Waist Circumference as an Independent Marker of Insulin Resistance: Evidence from a Nationwide Korean Population Study

<b>Background</b>: Waist circumference (WC) has become an essential diagnostic marker of metabolic syndrome; however, its accuracy in reflecting insulin resistance remains uncertain, particularly among Asian people who develop metabolic complications at lower levels of adiposity. <b>Methods</b>: We analyzed 20,202 adults (8886 men and 11,316 women) from the Korean National Health and Nutrition Examination Survey (KNHANES) 2015 and 2019-2021. All analyses accounted for the survey sampling weights and the complex survey design. WC, insulin resistance, and β-cell function were assessed using fasting insulin level, HOMA-IR, and HOMA-β. Associations between WC and insulin indices were evaluated using correlation analyses, stratification by WC deciles, and multivariable regression models adjusted for demographic and metabolic covariates. <b>Results</b>: WC was positively associated with HOMA-IR in both sexes (all <i>p</i> < 0.001). In the fully adjusted model, HOMA-IR remained significantly associated with WC [men, β = 0.014 (95% CI, 0.011-0.016); women, β = 0.009 (95% CI, 0.008-0.011)]. Subgroup analyses stratified by age revealed significant associations across all age groups. The association was stronger in older adults, particularly among men, whereas women exhibited significant associations across all ages with a larger age-related increase. <b>Conclusions</b>: WC is independently associated with insulin resistance in Korean adults, with stronger associations in older adults and consistent associations in women. These findings support WC as a simple yet pathophysiologically meaningful marker of metabolic risk and highlight the necessity of refining WC cutoff values for clinical screening of Asian populations and an age-specific approach for diagnosing metabolic syndrome.