In the first randomised controlled trial -- considered the gold standard of scientific analysis -- to directly compare a commercial weight-loss programme with standard care by family doctors, Weight Watchers was found to be more than twice as effective.

More people stuck to the Weight Watchers diet, they lost more weight and fat mass, and also shaved more off their waist measurements than those assigned to standard care.

Susan Jebb of Britain's Medical Research Council (MRC) Human Nutrition Research Unit, who led the study, said the results showed Weight Watchers is "a robust intervention that is generalisable to other economically developed countries."

"This kind of research is important so that we can identify clinically effective interventions to treat obesity," she said.

The study, published in the Lancet medical journal, 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.

Worldwide, around 1.5 billion adults are overweight and another 0.5 billion are obese, with 170 million children classified as overweight or obese. Obesity takes up between 2 to 6 per cent of healthcare costs in many countries.

In the weight loss study, which was funded by Weight Watchers International but run as an investigator-led trial with all data collection and analysis conducted by the independent research team, researchers assessed 772 overweight and obese adults in Australia, Germany and Britain.

Patients were randomly assigned to receive either 12 months of standard care as usually offered by the primary care team, or referred to and given a 12-month free membership for a Weight Watchers group in their neighbourhood.

As well as losing twice as much weight as those in the standard care group, patients referred to Weight Watchers were also more than three times as likely to lose 10 per cent or more of their initial body weight. Some 61 per cent of patients in the Weight Watchers group lost at least 5 per cent of their body weight, compared with 32 per cent in the standard care group.

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. 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.

"These important findings show that obesity treatment is effective and structured commercial programmes can enhance outcomes," said Nick Finer a consultant endocrinologist and bariatric physician at University College London Hospitals, who was not involved in the reseearch.

In a commentary on the study, Kate Jolly and Paul Aveyard of the school of health and population sciences at Britain's Birmingham University said cost-effectiveness was a key factor in determining whether commercial programmes like Weight Watchers become part of publicly funded health care.

They added that "the low cost of these programmes -- at present about 50 to 60 pounds for 12 weeks -- makes the case for incorporation intuitively appealing."

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

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

- Reuters