The review concluded that some obesity prevention interventions showed a positive impact on body mass index or weight status, but the evidence was based on a very small number of studies. The authors' cautious conclusion appeared reasonable, but due to potential for language and publication biases, and the unknown quality of the included studies, its reliability is unclear.

To evaluate the efficacy of obesity prevention interventions in adults.

MEDLINE via PubMed, EMBASE, PsycINFO, The Cochrane Library and Web of Science databases were searched from January 1996 to June 2006. Search terms were reported.

Studies that evaluated interventions aimed at primary prevention of obesity among adults (18 years or older) achieved by focusing on dietary intake or physical activity alone or in combination were eligible for inclusion. Obesity prevention was defined as the maintenance of body weight or the attenuation of weight gain among non-obese adults. The primary outcome of interest was change in body mass index (BMI) or body weight. Included studies had to evaluate primary prevention of obesity and include participants with an average BMI at start of study of less than 30kg/m^-2. Studies that evaluated interventions aimed at weight loss among overweight but not yet obese participants had to include participants with a BMI of 25 to 30kg/m^-2. Studies of weight maintenance interventions after weight loss or that included institutionalised adults, only patients with chronic conditions or those undergoing medical interventions such a bariatric surgery, drug interventions or in a preoperative setting were excluded. Studies had to have a minimum follow up of three months.

Most included studies focused on a combination of diet and physical activity change; the remaining studies focused on either dietary change or physical activity promotion alone. Most participants were women. The mean age of participants ranged from 22 years to 62.3 years. Most studies had outcomes measured by clinic staff; the rest had self-reported outcomes.

Two reviewers independently assessed papers for inclusion.

The authors did not state that they assessed validity.

Data on change in BMI or body weight from individual studies were extracted by one reviewer and checked by a second reviewer. No measures of variance were reported for these data.

Data were combined in a narrative synthesis with additional information provided in tables.

Nine Randomised controlled trials (RCTs) were included (n=51,438). Sample sizes ranged from 40 to 48,835. Follow-up ranged from four months to 7.5 years.

One RCT (n=48,835) reported a small but statistically significant greater reduction in BMI (p<0.001) and weight (p=0.01) for a dietary intervention compared to a usual diet control group at 7.5 years follow-up. For comparisons of combined dietary and physical activity against control groups, greater reductions for intervention groups were reported in BMI for two RCTs (n=575, p=0.046 and p<0.001), and weight loss for three RCTs (n=642, p=0.046, p=0.01 and p<0.001). No statistically significant differences were reported between groups for BMI reduction in six RCTs (n=2,023) and weight loss in five RCTs (n=575)

Some studies showed a positive impact on BMI or weight status, but the evidence of efficacy of obesity prevention interventions was based on a very small number of studies; therefore, more research was urgently needed to extend the body of evidence.

The review question was clear and supported by detailed inclusion criteria. Several relevant sources were searched, but no attempts were made to minimise publication bias. It was unclear whether language limitations were applied, although authors reported that only English-language studies were included in the review, which indicated potential for language bias. Appropriate methods to reduce reviewer errors and bias were used in the assessment of studies and extraction of data. Study validity was not assessed, so the results from these studies and any synthesis may not be reliable. A narrative synthesis was appropriate given the differences between studies in terms of intervention and study populations. However, the review excluded studies of patients with chronic conditions or undergoing medical or drug interventions and so the results may not be generalisable to these populations. Most included studies had small sample sizes. The authors' cautious conclusion appeared reasonable, but due to the potential for language and publication biases, and the unknown quality of the included studies, its reliability is unclear.

Practice: The authors did not state any implications for practice.

Research: The authors stated that further research was required to address strategies that focused on the underlying societal causes of obesity through action in sectors including transport, environment, employment conditions, education, health and food policies, and social and economic policies.

The 6th Framework programme of the Commission of European Communities (EUROCADET: SP23-CT-2005-006528, Contract Number 006528).

This is a critical abstract of a systematic review that meets the criteria for inclusion on DARE. Each critical abstract contains a brief summary of the review methods, results and conclusions followed by a detailed critical assessment on the reliability of the review and the conclusions drawn.