Calcium intake, physical activity and bone mass in pre-menopausal women

Thirty pre-menopausal women aged 35–40 years were assessed with respect to dietary calcium intake, physical activity and other factors related to bone loss (by questionnaire); bone mass in the mid and distal femur measured by computerized tomography; and leg strength and muscle size. Cortical thickness at mid-femur was negatively associated with breast-feeding (r=-0.40). Bone mass in the distal femur was negatively associated with age (r=-0.46) and positively associated with activity (r=0.44) and leg strength (r=0.42). There was no association of bone mass at either site with current dietary calcium intake (range 198–1511 mg/day), nor with body mass index (BMI), hand-grip strength, parity, oral contraceptive use, smoking, or alcohol intake. Thus, in older pre-menopausal women, activity apparently helps to maintain bone mass at the distal femur, a site of high trabecular bone content, while current dietary-calcium intake has no effect on bone mass in the femur. Activity was most strongly associated with bone mass in the distal femur when it was defined as the total number of hours spent standing or moving (i.e. other than sitting or lying down), regardless of the intensity of activity.
The strong negative association between bone mass in the distal femur and age in older pre-menopausal women is of concern in relation to long-term risk of osteoporotic related fracture, suggesting that factors relating to bone loss in this age-group should be studied longitudinally. The apparent loss of cortical bone in breast-feeders warrants further investigation.