JOURNAL OF BONE AND MINERAL RESEARCH Volume 5, Number 11, 1990 Mary Ann Liebert, Inc., Publishers Sex Differences in Peak Adult Bone Mineral Density PAUL J. KELLY,' LANCE TWOMEY,' PHILIP N. SAMBROOK,' and JOHN A. EISMAN' ABSTRACT Osteoporotic fractures are more common in women than men. Although accelerated bone loss following the menopause is recognized as of major importance, it is generally considered that a lower peak adult bone mass in females also contributes to their increased risk of osteoporosis in later life. To examine potential sex differences in peak adult bone mass we studied 29 pairs of dizygotic twins of differing within-pair sex in whom the female twin was premenopausal (mean age 37 years, range 21-55). Bone mineral density (BMD, g/cm2) was measured at the lumbar spine and femoral neck by dual-photon absorptiometry; 22 pairs also had BMD measured in the distal and 21 pairs in the ultradistal radius by single-photon absorptiometry. There was no significant difference in usual dietary calcium intake or tobacco consumption between the twin pairs. Consistent with accepted dogma, BMD at both radial sites were higher (+27%) in the males than their female cotwins. I n contrast, there was no sex difference (male versus female) in BMD (mean f SEM) in the femoral neck (0.96 f 0.02 versus 0.97 f 0.03), and surprisingly, the females had a greater lumbar spine BMD than their male cotwins (1.19 f 0.03 versus 1.26 f 0.03, p < 0.05). This difference was observed de- spite the fact that the males were taller (p = 0.033). If the femoral neck BMD values in the females were cor- rected for this difference in BMI, their values (0.99 f 0.03 g/cm2) were significantly higher than those in their male cotwin (p < 0.05). Alternatively if the lumbar spine BMD in the female twins was corrected for the sex difference in anterior-posterior vertebral dimension (measured in cadaveric vertebrae) the female val- ues were then 15% greater than those of their male cotwin. Therefore, a sex difference in peak adult bone density at the lumbar spine or femoral neck does not contribute to the higher incidence of osteoporotic frac- ture at these sites in aging women. Rather, the data suggest that before the menopause the female sex is asso- ciated with a bone density greater than expected on the basis of muscle strength or body size or weight. INTRODUCTION STEOPOROSIS is a common condition in postmenopau- 0 sal women, affecting up to half of the elderly female population, but it affects a smaller number of elderly men.''' The effect of bone loss following the menopause contributes significantly to the sex difference in the inci- dence of osteoporosis, but it is unknown whether females have a lower peak bone mass that may also contribute to their risk of osteoporosis in later life. Studies that have specifically assessed a sex effect on bone density and have found greater bone density in males than females have examined peripheral bone sites, such as the forearm." The possibility of a major sex effect on peak bone density at important axial bone sites has implications for the pre- vention of this disease. and influence. Twin pairs of differing within-pair sex provide a useful model for examining the effect of sex on bone density as they are controlled in part for the ge- netic effect. We studied bone density at the hip, spine, and forearm in twins pairs of differing within-pair sex in whom the female twin was premenopausal to assess the effect of sex on peak bone density. Bone density in adults is under strong 'Garvan Institute of Medical Research, St. Vincent's Hospital, Sydney, NSW 2010 Australia. 'School of Physiotherapy, Curtin University, F'erth, WA, Australia. 1169