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Deriving BMI-referenced, sex-specific fat mass index cutoff values in older Saudi adults: a cross-sectional secondary
Maram Alsadi1, Mahmoud M A Abulmeaty1, Aljazi Bin Zarah1
1Community Health Sciences Department, College of Applied Medical Sciences, King Saud University, Riyadh, Saudi Arabia.
Background/Objectives:
Age-related changes in body composition may reduce the ability of body mass index (BMI) to accurately reflect adiposity in older adults. Fat mass index (FMI), which expresses fat mass relative to height squared, may provide a more direct estimate of adiposity. However, BMI-referenced, sex-specific FMI cutoff values have not been well characterized among older Saudi adults. This study aimed to derive sex-specific cutoff values for FMI, body fat percentage (%BF), and fat mass corresponding to selected BMI thresholds among older Saudi adults.
Methods:
This cross-sectional secondary analysis included 1,954 adults aged ≥60 years who attended 15 randomly selected primary healthcare centers in Riyadh, Saudi Arabia. Anthropometric and bioelectrical impedance analysis-derived body composition data were analyzed. Receiver operating characteristic (ROC) curve analysis and the Youden index were used to derive cutoff values, and DeLong's test for correlated ROC curves was used to compare BMI-referenced discriminatory performance.
Results:
Across the examined BMI thresholds, FMI demonstrated high discriminatory performance relative to BMI in both men and women. For older men, FMI cutoffs corresponding to BMI > 27 kg/m2, BMI ≥ 30 kg/m2, and BMI > 31 kg/m2 were 7.90, 9.48, and 9.71 kg/m2, respectively. The corresponding values in older women were 10.30, 12.10, and 13.00 kg/m2. DeLong testing generally supported higher AUCs for FMI than for %BF and fat mass, although the difference between FMI and fat mass at BMI ≥ 25 kg/m2 in men was not statistically significant.
Conclusion:
This study provides BMI-referenced, sex-specific FMI cutoff values for older Saudi adults. FMI may serve as a complementary body composition index when BIA-derived fat mass is available. Because the cutoffs were derived using BMI as a pragmatic reference framework in a cross-sectional sample from primary healthcare in Riyadh, they should be interpreted as BMI-corresponding values rather than definitive clinical risk thresholds. External validation against clinical outcomes and reference methods for body composition is required before broad clinical implementation.
