Between 1990 and 2022, the global prevalence of obesity more than doubled, representing a critical public health concern. China has mirrored this trend, with rapidly increasing rates of overweight and obesity across all age groups. According to the 'Report on Chinese Residents' Nutrition and Chronic Diseases (2020)', 50.7% of Chinese adults are now classified as overweight or obese. Among children and adolescents aged 6 to 17 years, data from the 'Atlas of Nutrition and Health Status of Chinese Children' indicates that the prevalence of overweight and obesity has reached 26.5%. Obesity is associated with a broad spectrum of adverse health outcomes across the life course. In children, excess adiposity negatively affects skeletal maturation, neurocognitive development, and psychosocial well-being, while also increasing the likelihood of obesity and metabolic dysfunction in adulthood. In adults, obesity is a major risk factor for a range of non-communicable diseases (NCDs), including type 2 diabetes mellitus, hypertension, cardiovascular disease, non-alcoholic fatty liver disease, osteoarthritis, certain cancers, and all-cause premature mortality. Beyond its health implications, obesity imposes substantial economic and social burdens, including increased healthcare expenditure and reduced workforce productivity. The etiology of the obesity epidemic is multifactorial and remains under active investigation. Hypotheses center around a chronic imbalance between energy intake and expenditure, driven by behavioral, environmental, and physiological factors. Notably, decreased physical activity associated with sedentary lifestyles and increased consumption of energy-dense, nutrient-poor foods have been implicated as key contributors. However, the relative contributions of reduced energy expenditure versus increased energy intake remain insufficiently quantified at the population level. Accurate assessment of total energy expenditure (TEE) is therefore essential to elucidate the energy dynamics underlying the obesity epidemic. Current approaches for estimating population-level energy and food requirements are often based on indirect methods with limited precision. The doubly labelled water (DLW) technique, which quantifies TEE through measurement of isotope elimination rates (\^2H and \^18O), remains the gold standard for assessing free-living energy expenditure. However, its application has been predominantly confined to high-income countries with well-established research infrastructure. In contrast, the use of DLW in low- and middle-income countries-including China-remains minimal, resulting in critical data gaps that hinder the development of context-specific dietary recommendations and energy requirement models. To address these limitations, this study will apply the DLW method to measure TEE in healthy children and adults in China. In parallel, the study will assess key modulators of energy metabolism, including anthropometric and physiological parameters, gut microbiota composition, habitual physical activity, and ambient temperature exposure. The resulting dataset will provide high-resolution, population-specific evidence to inform national dietary reference intakes and support the formulation of evidence-based public health strategies aimed at obesity prevention and metabolic health promotion.