They are sold to the obese and desperate as a silver bullet that will restore a sense of self-worth, but crash diets risk damaging a person’s capacity to burn calories, research has revealed.
A study of contestants on a reality TV show which challenges obese people to get dramatically thinner revealed that they are now condemned to a lifetime battle with food as their bodies strive to regain their previous weight. To make things worse, they are no longer able to burn calories at the rate they once did.
The Biggest Loser, an American show that had a British offshoot from 2005 to 2012, has long been criticised by health professionals for irresponsible promotion of drastic weight loss through unsafe dieting and exercise.
The first study of contestants from the US series in 2009 has found that almost all have returned to their original weight, with many ballooning beyond their pre-show size — and not from lack of trying. Though many have admitted indulging in banned food and drinks, analysis of their metabolic rate shows they are now physically unable to process a normal number of daily calories.
Danny Cahill, the winner of season eight, shed 239lb, going from 30.7 stone to 13.6 stone in seven months. Six years later he weighs more than 21 stone despite eating just 800 calories a day — less than half of what another man his size could eat.
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Almost all of those examined by Kevin Hall, of the National Institute of Diabetes and Digestive and Kidney Diseases in Maryland, said they battled with hunger pangs and cravings that had risen dramatically since the show.
During the extreme bout of dieting, their leptin, a hormone that helps to control hunger, virtually disappeared, which would have made the contestants ravenous at all times. Once they started eating normally their leptin slowly began to increase but only to half its previous levels. They experienced greater hunger while their metabolisms were less able to burn energy from food.
Dr Hall’s six-year study revealed the extent to which the body fights extreme weight loss. “It’s frightening,” he told The New York Times. Experts have long known that losing weight quickly damages the body’s natural capacity to burn calories.
Rachel Batterham, of the centre for obesity research at University College London, said that tampering with the body’s predisposition to fight weight loss will result in uncontrollable weight gain in almost all cases. She added: “These changes do not reverse once the person returns to their previously higher weight — they continue.”