In the first randomized controlled trial of Weight Watchers participants, family doctors compared the commercial weight-loss program with standard care, and Weight Watchers was found to be more than twice as effective.

More people stuck to the Weight Watchers diet, and lost more weight and fat mass, than those assigned to standard care, researchers found.

Susan Jebb of Britain's Medical Research Council (MRC) Human Nutrition Research Unit, who led the study told reporters that results proved the Weight Watchers program to be a robust intervention that is generalizable to other economically developed countries.

The study, published Thursday in the British medical journal Lancet, comes in the wake of research last month which said obesity is a global epidemic that is fast replacing tobacco as the single most important preventable cause of costly chronic diseases like heart disease, diabetes and cancer.

This kind of research is important so that we can identify clinically effective interventions to treat obesity, said Jebb. Worldwide, around 1.5 billion adults are overweight and another 0.5 billion are obese, with 170 million children classified as overweight or obese.

In the weight-loss study, which was funded by Weight Watchers International, researchers assessed the health habits of nearly 772 overweight and obese adults in Australia, Germany and Britain.

[The study] proves that Weight Watchers is part of the solution to help transform the health of nations, said David Kirchhoff, CEO of Weight Watchers International.

Patients were assigned either 12 months of standard care, often offered by physicians, or a 12-month free membership for a Weight Watchers group in their neighborhood.

There is a clear need for practical treatment solutions that are proven effective, affordable and scalable to have a population-wide impact, Kirchhoff said in a statement.

About 61 percent of patients in the Weight Watchers group lost at least five percent of their body weight, compared with 32 percent in the standard care group, researchers noted.

According to the study, the average weight loss at 12 months was 5.1 kg for those using Weight Watchers versus 2.2 kg for those on standard care. And for those who completed the full 12 months, average weight loss was 6.7 kg on Weight Watchers versus 3.3 kg on standard care.

Obesity takes up between two to six percent of healthcare costs in many countries, according to the National Institutes of Health. Advocates of the weight-loss program added that the inexpensive rates - currently $65 for 12 weeks - of the program make it more appealing.