Obesity Transitions by Age, Period, and Cohort: A Population-Based Application of the Lancet Commission Framework in the United States and Korea

Background: While emerging global classification frameworks highlight the need to redefine obesity beyond body mass index (BMI) to improve prevention strategies and public health policy, population-level applications remain scarce. We examined trends in BMI-based and newly proposed Lancet Commission definitions of obesity in the United States and South Korea, two countries with contrasting obesity trends. Methods: Using representative data from the United States National Health and Nutrition Examination Survey (NHANES, 2003-2016) and the Korea National Health and Nutrition Examination Survey (KNHANES, 2007-2023), we conducted age-standardized and age-period-cohort (APC) analyses to compare obesity trends. Results: Newly defined obesity phenotypes demonstrated a greater proportion of at-risk individuals than BMI alone, particularly among United States women and Korean men. Excess adiposity in the United States showed early increases from 55.6% to over 75%, followed by stabilization, while Korea exhibited a sustained upward trend. APC analyses revealed more divergent patterns. These sex-specific patterns align with differences in visceral adiposity by sex and in East Asian populations. Period effects on obesity were characterized by a high and sustained burden in the United States, but a more gradual yet persistent upward trend in Korea. Preclinical obesity displayed a U-shaped cohort effect in the United States, with higher prevalence in cohorts born after 1975, whereas clinical obesity continued to rise in Korea's recent birth cohorts, especially among men. Conclusion: Our findings emphasize the utility of expanded obesity definitions and APC analysis in capturing sex-specific and generational patterns of obesity and related complications. They support adopting life-course, cohort-informed approaches in obesity prevention.