When I was working at a newspaper in Phnom Penh a few years ago, my Cambodian colleague went on a no ice diet. It was the hottest part of the year and our office ran on sweet iced coffees by day and iced beers by night. She refused both, as did many Cambodian women that year. Ice, she explained when asked, provided an unwelcome shock to the metabolism and therefore slowed it down.

When you encounter a fad diet whose logic you are unfamiliar with, its ridiculousness slaps you in the face. And yet at the exact same time as Cambodian women were sweating their way through the summer months, American and European media outlets were lending serious credence to another ice diet evangelist. According to the American gastroenterologist Brian Weiner, anyone hoping to lose weight shouldn’t cut...