Defining BMI cutoff points for detecting excess body fat in young adults

This study aimed to establish new body mass index (BMI) cutoff points for detecting excess body fat in 18-y-old males and females, using air displacement plethysmography (ADP, BodPod®) as the reference for measuring percent (%) fat mass.

A cross-sectional analysis was conducted using data from the 2004 Pelotas Birth Cohort (Brazil), including 3052 individuals at 18 y of age. Body composition was measured using ADP, with excess adiposity defined as body fat percentage ≥20% for males and ≥33% for females. BMI was calculated using the formula, body weight/height². Cutoff points were analyzed using the receiver operating characteristic curve (ROC) and the Youden index, comparing sensitivity, specificity, and predictive values of traditional versus proposed cutoff points.

The optimal BMI cutoff points for detecting excess body fat were ≥24.0 kg/m² for males and ≥23.5 kg/m² for females, showing similar accuracy to the traditionally adopted cutoff for overweight (≥25.0 kg/m²) and greater accuracy than the obesity cutoff (≥30.0 kg/m²). The sensitivity of the new cutoff was 2.4 times greater than that of the ≥30.0 kg/m² cutoff among males (75.8% vs. 31.9%) and 2.6 times greater among females (82.2% vs. 31.0%). The findings indicate that a BMI ≥30.0 kg/m² accurately detects fewer than one-third of young men and women with excess body fat.

The newly proposed BMI cutoff points (≥24.0 kg/m² for males and ≥23.5 kg/m² for females) optimize the detection of excess body fat and are more effective than traditional thresholds. These findings may facilitate early detection and support the implementation of public health interventions.