Waist to hip ratio predicts cardiometabolic risk beyond body mass index in metabolically unhealthy normal weight Indian women

Body Mass Index (BMI) is the dominant tool for cardiometabolic risk stratification in India, yet it captures total body mass rather than fat distribution. The Metabolically Unhealthy Normal Weight (MUNW) phenotype, characterised by normal BMI (18.5–24.9 kg/m2) with elevated Waist-to-Hip Ratio (WHR ≥ 0.85), represents a population of women who may carry clinically significant visceral adiposity that BMI screening may fail to detect. The magnitude of this diagnostic gap has not been quantified at a national level in India. This cross-sectional secondary analysis used aggregated subgroup-level data from India’s National Family Health Survey (NFHS-5, 2019–2021), covering 571 sociodemographic subgroups across 27 states and 2 union territories, representing 2,752,255 reconstructed women aged 15–49 years. Phenotype groups were constructed using the statistical independence assumption, with formal bias-bounding conducted to quantify the impact of BMI–WHR correlation on prevalence estimates: MHN (Normal BMI + Normal WHR), MUNW (Normal BMI + High WHR), MHO (High BMI + Normal WHR), and MUO (High BMI + High WHR). Binomial Generalised Linear Models estimated odds ratios for hypertension and blood glucose dysregulation. Reclassification was assessed using NRI and IDI. The MUNW phenotype was the single largest adiposity group (41.4%; n ≈ 1.14 million). Compared with MHN women, MUNW women had significantly elevated odds of hypertension (OR = 1.637; 95% CI 1.620–1.655), blood glucose dysregulation (OR = 1.620; 95% CI 1.598–1.643), and very high blood glucose (OR = 1.625; 95% CI 1.590–1.660), all p < 0.001. BMI attenuated WHR’s hypertension coefficient by 58.8%, yet WHR’s independent effect persisted across all models. Age-specific analysis revealed a perfect dose–response (Spearman ρ = 1.000; p < 0.001), with significant MUNW–hypertension associations from age 15–19 onwards. MUNW prevalence was highest in Jammu & Kashmir (55.6%), with significant regional heterogeneity (Kruskal-Walli’s p = 0.025). The IDI confirmed additional discriminative value of WHR beyond BMI (IDI = 0.000278). In this subgroup-level ecological analysis, more than 927,000 Indian women carry hypertension-relevant visceral adiposity invisible to BMI-based screening. These associations, while robust and consistent, are ecological in nature and require individual-level prospective validation. WHR represents a potentially clinically meaningful, low-cost addition to national cardiometabolic screening programmes, particularly in rural, tribal, and low-education populations where the diagnostic gap is largest and the public health cost of missing the MUNW phenotype is greatest.