Relative fat mass as a complementary anthropometric indicator for screening normal-weight obesity in Chinese college students: a cross-sectional study

Normal-weight obesity (NWO) may be overlooked in university-based health screening programmes that rely primarily on body mass index (BMI). Relative fat mass (RFM), a waist-based index incorporating height, waist circumference, and sex, may help identify excess adiposity among individuals classified as normal weight by BMI. This study evaluated whether RFM could serve as a complementary indicator for identifying NWO among Chinese college students. Methods: This cross-sectional study included 111 Chinese college students aged 17–20 years. Among them, 79 participants with normal BMI were included in the primary NWO analysis. Percentage body fat, assessed using bioelectrical impedance analysis, was used as the operational criterion for NWO classification. NWO was defined as BMI 18.5–23.9 kg/m² combined with elevated percentage body fat, using thresholds of ≥ 25% in men and ≥ 30% in women. Correlation analysis, between-group comparisons, receiver operating characteristic (ROC) analysis, exploratory logistic regression, and decision curve analysis were performed. The AUCs of RFM and BMI were directly compared using the DeLong test. Results: Among participants with normal BMI, 16 were classified as having NWO, including 1 man and 15 women. RFM showed the highest AUC for identifying NWO in the overall normal-BMI sample (AUC = 0.931, 95% CI: 0.878–0.984), and its AUC was higher than that of BMI (AUC = 0.798, 95% CI: 0.693–0.902; DeLong P = 0.030). In the exploratory female subgroup analysis, BMI showed the highest AUC, followed by waist circumference, whereas RFM and waist-to-height ratio showed identical AUC values. RFM did not outperform BMI among women with normal BMI (AUC: 0.852 vs. 0.912; DeLong P = 0.426). Conclusions: RFM showed higher single-index discriminatory performance than BMI in the overall normal-BMI sample, but this advantage was not evident among women with normal BMI. RFM should be considered a secondary complementary indicator rather than a replacement for BMI or waist-based indices in university-based screening.