Background Fat and lean mass have important implications for health and physical functioning. Studies have reported associations between low socioeconomic position (SEP) and high body mass index (BMI) in adulthood, but few have examined associations with fat and lean mass which BMI does not completely distinguish. We examined the associations between multiple indicators of SEP across life and objective measures of fat and lean mass in later adulthood.

Methods Data were from 1350 men and women in England, Scotland and Wales participating in the MRC National Survey of Health and Development who underwent dual-energy X-ray absorptiometry scans at age 60–64 years and had valid data for all indicators of SEP were included in analyses. Whole body fat and appendicular lean mass were ascertained and, to account for differences in height across SEP groups, height-adjusted indices were created. Associations of prospectively ascertained childhood SEP (paternal occupational class at 4 years), educational attainment (at 26 years) and adult SEP (household income at 60–64 years) with these outcomes were examined using the slope index of inequality (SII). Since high fat mass may increase muscle loading and lead to higher lean mass, associations with lean mass were adjusted for fat mass.

Results Low SEP was consistently associated with high fat mass index: childhood SEP (mean difference in fat mass index comparing the lowest with the highest SEP (SII) in males: 0.92kg/m^1.2, 95% CI –0.04 to 1.89; females: 2.49, 1.17 to 3.82), education (males: 1.61, 0.69 to 2.53; females: 2.20, 0.88 to 3.53), and adult SEP (males: 1.42, 0.47 to 2.38; females: 2.20, 0.88 to 3.53). In males, after adjustment for fat mass childhood SEP (0.16kg/m², –0.07 to 0.38) and education (0.09, –0.13 to 0.30) were not associated with lean mass index, however, low adult SEP was associated with low lean mass index (–0.21, –0.44 to 0.01). Low SEP was consistently associated with low lean mass index in females after adjustment for fat mass: childhood SEP (–0.24, –0.42 to –0.07), education (–0.36, –0.55 to –0.17) and adult SEP (–0.20, –0.38, –0.03).

Conclusion Low SEP across life was associated with high fat mass in early old age. In contrast, low SEP in adulthood in males and across life in females was associated with low lean mass (after adjustment for fat mass). Although further research is required to better understand the pathways underlying these associations, the socioeconomic patterning of these measures is likely to have important public health implications.