Lean mass and areal bone mineral density at the lumbar spine, femoral neck, and total forearm were measured in 215 volunteer female twin pairs (122 monozygotic, 93 dizygotic), aged 10-26 years, using dual energy X-ray absorptiometry. The study was conducted in Melbourne from 1990 to 1994. Under the classic twin model, there was evidence for a genetic component of variation in bone mineral density, adjusted for age or for age and lean mass, at all sites. Adjusting for lean mass almost halved the genetic variances in the adolescent years of peak growth, during which genetic variances peaked. Genetic variances were reduced in the late teenage years and increased in early adulthood. The latter may reflect gene-environment interactions or covariation. Importantly, there was evidence for environmental effects shared by twins on lumbar spine and femoral neck bone mineral density, even when adjusted for lean mass as well as age. These were greatest during the late teenage years, abated over the years when pairs started to live apart, and appear to be independent of lean mass during adolescence but not in early adulthood. In summary, the genetic and environmental etiology of bone mineral density is more complex than previously thought.
This cross-sectional twin study examined the influence of constitutional, lifestyle, and genetic factors on bone mineral density (BMD) in elderly women. BMD, at the lumbar spine, femoral neck, Ward's triangle, total hip, and total forearm, total body bone mineral content (BMC), and lean mass and fat mass were measured using dual energy X-ray absorptiometry in 69 volunteer female twin pairs (37 monozygotic [MZ], 32 dizygotic [DZ]) aged 60-89 years. Height and weight were measured. Medical history and lifetime tobacco and alcohol use were determined by questionnaire. In terms of within-pair differences, lean mass was independently associated with BMD at all sites. In contrast, fat mass was not associated with BMD at any site once allowance had been made for lean mass. Lifetime tobacco use was independently associated with BMD at the lumbar spine, total hip, and forearm. Total body BMC was independently predicted by lean mass, fat mass, tobacco use, and alcohol consumption. Age and the above independently predictive body composition and lifestyle factors accounted for 20-33% of variation in BMD. After allowing for these covariates, MZ and DZ correlations were consistent with about 75% of residual variation in BMD at the nonforearm sites being determined by genetic factors. For total body BMC, the covariates explained 75% of total variation, and genetic factors 76% of the residual variation. Therefore, at the proximal femur and lumbar spine, after taking into account the relation of BMD with lean mass and smoking, genetic factors appear to play a substantial role in explaining variation in BMD in elderly women.
This cross-sectional twin study aimed to quantify the roles of constitutional and lifestyle factors on bone mass in adolescent and young adult women. Areal bone density (BMD) at the lumbar spine, femoral neck, Ward's triangle, and total hip, total body bone mineral content (BMC), and lean mass and fat mass were measured using dual energy X-ray absorptiometry (DXA) in 215 female volunteer twin pairs (122 monozygotic [MZ], 93 dizygotic [DZ]) aged 10 to 26 years. Height, weight, menarchial history, dietary calcium intake, physical activity, current tobacco use, and alcohol consumption were determined by questionnaire. Mean BMD increased with age to around 16 years, when it reached a plateau. Within-pair differences in BMD at the lumbar spine (expressed as a percentage of the pair mean BMD) were univariately associated with pair differences in menarchial status (14 +/- 3%), height (0.7 +/- 0.1% per cm), weight (0.4 +/- 0.1% per kg), lean mass (1.0 +/- 0.1% per kg), and fat mass (0.5 +/- 0.1% per kg). Only menarchial status, height, and lean mass, however, were independent predictors. At the proximal femoral sites, within-pair BMD differences were associated with within-pair lean mass differences (1.0 to 1.1 +/- 0.2%/kg), and no other factor was significant. The same conclusions applied to within-pair differences in BMD/height. Total body BMC was independently associated with menarchial status, height, lean mass, and fat mass; the effects of the latter two variables were stronger in pairs both premenarchial. After adjusting for constitutional factors, no lifestyle factor was independently predictive. By reducing collinearity, the cotwin method clearly identified that lean mass, not fat mass, was the major independent determinant of bone mass at the hip, both pre- and postmenarche.
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