Rates of bone loss in postmenopausal women: Relationship to calcium intake, calcium absorption, serum estrogen, body mass and physical activity

Abstract

<p>A prospective 2-year study was designed to test the hypothesis that 3 core factors influence the postmenopausal decline in bone mineral: adequate supply of calcium to the skeleton, endogenous estrogen production by lean and fat tissue mass, and mechanical stress imparted by physical activity and body mass. The rate of change in bone mass (ΔBMD) was established for 61 postmenopausal women from semi-annual measurements of bone mineral density (BMD) at the proximal femur and lumbar spine using dual energy X-ray absorptiometry (DXA). Whole body BMD and body composition were also assessed annually using DXA. Serum estradiol was determined at baseline by radioimmunoassay. Calcium intake was evaluated using a food frequency questionnaire. Calcium absorption was measured using a single isotope technique. Grip strength was measured using a Jamar hand dynamometer. Aerobic fitness was determined using a submaximal 1-mile walk test. Habitual daily activity was assessed using a portable accelerometer. At baseline, the strongest associations were between BMD and body mass values. These explained 22 to 25% of the variance in BMD. Estradiol was an independent predictor of BMD of the whole body. No physical activity variable was independently predictive of BMD. Of the dietary variables, only tea consumption was independently predictive of BMD at the femoral and whole body sites. With age, bone loss was attenuated. At the lumbar spine, ΔBMD was also positively influenced by lean mass, weight gain, protein intake and increased calcium intake. The contribution of estradiol was borderline (p = 0.06). Weight gain was similarly influential at the femur. There was no positive influence of any physical activity measure on ΔBMD at any site. Lean mass and weight gain had the greatest positive influence on BMD and ΔBMD. A hormonal rather than mechanical explanation was favoured. Trabecular bone may also be responsive to dietary perturbations.</p>