Researchers compared the effectiveness of four popular commercial weight loss programmes with a control group. The study was sponsored by the BBC as part of its reality TV series, BBC Diet Trials.

The diets were the Slim-Fast Plan (a meal replacement approach), Weight Watchers pure points programme (an energy controlled diet with weekly group meetings), Dr Atkins' new diet revolution (a self-monitored low carbohydrate eating plan), and Rosemary Conley's eat yourself slim diet and fitness plan (a low fat diet and weekly exercise class). The control group was asked to maintain their current diet and exercise pattern.

Weight and body fat changes were monitored over six months and dieting behaviour was checked again at 12 months.

After six months, all diets resulted in significant loss of body fat and weight compared to the control group. Average weight loss was 5.9 kg and average fat loss was 4.4. kg (5-10% of body weight). The Atkins diet resulted in significantly higher weight loss during the first four weeks, but by the end, was no more or less effective than the other diets.

There were no significant differences in cardiac risk factors between the diet groups and the control group. The Atkins diet did not lead to substantial increases in cholesterol levels.

At 12 months, 158 participants (54% of the original sample) returned data. Only 58 (45%) were still keeping to their allocated diets (nine to Atkins, 20 to Weight Watchers, nine to Slim-Fast, 20 to Rosemary Conley). More participants in the unsupported programmes (Atkins diet and Slim-Fast) withdrew from the study than in the supported group based programmes, and weight rebound after the initial six months was higher in the unsupported programmes.

The authors conclude that clinically useful weight loss and fat loss can be achieved in adults who are motivated to follow commercial diets for a substantial period. People need to find a diet that best suits them.

"Our study provides data on how much weight patients can expect to lose by dieting," they write. "These data could help practitioners in managing patients' expectations of weight loss targets."

An accompanying editorial suggests that the challenge to researchers is to take weight loss studies to the next level by evaluating long term health outcomes, cost effectiveness, and novel strategies of improving adherence and weight maintenance.

bmj