DO GENETIC-FACTORS EXPLAIN ASSOCIATIONS BETWEEN MUSCLE STRENGTH, LEANMASS, AND BONE-DENSITY - A TWIN STUDY

Are the associations between muscle strength, lean mass, and bone mine ral density (BMD) genetically determined? Based on within-pair differe nces in 56 monozygotic (MZ) and 56 dizygotic (DZ) female twin pairs, m ean age 45 yr (range 24-67), BMD was associated with lean mass, indepe ndent of fat mass and height (P < 0.05). A 10% increment in femoral ne ck. (FN) BMD was associated with a 15% increment in lean mass (similar to 6 kg). BMD was associated with muscle strength (measured in 35 pai rs) before, but not after, adjusting for lean mass. Based on age-adjus ted cross-sectional analyses, same-trait correlations (+/-SE) in MZ pa irs were double those in DZ pairs: FN BMD (0.62 +/- 0.08, 0.33 +/- 0.1 2) and lean mass (0.87 +/- 0.03, 0.30 +/- 0.11; all P < 0.001), consis tent with a genetic hypothesis. The cross-trait correlation (r) betwee n lean mass and FN BMD in the same individual was 0.43 +/- 0.06. The c ross-trait cross-twin correlation between lean mass in one twin and FN BMD in the other was 0.31 +/- 0.07 in MZ pairs, similar to 75% of the cross-trait correlation (r) and 0.19 +/- 0.09 in DZ pairs (P < 0.001) . After adjusting for height and fat mass, the MZ and DZ cross-trait c ross-twin correlations were no different (0.16 +/- 0.08 and 0.13 +/- 0 .09, respectively). Therefore, genetic factors account for 60-80% of t he individual variances of both FN BMD and lean mass, and >50% of thei r covariance. The association between greater muscle mass and greater BMD is likely to be determined by genes regulating size.