Globally, childhood overweight and obesity is a public health problem. Although the rising trend in children's body mass index (BMI) has plateaued in some high-income countries, it has accelerated in low- and middle-income countries. It is especially true amongst Chinese children with the annual increase rate of obesity during 2010-2014 greater than any other periods from 1985 to 2010.

With the dramatic economic development in China, children are now growing up in an increasingly 'obesogenic' environment. For example, the availability and ubiquity of computers and smart phones promote sedentary time, and access to energy dense food and sugar sweetened beverages is now widespread. Effective childhood obesity intervention is urgently needed in China. Although over 20 intervention studies for overweight/obesity among children and adolescents have been conducted in China since the 1990s, most of them had moderate or serious methodological weaknesses. For example, they did not report the number of students, schools or districts initially approached to participate, raising the possibility of selection and recruitment bias. Additionally, although they stated the allocation of intervention and control were randomized, no description of the method of randomization was reported.

Given the relative lack of high-quality interventions for childhood overweight/obesity, the investigators designed a cluster randomized controlled trial to assess the effectiveness of a multicomponent one-academic-year intervention among 24 primary schools (approximately 1200 students) in the eastern (Beijing), middle (Shanxi) and western (Xinjiang) part of China.

The study aims to identify: 1) whether the school-based intervention will be effective for preventing excessive weight gain among children; 2) whether the intervention will be beneficial for improving healthy eating, physical activity and reducing sedentary behaviors among children.

1. Eligibility criteria for schools The trial recruited a total of 24 schools. A convenience sample of 8 schools were selected from Beijing, Shanxi (Changzhi City) and Xinjiang (Urumqi City) respectively.

1. Inclusion Criteria for schools:

   • school leader agreeing to implement this intervention; requiring school teachers who can implement this intervention (e.g. school doctors, physical education teachers, physical education teachers, etc.); having at least 50 children from Grade 4 (at the start of the intervention) per school;having a even number of schools within each district.

2. Exclusion Criteria for schools:

   • boarding schools;schools solely for children with special skills; schools of minor ethnic groups; similar programme (focus on weight gain prevention) would be conducted in schools during the study period; schools having a cancelling or relocation plan during the study period.

     2. Eligibility criteria for classes One class per school will be selected if the estimated number of students enrolled in the programme is no less than 40 within the selected class; two classes per school will be selected if the estimated number of students enrolled in the programme is less than 40 within the selected class.

     3. Eligibility criteria for students Eligible children will be those whose primary caregivers will provide written consent to participate the study. After collecting students' medical history from their parents, we will exclude individuals suffering from or having a history of any cardiovascular and metabolic diseases, asthma and disabilities that limit their ability to perform physical activity.

The assessors measuring childrens' weight and height will be blinded at follow-up phases.