Hormone replacement therapy and bone mineral density: a co-twin approach

Our objective was to estimate the difference in bone mass at clinically relevant sites within female twin pairs who were discordant for use of hormone replacement therapy (HRT).We studied 46 female twin pairs who were discordant for HRT use. Bone mineral content and density were measured at the lumbar spine, total hip, femoral neck, 13 total forearm, and the total body. HRT use, calcium intake, physical activity, alcohol intake, and lifetime smoking were determined by questionnaire.Within a pair, lumbar spine bone mineral density was significantly greater in past and current HRT users compared with nonusers (6.2% +/- 2.0%; P = 0.006). In those pairs who were currently using HRT, the within-pair difference in lumbar spine bone density was 7.8% +/- 2.1% (P = 0.002), and a significant within-pair difference in forearm bone density (5.1 +/- 2.1%; P = 0.02) was apparent. A significant difference (4.6%; P = 0.03) was observed in total body bone mineral content when an adjustment was made for age, lean mass, fat mass, and height.In keeping with randomized clinical trial findings, these results indicate that HRT in routine clinical use protects significantly against menopausal bone loss at the lumbar spine and the forearm. Our results also quantify the magnitude of the benefit of HRT on bone density that might be anticipated in clinical practice.