NHANES (CDC) — derived BRI percentiles
About this reference
No published study provides sex- and age-stratified Body Roundness Index (BRI) percentile tables for US adults. NHANES collects both waist circumference (BMXWAIST) and standing height (BMXHT) in the body measures examination, the two inputs to the Thomas 2013 BRI formula. FitnessNorms derived BRI percentiles by downloading public XPT microdata files, merging body measures with demographics, computing per-participant BRI as 364.2 − 365.5 × √(1 − (WC/(2π))² / (0.5×Ht)²) with WC and Ht in metres, and then computing weighted empirical quantiles (P5, P25, P50, P75, P95) for each sex × 5-year age bracket from 20-24 through 80+. Unlike the FitnessNorms WHtR and WHR pages, which pool multiple NHANES cycles, the BRI percentiles here use the 2021-2023 cycle alone (BMX_L / DEMO_L, weight WTMEC2YR). Pooling 2015-2016 + 2017-March 2020 + 2021-2023 was computed as a sensitivity check and rejected as the published primary because the 2015-2023 pool disagreed with the 2021-2023 cycle in 64 of 130 (sex × bracket × quantile) comparisons by more than 0.15 BRI units. The drift is directionally consistent: recent values are systematically higher in the upper tails, reflecting the rise in US adult adiposity over 2015-2023, which the BRI eccentricity formula amplifies. The largest single-cell difference is at male 45-49 P95 (11.52 in 2021-2023 vs 9.74 in the pool, an absolute difference of 1.78). Median absolute difference across all 130 cells is 0.145. Shipping the pool as primary would understate today's US adult distribution by an amount that varies cell by cell, with no honest way to summarise the drift in a single caveat, so the 2021-2023 single cycle is published instead. The pooled output is retained in the repository as an internal sensitivity artifact (scripts/nhanes_bri_pooled.SENSITIVITY.json). Weighted quantiles were computed using linear interpolation on the empirical cumulative distribution (analogue of Type 7). Sampling weights (WTMEC2YR) were applied to produce weighted percentile estimates; survey-design standard errors were not computed (this is an internal derivation, not a peer-reviewed publication).
Known limitations
- US population only. Percentiles reflect the US adult population, which has one of the highest obesity rates globally; US BRI distributions sit higher than reference values from leaner populations. No widely accepted clinical BRI cut-off exists. Recent papers use study-specific ROC thresholds for different outcomes (≥4.2 for diastolic dysfunction; quartile splits for cancer studies; other values for diabetes, NAFLD, etc.), which is why this page reports percentile distributions rather than risk categories.
- This is an internal derivation by FitnessNorms from CDC public microdata, not a peer-reviewed publication. Weighted empirical quantiles are used; survey-design-correct standard errors are not reported.
- Single-cycle sample (n=5,754) is smaller than the comparable WHtR pool (n≈18,700) and WHR pool (n≈13,727), so the P5 and P95 tails are noisier per cell. Per-cell n ranges from 136 (male 80+) to 397 (female 60-64). Single-cycle was chosen over multi-cycle pooling because cross-cycle drift in the BRI distribution is large enough that pooling would mask a real population change rather than smooth random noise.
- RIDAGEYR is top-coded at 80 in NHANES public data; the 80+ bracket includes all participants aged 80 and over.
- Cross-sectional design: values reflect population averages at the time of survey, not individual trajectories.