Wondering which of the many "brand" diets is the most effective? It may not really matter which you choose, say researchers who've found they all allow people to lose around the same amount of weight in the first year and then generally see people putting some of it back on.

That's the conclusion of a review of randomized controlled trials of the Weight Watchers, Atkins, South Beach and Zone diets, says study lead author Renee Atallah of McGill University in Montreal.

Available studies to date have been unable to definitively show whether one branded diet was better than others, Atallah says of the study's review, and extensive clinical trials of all four diets would be needed to settle that question.

"These trials [would] need to assess both long-term weight loss and changes in other heart disease risk factors so that we better understand the potential benefits with their use," she says.

In their review, the researchers found that dieters on Atkins, Weight Watchers and Zone lost similar but small amounts of weight over the first year of dieting.

People following the Atkins diet lost an average of 4.6 to 10.3 pounds, Weight Watchers participants shed an average of 6.6 pounds, while Zone dieters dropped an average of 3.5 to 7 pounds.

People in a comparison group who were on any form of non-branded diets lost around 4.9 pounds, the researchers reported.

A review of other health measures such as cholesterol levels, blood sugar numbers, blood pressure, or other cardiovascular risk factors, the Atkins, Zone and Weight Watchers diets showed no difference, the researchers said.

In North America, people spend more than $66 billion a year on these branded diets or others in an effort to lose weight and keep that weight off.

"What is incredible to me is that dieting is a multibillion-dollar industry, and if we look at the amount of data out there, it's miniscule in comparison to the market," says McGill researcher Mark Eisenberg. "There are very few well-done studies, and most of them look at the short term. And weight loss is obviously a long term issue."

Other experts not involved in the study noted that all four diets reviewed had elements in common and featured proven techniques for weight loss.

"They almost all promote non-starchy vegetables, low-calorie high-volume foods," explains Lindsay Malone, a registered dietitian at the Cleveland Clinic, "Most shy away from junk foods."

Dietary tracking, a recognized component of successful weight loss strategy, was utilized in all four diets, she says.