A man who claims to have lost more than 60 pounds by eating only from McDonald's has now become a brand ambassador for the fast food chain.
Iowa high school science teacher John Cisna ate McDonald's for every meal for three months, as part of an experiment for his class. And when he lost 37 pounds, he carried on.
After another three months, he found he'd slimmed down from his original 20 stone to a slim 15 stone 8 pounds. He also lowered his cholesterol and lost a total of 21 inches off his chest, waist and hips.
And Cisna is now travelling the US giving speeches about his weight loss and promoting his book, My McDonald's Diet.
"Today, John serves as an official brand ambassador for McDonald's and travels the country sharing his story about the importance of choice and balance," a McDonald's spokeswoman told ABC News.
He's not paid by McDonalds, and continues to work part-time as a substitute teacher.
But Cisna's achievement was harder than it seems. He and his students used the McDonald's meal planner to create his diet, which included rather more fruit, porridge and salads than it did Big Macs.
However, according to Tim Spector, professor of genetic epidemiology at of King's College London, limiting your diet can make you fatter by reducing the number and variety of bacteria in your gut.
In his new book, The Diet Myth, he explains that changes in our attitudes to food have meant we are no longer exposed to the microbes that burn calories and help regulate our weight. A diet of junk food can dramatically reduce healthy gut microbes in only two days, he says.
In one experiment, Spector's son, Tom, spent 10 days eating nothing but McDonald's chicken nuggets, fries, burgers and Coca-Cola.
"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," the 23-year-old genetics student told The Australian.
"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."
The greater the variety of food, the greater diversity of microbes; but some foods are better than others. For a healthy gut, says professor Spector, we should all be eating garlic leeks and celery - and drinking coffee and Belgian Beer.
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