This is a protocol for a Cochrane Review (intervention). The objectives are as follows:

The overall aim of the review is to determine the effectiveness of interventions to prevent obesity in children under two years old. This includes to assess obesity prevention interventions that are conducted during this age bracket (in children under two years old only) and to measure any impacts of these interventions during this age bracket and at any follow-up periods.

The four objectives are:

1. to evaluate the effects of interventions that aim to modify breastfeeding, formula feeding and other dietary intake on changes in zBMI score, BMI and serious adverse events among infants and children under two years old;

2. to evaluate the effects of interventions that aim to modify movement behaviours (physical activity, sedentary behaviour, sleep, play or structured exercise) on changes in zBMI score, BMI and serious adverse events among infants and children under two years old;

3. to evaluate the combined effects of interventions that aim to modify breastfeeding, formula feeding or other dietary intake and movement behaviours on changes in zBMI score, BMI and serious adverse events among infants and children under two years old;

4. to compare the effects of interventions that aim to modify dietary interventions with those that aim to modify movement behaviours on changes in zBMI score, BMI and serious adverse events among infants and children under two years old.

The secondary objectives are designed to explore if, how, and why the effectiveness of interventions on zBMI/BMI varies depending on the following PROGRESS factors.

• Place of residence

• Race/ethnicity/culture/language

• Occupation

• Gender/sex

• Religion

• Education

• Socioeconomic status

• Social capital

The PROGRESS acronym is intended to ensure that there is explicit consideration for health inequity, the unfair difference in disease burden, when conducting research and adapting research evidence to inform the design of new interventions (O'Neill 2014). The PROGRESS acronym describes factors that contribute to health inequity. Recent work on race and religion in the UK suggests that consideration of these factors is critical to the design of new interventions (Rai 2019).

We will also collect, from RCTs, information about the costs of interventions so that policymakers can use the review as a source of information from which they may prepare cost-effectiveness analyses.