Logo: to the web site of Uppsala University

Background Bone mineral density (BMD), bone size and bone turnover are independent determinants of fractures in elderly. Earlier twin studies of these phenotypes have revealed high heritability for BMD and bone area, and more moderate heritability for bone-turnover markers. No previous Scandinavian study has evaluated the genetic and environmental contribution to the variance of these phenotypes, despite the fact thatSweden andNorway have the highest osteoporotic fracture incidences worldwide.

Methods Participants were selected from the Swedish Twin Registry. All intact like-sexed twin pairs born in 1965 or earlier and living in thecounty ofUppsala were invited to participate. A total of 102 twin pairs, 45 monozygotic and 57 dizygotic, accepted the invitation to participate. All twins underwent measurement of bone mineral density and bone area using dual-energy X-ray absorptiometry DXA. Hip geometry was also calculated. Markers for bone formation (osteocalcin) and bone resorption (CrossLaps) were measured in serum.

Results We observed a strong heritability for BMD at the lumbar spine (0.85; 95% CI 0.54-0.90), the femoral neck (0.75; 95% CI 0.62-0.83), and the proximal femur (0.84; 95% CI 0.74-0.90). The values for bone area were approximately similar to those for BMD. Bone turnover markers had a slightly lower genetic influence with a value of 0.69 (0.53-0.80) for osteocalcin and 0.58 (95% CI 0.33-0.75) for CrossLaps. As a comparison, the heritabilities of body height and weight were 0.95 and 0.82, respectively.

Conclusion The high genetic influence on bone phenotypes among Swedish middle-aged and older men and women should encourage further work on the identification of specific genetic pathways. Continuing research in this area could reveal the mechanisms behind the strong genetic susceptibility of bone-related phenotypes.