Fatness biases the use of estimated leg length as an epidemiological marker for adults in the NHANES III sample

Background We analyse the NHANES III sample to assess the suitability of measured stature and sitting height to estimate leg length (tibia + femur) and predict fatness. High rates of overweight in the United States population may lead to greater gluteo-femoral fat mass which will increase sitting height and artificially decrease estimates of both absolute and relative leg length. Methods The analyses include 3076 women and 3233 men, 20.0-49.9 years of age of White, Black or Mexican-American ethnicity. The poverty index ratio, measured stature, sitting height, upper leg length, weight, four skinfolds, buttocks circumference and elbow, biacromial and biiliac breadths were extracted from the database. The sitting height ratio, % body fatness, % upper leg length (ULL/stature), and other indices were estimated. Correlation and principle component analysis were used to assess the relationship between measures of body fatness, relative leg length and the other variables. Results For adults in the NHANES III body fat is more strongly correlated with buttocks circumference (r = 0.87 and 0.78 for women and men), than with any measure of estimated leg length (r's range from -0.28 to -0.10 for both sexes). Principle components analysis separates fatness, stature and estimated leg length into uncorrelated factors for this sample. Conclusion Reports of a negative association between leg length and fatness for adults of the NHANES III are likely spurious, due to greater gluteo-femoral fat thickness increasing sitting height. Future rounds of the NHANES, and similar surveys in other nations with high body fat populations, should measure lower extremity length directly to better assess its relationship to health and disease risk.