Fast food kills bugs that keep you thin

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FAST food diets make people fat not just through excess calories but by killing off the gut bacteria that help people burn off their excess energy, a leading British researcher has found.

Diets with just a few highly processed ingredients, rather than the dozens of foods found in a healthy diet, are so toxic to these bacteria that it takes only a few meals to wipe out hundreds of the species humans need to maintain healthy digestion, according to research by Tim Spector, professor of genetic epidemiology at King’s College London.

The findings emerged from a series of studies in which Spector, whose book on the topic, The Diet Myth, is published this week, investigated links between gut bacteria and health. He was seeking insights into why some people put on weight while others stay slim on similar diets — and why less than 5% of fat people succeed in long-term weight loss.

In one study, Spector’s 23-year-old son, Tom, agreed to spend 10 days eating just McDonald’s burgers, chicken nuggets, chips and Coke.

“Before I started my father’s fast food diet there were about 3,500 bacterial species in my gut, dominated by a type called firmicutes,” said Tom Spector, a genetics student, who gained 4lb during the experiment.

“Once on the diet I rapidly lost 1,300 species of bacteria and my gut was dominated by a different group called bacteroidetes. The implication is that the McDonald’s diet killed 1,300 of my gut species.”

For his father, this supported other research suggesting the reason almost twothirds of Britain’s adults are obese or overweight is far more complex than eating too much.

“Microbes get a bad press, but only a few of the millions of species are harmful and many are crucial to our health,” Spector said. “Microbes are not only essential to how we digest food; they control the calories we absorb and provide vital enzymes and vitamins.”

Spector is well placed to investigate such issues. He oversees Twins UK, a registry of 12,000 twins who are being tracked and tested throughout their lives to tease out the relative importance of genetic and environmental factors in a range of complex diseases including obesity.

“What is emerging is that changes in our gut microbe community, or microbiome, are likely to be responsible for much of our obesity epidemic, and consequences like diabetes, cancer and heart disease,” he said.

“It is clear that the more diverse your diet, the more diverse your microbes and the better your health at any age.”

Spector’s research reflects growing global interest in the relationship between gutdwelling bacteria and human health. Scientists have long known that human intestines harbour 3lb of bacteria but the numbers and diversity made research hard.

This is changing, largely because of the advent of highspeed genome-sequencing machines, which mean samples taken from the gut can be rapidly and cheaply analysed. In large-scale studies such variations can also be linked to conditions including obesity, allergies and asthma.

Professor Rob Knight, of the University of Colorado Boulder, who collaborates with Spector, began investigating the links between gut bacteria and health after he and his wife suffered a severe stomach infection during a trip to Peru. In the months afterwards he lost 80lb, going from obese to a healthy weight — a change he suspects was linked to the change in his intestinal flora.

Since then he has launched his own research programme, becoming a leader in the field, and founded American Gut, a project that offers people the chance to have the genomes of their gut bacteria analysed cheaply if they make the data available to researchers.

His recently published book, Follow Your Gut, sets out some of the findings from his and other laboratories. One was the discovery that gut bacteria vary not just between people but between countries too, because of variations in national diet. Another was that transferring bacteria from the gut of an obese human to a lean mouse made the animal obese as well.

Such experiments lead to a highly controversial possibility: that obesity is at least partially infectious. In his book, Spector describes how lean mice put into cages with fat ones also became obese.

The findings are powerful evidence for gut bacteria playing a key role in controlling obesity and suggest there might be certain combinations that could aid weight loss.

Other scientists have used antibiotics to target the bacteria that proliferate on a high-fat diet, leading to weight loss in mice.

Such research is in its early days but expanding fast. Spector recently founded the British Gut Project, the UK equivalent of Knight’s study, but some early practical conclusions are emerging already.

Healthy diets are not about excluding things such as fat or sugar, or about eating lots of protein, a currently popular idea. Instead, said Spector, they should simply be as diverse and natural as possible.

One finding is that Belgian beer, garlic, coffee, leeks and celery are perfect foods to increase gut microbes.

“Fifteen-thousand years ago our ancestors regularly ingested around 150 ingredients in a week. Most people nowadays consume fewer than 20 separate food types and many, if not most, are artificially refined. Most processed food products come, depressingly, from just four ingredients: corn, soy, wheat or meat,” said Spector.

“The promotion of restrictive diets that depend on just a few ingredients will inevitably lead to more reduction in microbe diversity and to ill health.”