Interaction of Nutritional Calcium and HRT in Prevention of Postmenopausal Bone Loss: a Prospective Study

The aim of this study was to investigate the interactive effects between nutritional calcium (Ca) intake and hormone replacement therapy (HRT) on bone loss. The study population, 937 peri- and postmenopausal women, was selected from a random sample (n = 2025) of the OSTPRE-study cohort (n = 13,100) in Kuopio, Finland. Of them, 545 women had never used HRT and 392 women reported its use during the follow-up period of 6 years. Women were divided in groups according to self-reported daily nutritional Ca intake (mg/day): <648 (1st), 648-927 (2nd), >927 (3rd). Bone mineral density of the lumbar spine and femoral neck was measured with dual X-ray absorptiometry at baseline in 1989-91 and at the 5-year follow-up in 1994-97. According to analysis of variance, there were no statistically significant differences in annual bone loss rate between Ca intake tertiles in HRT never users. In HRT users the annual bone loss at the femoral neck was significantly lower in the third tertile than in the second and first tertiles. In a linear regression model, Ca intake prevented femoral bone loss in HRT users (P < 0.001) but contrast had no effect in never users. At lumbar spine, the corresponding Ca effect was weak (P = 0.063). Adjustment for potentially modifying parameters did not change these effects. In addition, HRT prevented femoral bone loss only among women with the highest Ca intake. At the lumbar spine, the difference between HRT users/non-users was significant in all tertiles but was greater in the second and third tertiles than in the first. In conclusion, nutritional Ca intake may protect HRT users from bone loss and vice versa, low nutritional calcium intake may be a risk factor for non-response to HRT.