Childhood obesity is a significant global health concern, affecting an estimated 39 million children under five in 2020. Obesity in childhood is linked to various short-term health problems (asthma, sleep apnea, high blood pressure) and strongly predicts adult obesity, increasing the risk of type 2 diabetes, cancers, and cardiovascular disease. Early intervention is crucial to establish and maintain healthy growth trajectories and reduce the substantial social, health, and economic burden. While numerous early childhood obesity prevention trials are underway, heterogeneity in reported outcomes hinders the synthesis of findings and understanding of effective interventions. Core outcome sets (COS) are agreed-upon minimum sets of outcomes for measurement and reporting, improving cross-study comparisons. A COS for infant feeding interventions already exists, but a broader COS encompassing children from birth to five years and addressing multiple risk factors is needed. This study aimed to develop the COS-EPOCH for trials of early childhood obesity prevention interventions targeting lifestyle-related factors (diet, physical activity, sedentary behavior, sleep, parental practices) across various settings (community, home, early childhood education and care).

The study draws upon a previous systematic scoping review of randomized controlled trials (RCTs) of early childhood obesity prevention interventions. This review identified 18 outcome domains and 221 unique outcomes from 161 studies. A substantial number of these outcomes were merged or omitted to reduce the burden on participants in subsequent stages of COS-EPOCH development, guided by a priori decisions in consultation with a steering group of experts.

The COS-EPOCH development followed the COS-STAD and COS-STAR recommendations. The three-stage process involved: (1) a systematic scoping review of RCTs to identify reported outcomes; (2) a three-round e-Delphi study with stakeholders to prioritize outcomes; and (3) an online consensus meeting to finalize the core outcome set. Stakeholders included parents/caregivers, policymakers/funders, researchers, health professionals, and community/organizational stakeholders. The e-Delphi employed a Likert scale (1-9) to rate outcome importance. Consensus for inclusion was defined as ≥75% of participants in each stakeholder group rating an outcome as 'critically important' and <15% rating it as 'not that important'. Exclusion criteria were similarly defined. The consensus meeting used a nominal group technique (NGT) to discuss e-Delphi results and reach final agreement on the COS. A total of 206 participants from 25 countries participated in round 1 of the e-Delphi study, with participation rates declining in subsequent rounds. The consensus meeting involved nine stakeholders using electronic anonymous voting via Mentimeter.

The systematic scoping review identified 18 outcome domains and 221 unique outcomes. After omitting or merging outcomes considered redundant or lacking clarity, 112 were included in the first round of the e-Delphi. Round 1 yielded consensus for inclusion of five outcomes; round 2 added one more, and round 3 added four more. In total, 10 outcomes reached consensus for inclusion after three rounds of the e-Delphi survey. The consensus meeting finalized the COS-EPOCH, including 22 outcomes across nine domains: anthropometry (child weight-based anthropometry, infant tummy time); dietary intake (child diet quality, dietary intake, fruit and vegetable intake, non-core food intake, non-core beverage intake, meal patterns); sedentary behavior (child screen time, child time spent sedentary); physical activity (child physical activity); sleep (child sleep duration); outcomes in parents/caregivers (parent/caregiver physical activity parenting practices, parent/caregiver sleep parenting practices, parent/caregiver nutrition parenting practices); environmental (food environment, household food security, family meal environment, sedentary behavior or physical activity home environment, early childhood education and care environment); emotional/cognitive functioning (child wellbeing); and economic (economic evaluation). Notably, dietary intake outcomes complement those in an existing COS for infant feeding interventions (≤1 year).

The COS-EPOCH provides a standardized minimum set of outcomes for early childhood obesity prevention trials, addressing the significant heterogeneity in current research. Inclusion of outcomes across multiple levels of the ecological model (individual, family, environment, policy) reflects the complexity of childhood obesity determinants. The rigorous methodology, using established guidelines and a multi-stage approach involving diverse stakeholders, enhances the validity and applicability of the COS-EPOCH. The COS's utility is expected to streamline intervention evaluation, facilitate better synthesis of findings across studies, and ultimately enhance the efficiency and impact of research efforts. While the COS recommends 22 outcomes, trialists may tailor its application based on specific intervention foci and resource limitations; however, deviations should be clearly justified.

The COS-EPOCH, comprising 22 outcomes across nine domains, provides a standardized framework for data collection and reporting in early childhood obesity prevention trials. Future work will focus on identifying and recommending validated measurement instruments for each outcome to further enhance the COS-EPOCH's practical utility and ensure consistent and comprehensive evaluation of interventions. The broader adoption of the COS-EPOCH should lead to more efficient knowledge synthesis, inform future intervention development, and accelerate the translation of research findings into practice.

The study's limitations include lower than expected e-Delphi participation from some stakeholder groups, particularly parents/caregivers, potentially influenced by COVID-19 pandemic disruptions. The predominantly Western representation of participants (Australia, UK, USA) in the e-Delphi and the Australian-centric nature of the consensus meeting warrant further investigation of the COS-EPOCH's applicability in low- and middle-income countries and non-English-speaking contexts. While a steering group of experts provided oversight, the absence of a non-academic stakeholder member on the group is a further limitation.