Next time you shuffle into McDonald's because you can't be bothered to cook, you might want to make a mental note of this: eating junk food kills gut bacteria that can help you stay slim.
However, eating a diet based on processed foods - rather than a balanced, healthy diet - can wipe out the number of these illness-busting stomach flora by a third.
Professor of genetic epidemiology at King's College London, Tim Spector, found that microbes living in the human body make up 90% of living cells and when disrupted could be a major cause of obesity.
The human gut contains around 3,500 difference microbial species.
In a new book called The Diet Myth - published by Weidenfeld and Nicolson - he researched the links between microbes, food and health in an attempt to an gain insight into the burgeoning obesity crisis.
Currently, almost two thirds of adults in Britain are overweight.
Spector's research found sweeteners in diet fizzy drinks had adverse affects on metabolism, leading to weight gain, while fasting diets such as the 5:2 diet could benefit microbes and metabolism.
He also found some elements were down to genetics, with a third of people naturally possessing microbes which prevented them getting fat, while genes even determined some people's preference for salads, broccoli or garlic.
Spector used his 23-year-old son Tom, who is a genetics student, as the guinea pig for his research.
Tom spent 10 days on a fast-food-only diet of McDonald’s hamburgers, chips, chicken nuggets and Coca Cola.
The 23-year-old said: “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.
“Once on the diet I rapidly lost 1,300 species and my gut was dominated by a group called bacteriodetes. The implication is that the McDonalds diet killed 1,300 of my gut species.”
Spector added that using his son as his research subject was a "massacre in the name of science".
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In addition to Spector's research, a new report by London's Overseas Development Institute (ODI) found prices of fruit and vegetables had risen 30% in the UK since 1980 while those of processed foods, meat and dairy had dropped 25% - ice cream experienced the biggest price fall.
In its report, entitled The Rising Cost of a Healthy Diet, the ODI analysed data from five countries, including the UK and USA.
The researchers said: "Typical UK diets are not balanced in accordance with dietary recommendations, with excessive consumption of grains and other starchy foods, protein-rich foods, oils, fats, and sugar - coupled with particularly low intake of fruit and vegetables."
And they suggested there may be a case for taxation of certain foods, although it would come down to "political appetite".
They said: "In terms of what might be taxed and subsidised, this report suggests that energy-dense foods might be taxed, while fruit and vegetables whose prices often rise compared to other foods, might be subsidised."
Meanwhile a statement from publishers Weidenfeld and Nicolson, said: "Compared to our recent ancestors who lived outside cities, with rich and varied diets and without antibiotics, we have only a fraction of the diversity of species of microbes living in our guts. Scientists are only now starting to understand the long-lasting impact this has on all of us.
"Only by understanding what makes our own personal microbes tick and interact with our bodies can we overcome the confusion of modern diets and nutrition to regain the correct balance of our ancestors."
The Diet Myth is published on Thursday, priced at £14.99.