Objective: To identify environmental factors associated with bone loss in adult male twins and to determine the extent to which shared environmental characteristics affect estimates of the genetic influence on bone loss.
Design: A 16-year cohort study.
Setting: A midwestern university hospital.
Participants: One hundred and eleven male veterans of World War II or the Korean conflict, born between 1916 and 1927. All were twins, with the sample comprising 48 pairs and 15 persons whose twin brothers were deceased or seriously ill.
Measurements: Bone mass and environmental characteristics (cigarette smoking, alcohol consumption, physical activity, dietary calcium intake, use of thiazide diuretics) measured at baseline and 16 years later.
Results: Rates of radial bone loss averaged 0.45% per year. Those who both smoked and used alcohol at levels greater than the median for the population had a rate of bone loss (10% in 16 years) twice the rate of those who were below the median level for both variables (5% bone loss, P = 0.003). Rates of bone loss were correlated within twin pairs, and these correlations were diminished 25% to 35% by adjustments for environmental influences on bone loss. However, statistically significant within-pair correlations remained (r = 0.4), which did not differ between monozygotic and dizygotic twin pairs after adjustments for smoking, alcohol use, dietary calcium intake, and exercise.
Conclusions: Bone loss in men during mid-life is determined, at least in part, by environmental factors, including smoking, alcohol intake, and, possibly, physical activity. Rates of bone loss were similar within twin pairs, apparently because of a shared environment.