Temporal trends in physical activity and diet quality among Korean adults (2018–2021)

Background Physical activity and diet quality are key determinants of chronic disease prevention and overall health. However, their population-level time trends and between-year differences in cross-sectional associations remain underexplored. Aims This study examined secular trends in physical activity and diet quality among Korean adults from 2018 to 2021 and assessed whether the cross-sectional association between adherence to WHO physical activity guidelines and diet quality varied across survey years. Methods Data were drawn from 18,976 adults aged 19–79 years participating in the Korea National Health and Nutrition Examination Survey. Physical activity was assessed using the Korean version of the International Physical Activity Questionnaire-Short Form and classified according to the WHO guideline (≥150 min/week). Diet quality was measured using the Korean Healthy Eating Index (KHEI). Logistic regression was used to examine the association between physical activity adherence and higher diet quality, adjusting for age, gender, smoking, alcohol use, and body mass index. Meta-regression evaluated the between-year variation in the cross-sectional association. Results Physical activity levels remained stable across survey years, whereas diet quality significantly declined ( P linear = 0.04), particularly among women, non-smokers, alcohol consumers, and individuals with underweight or overweight. In year-specific cross-sectional models, meeting the physical activity recommendation was associated with 31% higher odds of higher diet quality (OR = 1.31, 95% CI = 1.16–2.48, p < 0.001). This association did not significantly change over time. Conclusions From 2018 to 2021, diet quality declined among Korean adults despite stable physical activity levels. Although physical activity adherence was consistently associated with higher diet quality, no significant temporal trends were observed in this relationship.