Sustainable diets are linked to co-benefits for human and planetary health, such as decreased environmental impacts by reducing carbon dioxide emissions and improved cardiometabolic health. However, the applicability of sustainable diets to obtain co-benefits has been investigated in adults but not for children and adolescents, who have unique nutritional requirements. To address this, co-benefits of sustainable diets will be estimated in children in ALSPAC. Specifically, we will assess the environmental impacts from diets (e.g., carbon dioxide, water usage, and biodiversity) in children from ALSPAC and associated cardiometabolic health parameters (e.g., body mass index) and estimate the reduction of environmental impacts from switching to a more sustainable diet (e.g., EAT-Lancet reference diet). Uniquely, this project will have in depth assessment of associated co-benefits from reducing ultra-processed foods, a topic of much debate in terms of their true environmental impact. Results from this study can be used to inform sustainable dietary targets specific for children and adolescents and associated policies in the United Kingdom.