Normal-Weight Obesity and Hidden Cardiometabolic Risk in Young Adults: Implications Beyond BMI-Based Classification

Background: BMI is widely used for obesity classification, yet it does not adequately reflect adiposity distribution and related metabolic risk. Normal-weight obesity (NWO), defined as excess adiposity despite normal BMI, has emerged as a clinically relevant phenotype associated with increased cardiometabolic risk. However, its prevalence and implications in young populations remain insufficiently characterized. Objective: To evaluate the prevalence of normal-weight obesity and its association with cardiometabolic risk markers in a cohort of young adults undergoing occupational health assessments. Methods: A cross-sectional study was conducted in 12,874 adults aged 22–30 years undergoing routine occupational health assessments. NWO was defined using a strict criterion (normal BMI with a waist-to-height ratio ≥ 0.5) and an expanded definition including additional adiposity markers derived from body fat percentage and visceral fat estimates. Anthropometric, adiposity-related, biochemical, and lifestyle data were collected using standardized protocols. Multivariable logistic regression models adjusted for age and sex were used to assess associations between NWO and cardiometabolic risk markers. Results: Among individuals with normal BMI (n = 9290), the prevalence of NWO was 1.79% (n = 166) using the strict definition and 2.30% (n = 214) with the expanded definition. Compared with metabolically healthy normal-weight individuals, those with NWO exhibited higher triglycerides, fasting glucose, and atherogenic lipid indices, along with lower HDL cholesterol (all p < 0.05). In multivariable analyses, NWO was independently associated with elevated triglycerides (OR 4.99; 95% CI 2.90–8.58), an unfavorable triglyceride-to-HDL ratio (OR 7.44; 95% CI 5.19–10.65), and impaired fasting glucose (OR 3.87; 95% CI 2.19–6.82). Associations were consistent across sensitivity and sex-stratified analyses. Conclusions: Normal-weight obesity is present in a measurable proportion of young adults and is associated with an unfavorable cardiometabolic profile despite normal BMI. The triglyceride-to-HDL ratio was consistently associated with normal-weight obesity and showed marked differences across adiposity-defined phenotypes. These findings highlight the limitations of BMI-based classification alone and suggest that additional anthropometric and metabolic markers may help identify individuals with less favorable cardiometabolic characteristics.