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Dual-Energy X-Ray Absorptiometry-Measured Lean Soft Tissue Mass: Differing Relation to Body Cell Mass Across the Adult Life Span

¹ St. Luke's-Roosevelt Hospital, Columbia University College of Physicians and Surgeons, New York.
² Department of Biostatistics, University of Alabama at Birmingham.

Address correspondence to Steven B. Heymsfield, MD, Weight Control Unit, 1090 Amsterdam Ave., 14th Floor, New York, NY 10025. E-mail: sbh2{at}columbia.edu

Lean soft tissue (LST) measured by dual-energy X-ray absorptiometry (DEXA) is used as a metabolic measure in aging research despite evidence of extracellular fluid expansion and a corresponding reduction in body cell mass (BCM) in older participants. We investigated the hypothesis that the fraction of LST as BCM is smaller with greater age. Men and women (n = 2043) had DEXA and ⁴⁰K-counting for body potassium and BCM measured on the same day. Both BCM and LST were lower with greater age but the relative lowering was larger for BCM. A multiple linear regression model was fitted with BCM/LST as the dependent variable, and age, sex, and interaction terms as independent variables. Men had a mean BCM/LST greater (p <.001) than women; quadratic and cubic age terms were also significant or approached significance. Thus, the fraction of LST as BCM is smaller in older adults, a finding that has implications for the interpretation of DEXA results.