Regulation of Protein Metabolism in Middle-Aged, Premenopausal Women: Roles of Adiposity and Estradiol1

The age-related loss of fat-free mass (FFM) is accelerated in women during the middle-age years and continues at an increased rate throughout the postmenopausal period. Because protein is the primary structural component of fat-free tissue, changes in FFM are largely due to alterations in protein metabolism. Knowledge of the hormonal and physiological correlates of protein metabolism in middle-aged women, therefore, has important implications for understanding the mechanisms underlying changes in FFM. We measured leucine kinetics (expressed relative to FFM: μmol/kg FFM·h) in 46 middle-aged, premenopausal women (mean ± sd, 47 ± 3 yr) after an overnight fast (i.e. basal) and during euglycemic hyperinsulinemia (40 mU/m2·min) using a 5.5-h infusion of[ 1-13C]leucine. Additionally, we measured insulin-stimulated glucose disposal by euglycemic hyperinsulinemic clamp, body composition by dual energy x-ray absorptiometry, abdominal fat distribution by computed tomography, and hormone levels by RIA as possible cor...