"The reason [the companies] aren’t disclosing success rates is because they are bleak," says Allan Gelibter, who treats obesity and eating disorders at St. Luke’s-Roosevelt Hospital in New York City.

The low success rates have provoked critics to argue that the industry profits off destined-to-fail diets.

graphic: slim-fast

Slim-Fast Foods company has sold meal replacements for over two decades.

Cogan reviewed 50 studies on the effectiveness of weight-loss diets and found uninspiring results. In most programs, within half a year subjects gained an average of half of the weight they lost back.

There is a conflict of interest for obesity experts, who often rate the diets and serve on government panels while working as consultants to the diet industry at the same time, she says.

"We have an understanding that experts are neutral, but when it comes to this issue, they are not," Cogan says.

But the publicized failure rates may be slightly inflated because studies of diet success are often performed on patients at hospitals seeking hard core treatment, Gelibter says. These patients are most likely to have tried to lose weight and failed before.

"But the diet companies definitely have a commercial interest in keeping them [statistics] under wraps," he says.