Calcif Tissue Int (1987) 40:133-136 Calcified Tissue International 9 1987 Springer-Verlag New York Inc. The Effect of an Oral Calcium Load on Plasma Ionized Calcium and Parathyroid Hormone Concentrations in Osteoporotic Postmenopausal Women M. Horowitz, H. A. Morris, T. F. Hartley, A. G. Need, J. Wishart, P. Ryan, and B. E. C. Nordin Departments of Medicine, Endocrinology, and Community Medicine, Royal Adelaide Hospital and Division of Clinical Chemistry, Institute of Medical and Veterinary Science, Adelaide, South Australia, 5000 Summary. Plasma ionized calcium (IC) and para- thyroid hormone (PTH) concentrations were mea- sured in 31 osteoporotic postmenopausal women at hourly intervals for 5 hours after a 1 g oral calcium load. Fifteen subjects had normal radiocalcium ab- sorption and 16 subjects were malabsorbers of cal- cium. IC rose and PTH fell after the calcium load in both groups with a plateau at 3-4 hours, and the rise in IC was greater (P < 0.01) in the normal ab- sorbers. There was a nonsignificant trend for the fall in PTH to be greater in the normal absorbers. In the group as a whole the mean increase in IC (above baseline) at 4 hours was directly related to calcium absorption (P < 0.025) and the mean change in PTH was inversely related to calcium ab- sorption (P < 0.05). These results demonstrate that in subjects with postmenopausal osteoporosis the responses of IC and PTH to an oral calcium load are a function of calcium absorptive status. Key words: Osteoporosis -- Calcium absorption -- Calcium supplements -- Hydroxyproline -- Para- thyroid hormone. Although calcium supplementation is widely used in the treatment of osteoporosis, its value is fre- quently questioned [1, 2], mainly because the re- suits of prospective studies in normal postmeno- pausal women have been rather marginal [3, 4]. If calcium supplementation were to delay bone loss it Send reprint requests to Dr. Michael Horowitz, Michell Senior Research Fellow, Department of Medicine, Royal Adelaide Hos- pital, North Terrace, Adelaide, South Australia Australia 5000 would be expected to do so by inhibiting bone re- sorption. We have previously demonstrated, in un- treated osteoporotic postmenopausal women with normal calcium absorption, that both acute (8 days) [5] and chronic (6-12 weeks) [6] administration of a calcium supplement (1 g/day) results in a significant reduction in the fasting urinary hydroxyproline/cre- atinine ratio, indicating suppression of bone resorp- tion. However, in osteoporotic women with malab- sorption of calcium, administration of a calcium supplement for 6-12 weeks does not affect the fasting urinary hydroxyproline/creatinine ratio [7]. In this study, plasma ionized calcium and parathy- roid hormone concentrations have been measured in osteoporotic postmenopausal women with both normal and impaired calcium absorption at hourly intervals for 5 hours after an oral calcium load. Materials and Methods The studies were performed on 31 randomly selected postmeno- pausal women aged 58-79 years with varying degrees of osteo- porosis. Vertebral compression fractures were present in 29 pa- tients and the two other patients had low vertebral densities measured by C-T scanning [8] and previous peripheral bone fractures. No patient had clinical evidence of a fracture in the past 6 months or had had previous treatment for osteoporosis. The patients were admitted to a metabolic ward for the studies. After a 12 hour overnight fast, a venous blood sample was taken at 8 AM for measurement of plasma parathyroid hormone (PTH) and ionized calcium (IC), immediately followed by the adminis- tration of a 1 g calcium supplement (Sandocal-Sandoz). This supplement contained 5.23 g of calcium lactate gluconate and 0.8 g of calcium carbonate in an effervescent tablet. Further blood samples were then taken at 9, 10, and 11 AM, at noon, and at 1 PM. Plasma IC (normal range 1.17-1.27 retool/liter) was mea- sured with a radiometer ICA, (ionized calcium analyser). PTH was measured with a midregion PTH specific radioimmunoassay [9] (INC, Stillwater, MN) (normal range <75 pmol/liter).