In a study published in the International Journal of Obesity, they found that women who stuck with the programme lost 19.5kg, while men trimmed their weight by 25.8kg.
But more than 40% quit before the year was up. And even among completers, three-quarters of the weight they'd lost had crept back after three years.
"Weight regain remains the Achilles' heel of all weight loss therapies," said Dr Thomas Wadden, who runs the Centre for Weight and Eating Disorders at the University of Pennsylvania in Philadelphia, US, but wasn't involved in the new work.
In the new study, Dr Stephan Bischoff of the University of Hohenheim in Stuttgart and colleagues used a number of lifestyle changes, including a low-calorie diet, behavioural therapy, group meetings, nutritional counselling and exercise - a weight loss programme franchised by Nestle as OPTIFAST52.
The researchers, all of whom work for OPTIFAST centres, signed up more than 8,000 obese participants at dozens of centres across the country.
At the start of the programme, women weighed about 112kg on average and men weighed 136kg. Combining all participants, regardless of whether or not they finished the programme, women lost an average of 15kg, with men shedding about 5kg more.
Completers achieved best results
Those who stuck it out, about six out of every ten participants, lost 53% of their excess weight. That's close to the weight loss achieved by surgery - about 60% - the researchers note.
But when the researchers tracked a sample of 300 people three years after they'd completed the programme, it turned out they had regained most of their original weight. According to experts, that's less likely to happen after surgery.
Still, Dr. Bischoff said, a fifth of the participants were able to keep their new weight without further help.
Side effects rare
Potential side effects from the programme were rare, with the most common ones - hair loss and constipation - reported by less than 1% of the participants.
Whether those problems are a real consequence of the Nestle program is still uncertain, because the study didn't include a control group. And there are other problems with the programme, said Stanley Heshka, a nutrition researcher at Columbia University in New York.
Although OPTIFAST52 appears to work in the short term, he told Reuters Health by email that it is not a practical solution to America's obesity problem. Obese Americans tend to be poor, while intensive programmes like Nestle's tend to be "very costly".
University of Pennsylvania's Wadden does see promise in weight loss programmes, but said they need to do a better job of helping participants keep the pounds off.
"For every month that you spend losing weight, you should spend another month learning how to keep the weight off," he said. - (Eric Schultz/Reuters Health, July 2011)