1. Bone mineral content of the second, third and fourth lumbar vertebrae was determined in normal women and women with clinical osteoporosis by using dual-photon ( 153 Gd) absorptiometry. 2. A cross-sectional study of 70 normal women (aged 19–88 years) showed a bone loss of 44% from the age of around 34 years throughout life. 3. Longitudinal data from 59 normal women confirmed that the vertebral bone loss started before the menopause. An accelerated bone loss amounting to nearly 6% per year was seen immediately after the menopause. The bone loss of older women was slower. 4. Mean lumbar bone mineral content of 36 women (aged 48–93 years) with recent fractures of their femoral neck after minor trauma equalled that of age-matched normal women. Lumbar bone mineral content of the women with intratrochanteric femoral neck fractures was lower than that of the women with medial femoral neck fractures. 5. Mean lumbar bone mineral content of 72 women (aged 58–89 years) with primary osteoporosis was 41% lower than that of normal premenopausal women and 18% lower than that of age-matched controls. A weak inverse relationship between lumbar bone mineral content and the number of compression fractures was found. A weak inverse relationship between lumbar bone mineral content and the number of compression fractures was found. 6. Women with lumbar bone mineral content values below the 95% confidence limits for normal premenopausal women are at risk of future vertebral compression fractures, the fracture risk being inversely related to lumbar bone mineral content.
1. Normal subjects showed an average increase in serum ionized calcium (Ca 2+ ) concentration of 0·11 mmol/l in peripheral venous blood 10 min after onset of bicycle exercise at 70% of maximum aerobic capacity. The corresponding mean rise in serum total calcium concentration was 0·21 mmol/l. 2. The change in serum Ca 2+ as result of acidification was studied in 20 normal subjects by carbon dioxide equilibration in vitro followed by measurement of serum Ca 2+ . The log serum Ca 2+ was inversely proportional to serum pH. 3. The Δlog serum Ca 2+ /ΔpH in vitro was similar to the Δlog serum Ca a+ /ΔpH in vivo during exercise, this ratio, however, being somewhat greater during the first minute of exercise. 4. Serum Ca 2+ returned to normal values about 20 min after stopping exercise as the pH returned to normal, but the fall immediately after stopping exercise was more pronounced than that due to the change in pH, as predicted from the studies in vitro. 5. Blood lactate concentration rose from 0·86 to 8·41 mmol/l after 10 min exercise, but the rise in blood lactate during exercise was slower than the rise in serum Ca 2+ . Also the fall during the recovery period was delayed compared with the fall in serum Ca 2+ . 6. It is suggested that the rise in serum Ca 2+ during severe muscular exercise might be important for the physiological adaptations during work, and for bone metabolism.