Jonathan Leake

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 two-thirds 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.”

“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.”

The Diet Myth: The Real Science Behind What We Eat, by Tim Spector, is out on May 14»

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