Over one-quarter of children in Wales aged four-to-five are overweight or obese. Children who are overweight or obese may experience health issues during childhood or adolescence, or both. They are also more likely to be overweight or obese through to adulthood. A wide range of biological (e.g. genetics), psychological, environmental (e.g. barriers to physical activity and access to healthier foods) and societal (e.g. lifestyle and peer influence) factors can potentially be associated with childhood obesity or overweight. Evidence reviews of factors associated with childhood obesity or overweight already exist, but these examine the influence of the above factors across a range of ages. No up-to-date review focuses specifically on factors associated with obesity or overweight when both the factor and its impact on weight status were studied in children under five years of age. We aimed to review existing reviews to identify the factors associated with obesity and overweight, specifically when impact on weight status was reported in children under five years of age. The review included evidence available up until December 2023, and 30 systematic reviews were identified. A wide range of biological, psychological, environmental and societal factors were consistently found to be associated with an increased risk of childhood obesity and/or overweight. Factors consistently found to be associated with a decreased risk of childhood obesity and/or overweight were breastfeeding and larger household size. Most of the high-certainty evidence related to childhood overweight. No high-certainty evidence was identified on the association between factors and childhood obesity specifically, and further research studies are needed. The high-certainty evidence supports helping overweight women (who are thinking about having a baby or trying to conceive) to lose weight, reducing rapid weight gain during the first 12 months of life, providing opportunities for children of working mothers to access healthier foods and be more physically active. The moderate-certainty evidence supports promoting breastfeeding, reducing rapid weight gain during the first 13 months of life, monitoring the childs growth rate during the first two years of life (particularly for babies with catch-up growth), promoting baby-led weaning, reducing consumption of sugary drinks, and educating and supporting the wider caregivers to provide healthier foods and opportunities for play and physical activity. The findings from this review may differ from other reviews conducted to inform practice. This review only reports on obesity or overweight outcomes where they are measured before the age of five years, whereas previous reviews have measured these outcomes over a wider age range. This review also focusses on evidence that specifically classifies children as overweight or obese using body mass index (or other well-accepted measures for children under two years).

The authors have declared no competing interest.

The authors and their Institutions were funded for this work by the Health and Care Research Wales Evidence Centre, itself funded by Health and Care Research Wales on behalf of Welsh Government.

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