Commercial weight loss diets are all similarly effective at 6 months, and those with support are more effective at 12 months, according to the results of the randomized BBC “diet trials” reported in the June 3 issue of the BMJ. Participants in these trials were filmed as part of a BBC television series.

“Most adults in the United States diet at some time, and trends in the United Kingdom are similar,” write Helen Truby, MD, from the University of Surrey, Guildford, United Kingdom, and colleagues. “Long term success rates are poor, with 50% of weight loss being regained within one year. Although commercial diets provide consumers with a plethora of choice, data on their comparative efficacy are limited.”

Using a community-based sample of otherwise healthy overweight and obese adults, this 6-month, multicenter, unblended controlled trial compared 4 popular commercial weight loss programs with a control group. These diets were the Slim-Fast plan (a meal replacement approach), Weight Watchers pure points program (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 a weekly group exercise class). Primary endpoints were changes in weight and body fat during 6 months.

Compared with the control group, all diets were associated with a significant loss of body fat and weight during 6 months, but outcomes in the 4 diet groups did not differ significantly from each other. Intent-to-treat analysis revealed that average weight loss was 5.9 kg, and average fat loss was 4.4 kg during 6 months. Although the Atkins diet resulted in significantly higher weight loss during the first 4 weeks, it was no more or less effective than the other diets by the end of the study.

At 12 months, 158 participants (54% of the original sample) returned data, and only 58 (45%) were still compliant with their assigned diets (9 to Atkins, 20 to Weight Watchers, 9 to Slim-Fast, 20 to Rosemary Conley). More participants in the unsupported programs (Atkins diet and Slim-Fast) withdrew than in the supported programs (P = .04), and weight rebound after the initial 6 months was higher in the unsupported programs. All diets resulted in a clinically useful weight loss of around 10% after 12 months in participants who had kept to their original diet.

“Clinically useful weight loss and fat loss can be achieved in adults who are motivated to follow commercial diets for a substantial period,” the authors write. “Given the limited resources for weight management in the NHS [National Health System], healthcare practitioners should discuss with their patients programmes known to be effective.”

Study limitations include varied compliance with each diet, small number of participants, and effect of media interest on motivation to meet goal weights and patients’ expectations of weight loss.

One of the authors has disclosed receiving consulting fees for serving on the scientific advisory panel of Slimming World.

In an accompanying editorial, David Arterburn, MD, MPH, from the Group Health Center for Health Studies in Seattle, Wash, notes the high costs of some commercial diets and the low rate of ongoing compliance with the assigned diets at 1 year.

“The challenge to researchers in obesity is to take weight loss studies, especially those involving commercial programmes and private funding, to the next level,” Dr Arterburn writes. “‘Diet Trials II’ would serve us best by evaluating long term health outcomes, cost effectiveness, and novel strategies of improving adherence and weight maintenance. Such strategies might include economic incentives for participants and researchers collaborating with employers and healthcare providers.”