Studies were eligible if they were randomised controlled trials (RCTs) with at least 12 weeks of follow-up that studied adults with a mean/median body mass index (BMI) of 30kg/m2 or more, a mean/median age of 40 years or more and at least one additional risk factor for morbidity. Interventions had to be behavioural interventions aimed at changing diet and/or physical activity. Outcomes included behavioural outcomes, weight and risk factors (total cholesterol, low density lipoprotein (LDL) cholesterol, high density lipoprotein (HDL) cholesterol, triglycerides, blood pressure, glycated haemoglobin (HbA1c), fasting plasma glucose).
Most of the included studies were conducted in USA; others came from Canada, UK, Australia, Finland and the Netherlands. About half of the studies examined participants with type 2 diabetes; others included participants with risk factors such as hypertension, impaired glucose tolerance and cardiovascular disease. Eleven studies were in women only. Mean age (standard deviation (SD)) of participants was 55±6.8 years. Mean BMI (SD) was 33.1±2.2 kg/m2.
Interventions aimed at both diet and physical activity behaviour were used in 75% of studies, 14% of the studies were focused on diet only and 11% were focused on physical activity only. Of 51 dietary treatment arms, 20 provided general healthy eating advice and 30 used prescribed diets (reduced calorie, low fat, Weight Watchers, Ornish). Physical activity interventions included physical activity recommendations and supervised exercise classes and groups. Most studies compared interventions against usual care or waiting list controls and a small number compared against less intensive interventions. Study duration ranged between two and 36 months (mode six months). Follow-up ranged from three to 36 months (mode 12 months). Frequency of contact ranged from once every four months to twice a week (mean (SD) 4.6±6.5 times per month).
The first 200 references identified were screened independently by two reviewers and differences were resolved by discussion; the remaining references were screened by one reviewer.