones that would force them down different routes of personality or disease?” he asked. He began to look into epigenetics: the power of outside factors on the way genes can get switched on and off.

But epigenetics couldn’t explain all of the differences they were seeing. Weight between twins could differ by up to 10kg, for example. “We could explain only a small part of that 10kg difference with the epigenetics,” says Tim, “so we knew there was something going on that wasn’t down to the genes.”

Tim and his team hunted for another explanation and made a fascinating discovery: “The one thing that really came up most was that they really differed in their gut microbes,” says the scientist.

No two people’s vast repertoire of gut bacteria – even in identical twins – is the same, says Tim.

“If you think of them as an organ in your body that’s like a chemical factory, that differs between all of us, suddenly you start to understand why a lot of where we’ve gone wrong over the last thirty years in nutrition suddenly starts to make sense.”

“All of us know that some people have responded to some diets, others not, and yet we’ve got this sort of dogma that we must all be the same and if we fail to lose weight it’s our fault,” he says. But it’s our unique mix of gut bacteria that dictates our highly individual responses to different foods, he argues.

Tim asked twins in the study to provide stool samples, so they could measure their microbes. They collected lots of these samples, sequenced them, and then looked at twins where one was overweight and one was skinny.

“We found in every case the skinnier twin had a more diverse microbiome (greater numbers of different species), and they also nearly always had high numbers of a couple of microbes that just stuck out of the crowd,” says Tim. They go by the name of Christensenella and Akkermansia.

“It turns out that these are really beneficial microbes,” says Tim. When they put Christensenella into sterile mice, they could “stop them getting fat” – an association that has been confirmed by several other similar projects. “This is showing that a beneficial microbe can have an effect on our guts to in some way change our metabolism to stop us putting on weight.”

“There’s probably hundreds of thousands of microbes or strains like that that in combination can have this beneficial effect,” he says.

To ensure we have diverse gut microbes, we need a diverse diet, says Tim.

“We found that the optimum diversity, across 11,000 people, was whether you ate 30 plants a week or not.” That sounds like an awful lot, he admits, but this doesn’t mean drinking 30 kale smoothies a week. The key is to “go back to biology” and remember what a plant really is. “It’s also a peanut, it’s a seed, it’s a bit of turmeric,” says the scientist, “and each plant will help promote the growth of a different set of bacteria or strains of bacteria.”

“It’s this diversity of our diet of unprocessed, different foods, that was the best predictor of a healthy gut. It didn’t matter if you were vegan, vegetarian, keto, intermittent faster,” he states. “This was the thing that you need to have on your plate. Once you got that, you could do anything else.”

When the pandemic struck, The ZOE team were able to very swiftly adapt the technology to help shed light on the spread of COVID and the different symptoms that people were developing. It became known as the . Within 24 hours a million people had downloaded it, and up to four and a half million people have now used the app. “It’s a unique citizen science project,” states Tim.

Tim has now turned his attention to how diet can influence COVID outcomes. “Because we had this amazing database of people, we decided to do a mass diet quality survey in the U.S. and the U.K.,” he explains. They had over a million people reply.

Results showed that “the quality of the diet people were on, not the calories, was instrumental in reducing the severity of COVID: the number of hospitalisations or people who had long-term symptoms.”

This should be a wake-up call, argues the scientist. “Everything starts to come together about the importance of your diet on your immune system via your gut microbes, and how this has been so neglected.” He poses that the reason why the U.S. and the U.K. “have done so badly in terms of hospitalisations and deaths from COVID, compared to other countries, could well be just the high levels of ultra-processed foods that we’re eating, which are still being labelled as healthy.”

“We know diet is important in immune function,” says Tim. “We know that diet quality has an effect on the gut microbiome, and we know that the microbiome and the immune system are absolutely linked.” The data is observational, has flaws, and “absolutely needs a randomised control trial,” he admits, “but if I was advising people now, I don’t think it’s really dangerous to say your diet quality is one of the things we can all do to improve our health and reduce risk of COVID.”

The information contained in this article was correct at the time of broadcast on 19 October 2021.

Listen to The Life Scientific to learn more about the need for personalised diverse diets and how by looking after your gut, it will look after you.