DXA Body Composition: NHANES Microdata Derivation
The six DXA body-composition pages (body fat %, lean mass index, appendicular lean mass index, fat mass index and the two trunk fat ratios) are a FitnessNorms derivation from public CDC NHANES microdata for all US adults. Before any value was published, the pipeline had to reproduce the National Center for Health Statistics' own published percentiles for the same scans.
Files and cohort
- Cycles: 1999-2000 (
DEMO / BMX / DXX), 2001-2002 (_B), 2003-2004 (_C). - DXA records: 22,133 participants; 14,213 aged 20 and over, all with a positive examination weight.
- Excluded, no DXA data: 1,122 adults: 772 not scanned because pregnant; 8 not scanned, weight over 300 lb; 342 not scanned for another reason (no time, refusal or pregnancy test not completed); status as recorded in the NHANES DXX codebook.
- Imputed without a valid scan: 1,265 adults in the analytic cohort have no valid scan and DXA values fully imputed by NCHS: 10 scan completed but invalid; 158 not scanned, weight over 300 lb; 8 not scanned, height over 6 ft 5 in; 1,089 not scanned for another reason (no time, refusal or pregnancy test not completed).
- Analytic cohort: 13,091 adults. The three height-based indices also exclude 134 adults without a height measurement (12,957 remain).
Weights, imputation and estimator
- Weights: six-year examination weights per NCHS guidance: the four-year weight x 2/3 for 1999-2002 and the two-year weight x 1/3 for 2003-2004.
- Imputation: NHANES released five imputed versions of the DXA data. Every percentile is computed within each version and the five are averaged; each participant must appear in all five.
- Percentiles: survey-weighted empirical quantiles with midpoint cumulative weights normalised by the total weight, by sex and 5-year age bracket (20-24 to 85+). These are bracket aggregates, not curve values at an exact age.
Definitions
- Lean mass is lean soft tissue excluding bone mineral content, as in the NCHS tables. Kelly et al. 2009's lean values behave as if bone mineral were included, so these lean percentiles run about 4% lower than the Kelly-based values this site published before October 2026.
- Indices divide kilograms by height in metres squared. The trunk-to-leg fat percent ratio uses the combined fat percentage of both legs (leg fat / leg total mass).
Reproduction of the NCHS tables
Target: Borrud LG, Flegal KM, Looker AC, et al. Body composition data for individuals 8 years of age and older: U.S. population, 1999-2004. Vital Health Stat 11(250). NCHS, 2010. Thirteen of its tables (total, trunk, arm and leg fat and lean, and percent fat) were compared for adults by sex and age group (20-39, 40-59, 60-79, 80+), using the same estimator as the published pages.
- Sample sizes: 104 of 104 exact.
- Percentiles: 936 of 936 within the pre-registered tolerance of two units of the last printed digit; 18 sit exactly at the limit, 17 of them in the two percent-fat tables.
- Offset: our values are slightly higher than the printed ones throughout, by about 0.06 units on average in the kilogram tables and 0.10 in the percent-fat tables. Truncation of the printed values would account for 817 of the 936 cells, so it is not a complete explanation, and the rest of the difference is unexplained. Nothing was adjusted to reduce it.
Comparison with Kelly et al. 2009
Kelly et al. 2009 fitted smoothed curves separately for non-Hispanic White, non-Hispanic Black and Mexican American adults from the same scans. For comparison only, we computed each group's bracket medians with this pipeline and set them against Kelly's curve value at the bracket midpoint, in cells with at least 100 people. The two are different kinds of estimate, so differences are expected; the lean measures also differ in their bone-mineral definition.
| Metric | Cells compared | Median difference | Largest difference |
|---|---|---|---|
| Body fat % | 59 | 1.2% | 4.7% |
| Lean mass index | 58 | 4.5% | 6.9% |
| Appendicular lean mass index | 58 | 5.3% | 9.1% |
| Fat mass index | 58 | 2.2% | 9.7% |
| Trunk-to-limb fat mass ratio | 59 | 1.0% | 4.8% |
| Trunk-to-leg fat percent ratio | 59 | 1.4% | 3.9% |