The relationship of vitamin D and selected nutrient intakes, sex hormone binding globulin and markers of bone turnover to bone mineral density in exercising and non-exercising postmenopausal women taking or not taking HRT

Abstract

The loss of bone mineral density (BMD) plays a major role in the increased incidence of osteoporosis in aging women and, consequently, strategies to maintain BMD are critical to quality of life for these women. The role of vitamin D in the accrual and maintenance of bone mineral and its relationship to the incidence and severity of osteoporosis is not well understood. By measuring serum and intake levels this study investigates the relationship of vitamin D to baseline BMD and changes in regional and total body BMD over 1 y. The role of sex hormone binding globulin (SHBG) is also investigated. Because SHBG binds with both estrogen, an antiresorptive agent, and testosterone, a bone formation agent, lower serum SHBG concentrations may promote a greater bioavailability of estrogens and androgens, which could decrease resorption, stimulate formation and increase BMD. Women who were 3-10 y postmenopausal, aged 55 ± 5.1 y, and taking or not taking hormone replacement therapy (HRT) were randomized into exercise and non exercise groups: (1) No HRT, no exercise; (2) HRT, no exercise; (3) No HRT, exercise; and (4) HRT, exercise. The number of subjects per group at the end of one year was 25, 19, 27 and 20, respectively. The thrice weekly exercise regimen, consisting mainly of weight lifting and weight bearing activities, lasted for 1 y. Vitamin D deficiency was found in 3% of the subjects, Serum 25(OH)D₃ concentrations had inverse relationships with changes in BMD in the femoral neck (P < 0.05) and trochanter (P = 0.07). When subjects were grouped according to HRT status, BMD at baseline and one 1 y was never positively related to serum 25(OH)D₃ concentrations in HRT users, Subjects having greater than 80 nmol/L 25(OH)D₃ had significantly decreased concentrations of serum osteocalcin and urinary deoxypyrodinoline (Dpd) crosslinks (P < 0.05). Exercise had no effect on serum content of 25(OH)D₃. Serum concentrations of SHBG were not significantly related to BMD at any site, nor did they show a decrease with exercise even when HRT status was taken into account. Significant inverse relationships (P < 0.05) were found between SHBG, sex hormone indices (Estrone/SHBG; Estradiol/SHBG) and bone turnover markers, osteocalcin and Dpd crosslinks/creatinine.