BMC Public Health 2005, 5:41 http://www.biomedcentral.com/1471-2458/5/41 Background Advancing age is associated with a number of changes in body composition. Notable among these changes is the reduction in FFM, that occurs primarily as a result of losses in skeletal muscle mass [1], a condition referred to as sar- copenia. For example, older men aged 80 years or older have 25 percent less leg muscle mass than young men between the ages of 20 and 29[2]. FFM has been closely linked with reduction in muscle strength in the aged. Fur- ther, a greater reduction in both muscle mass and strength have been reported in older nursing residents with history of falls. Thus reduction in muscular strength is associated with increased risk of falls and hip fracture, and preva- lence of disabilities[3]. Independent of fall risk, low FFM has been linked to higher rates of all-cause mortality in women in the United States[4]. Therefore, losses in FFM with advancing age have important health implications. Higher FFM and lean mass to height ratio in Blacks than Whites have been reported [5], but mostly in young non- representative samples. There is limited information on national estimates, and on the existence of differential influence of ethnicity on the decline in FFM with increas- ing age. Given the inverse association of FFM with all cause mortality[6] and the reported higher FFM in Blacks, lower mortality rates in Blacks would be expected. To the converse, both all cause mortality[7] and FFM are know to be higher in Blacks, suggesting that the relationship of FFM with ethnicity and all cause mortality may be due to reasons other than differences in absolute FFM. We hypothesized that black men and women will have higher degree of age-related decline in FFM and FFMI despite higher mean values in Blacks, using data from NHANES III. The characterization of changes in FFM across several decades of life is crucial to our ability to reduce the burden of sarcopenia, osteoporosis, falls and hip fracture, as well as associated mortality. Methods In order to characterize FFM in older populations in the U.S., we analyzed data from NHANES III. The data for NHANES III were collected by the National Center for Health Statistics of the Centers for Disease Control and Prevention between 1988 and 1994, using a national multistage, probability sample of the civilian non-institu- tionalized population of the United States. The initial NHANES III sample consisted of 33,994 individuals, 2 months to 99 years of age [8]. The survey design ensured over-sampling of Blacks, and those 60 years and older. Data were collected from each participant through face-to- face interview, physical examination, and laboratory anal- yses. The overall survey design and provisions for informed consent have been previously detailed [8]. Subjects in the present study included 8,679 non-institu- tionalized Whites and Blacks over the age of 40 years. Of this number, complete data on anthropometric measure- ments were only available for 5,803 persons. Age was cat- egorized as decades of life up to the age of 79 years, while persons ages 80 and older were separately grouped to avoid over-stratification and loss of statistical power. We also categorized age by increments of 5 years for the pur- pose of graphical representations only. Anthropometric measurements Weight and standing height were determined according to standardized procedures [9]. Weight was recorded to the nearest 0.01 kg, while height was recorded to the nearest 0.1 cm. Body mass index (BMI) was calculated as weight in kg divided by height in meter squared (m2). Except for a few measurements that were taken on the left side of the body because of examinees' limitations, the rest of the anthropometric measurements were taken on the right side of the body using standard procedures and were recorded to the nearest 0.1 mm [9]. All skinfolds were measured using Holtain skinfold calipers on the right side of the body. With the examinee standing erect and the arms hanging freely at the sides, the triceps skinfold was measured on the posterior surface at the midpoint of the right upper arm. The suprailiac skinfold was measured at a 45-degree angle at the marked iliac crest. Measurement of the thigh skinfold was taken from a vertical line over the quadriceps muscle at midline of the thigh, and half way between the top of the patella and the inguinal crease [9]. All examiners were trained in standard procedures for obtaining measurements. Out-of-range measurements were verified to differentiate errors in measurement from high values for very small or very large examinees. Because relevant anthropometric measurements were available only for the triceps, suprailium and thigh, body density was estimated from the sum of 3 skinfolds, (tri- ceps, suprailium, and thigh) and age, based on the equa- tion of Jackson and Pollock (body density = 1.0994921- 0.0009929 * (triceps + suprailium + thigh) + 0.0000023 * (triceps + suprailium + thigh)**2 - 0.0001392 * (age)) [10], and percent body fat was estimated from the Brozek equation (percent body fat=((4.570/body density) - 4.142) * 100) [11]. FFM was calculated by subtracting body fat from body weight. Based on the analogy of BMI and the high correlation between FFM and height, FFMI was derived by dividing FFM (kg) by the square of height (m2)[12]. Statistical analyses Because our primary interest was in how ethnicity influ- enced age-related differences or decline in FFM by gender, initial analyses were conducted for each of the 4 demo- graphic subgroups formed by the combination of sex and Page 2 of 9 (page number not for citation purposes)