Intestinal calcium absorption assessed by a double-isotope method, decreased significantly with aging in 94 normal subjects (r = -0.22, P < 0.025). In 52 untreated patients with postmenopausal osteoporosis, calcium absorption was significantly lower than normal when either age or habitual calcium intake was used as a covariable (P < 0.001). Serum 25-hydroxyvitamin D (25-OH-D) and 1,25-dihydroxyvitamin D (1,25(OH)(2)D) were measured in 44 normal subjects and 27 osteoporotic patients. For all normals, calcium absorption and serum 1,25(OH)(2)D were positively correlated (r = 0.50, P < 0.001). In nonelderly normal subjects (ages 30-65 yr), dietary calcium intake correlated inversely with both calcium absorption (r = -0.39, P < 0.01) and with serum 1,25(OH)(2)D (r = -0.50, P < 0.01). Both osteoporotic patients and elderly normal subjects (ages 65-90 yr) differed from nonelderly normals in that these correlations were not present. In addition although serum 25-OH-D was normal, serum 1,25(OH)(2)D was significantly decreased in both osteoporotic patients and elderly normals (P < 0.001). In osteoporotic patients, calcium absorption increased significantly (P < 0.001) after 7 d administration of a small dose (0.4 mug/d) of synthetic 1,25(OH)(2)D(3). In osteoporotics mean serum immunoreactive parathyroid hormone was either normal (COOH-terminal assay) or low (NH(2)-terminal assay) relative to age-matched controls, and mean serum phosphate was increased. The data suggest that inadequate metabolism of 25-OH-D to 1,25(OH)(2)D contributes significantly to decreased calcium absorption and adaptation in both osteoporotics and elderly normal subjects. In patients with osteoporosis this abnormality could result from a decrease in factors that normally stimulate 1,25(OH)(2)D production, such as the decreased parathyroid hormone secretion and increased serum phosphate demonstrated in this group. In elderly subjects a primary abnormality in metabolism of 25-OH-D to 1,25(OH)(2)D, analagous to that seen in aging rats, cannot be excluded.

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Intestinal Calcium Absorption and Serum Vitamin D Metabolites in Normal Subjects and Osteoporotic Patients: EFFECT OF AGE AND DIETARY CALCIUM

Ca(2+) is an essential ion in all organisms, where it plays a crucial role in processes ranging from the formation and maintenance of the skeleton to the temporal and spatial regulation of neuronal function. The Ca(2+) balance is maintained by the concerted action of three organ systems, including the gastrointestinal tract, bone, and kidney. An adult ingests on average 1 g Ca(2+) daily from which 0.35 g is absorbed in the small intestine by a mechanism that is controlled primarily by the calciotropic hormones. To maintain the Ca(2+) balance, the kidney must excrete the same amount of Ca(2+) that the small intestine absorbs. This is accomplished by a combination of filtration of Ca(2+) across the glomeruli and subsequent reabsorption of the filtered Ca(2+) along the renal tubules. Bone turnover is a continuous process involving both resorption of existing bone and deposition of new bone. The above-mentioned Ca(2+) fluxes are stimulated by the synergistic actions of active vitamin D (1,25-dihydroxyvitamin D(3)) and parathyroid hormone. Until recently, the mechanism by which Ca(2+) enter the absorptive epithelia was unknown. A major breakthrough in completing the molecular details of these pathways was the identification of the epithelial Ca(2+) channel family consisting of two members: TRPV5 and TRPV6. Functional analysis indicated that these Ca(2+) channels constitute the rate-limiting step in Ca(2+)-transporting epithelia. They form the prime target for hormonal control of the active Ca(2+) flux from the intestinal lumen or urine space to the blood compartment. This review describes the characteristics of epithelial Ca(2+) transport in general and highlights in particular the distinctive features and the physiological relevance of the new epithelial Ca(2+) channels accumulating in a comprehensive model for epithelial Ca(2+) absorption.

https://www.physiology.org/doi/pdf/10.1152/physrev.00003.2004

Calcium Absorption Across Epithelia

Bone status and fracture rates in two regions of Yugoslavia.

With the aim of preventing postmenopausal bone loss, a placebo-controlled double-blind trial of 2 years duration was performed. We randomized 315 healthy volunteers in their early natural menopause to seven treatment and three placebo groups: 17 beta-oestradiol, oestriol and sequential norethisteron (hormones); bendroflumethiazide 5 mg/day (thiazide); hormones and thiazide; sodium fluoride 20 mg/day; vitamin D3 2000 IU/day (D3); fluoride and D3; and 1 alpha (OH) vitamin D3 0.25 microgram/day (1 alpha D3). All participants were given daily calcium supplement of 500 mg. Every 3 months we measured the bone mineral content (BMC) of both forearms by photon absorptiometry and chemical quantities in blood and 48 h urinary collections. The study was completed by 264 (84%). The combined placebo groups showed a linear fall in BMC reaching 3.3% after 2 years (P < 0.001). Hormones and hormones and thiazide led to a 2.5% gain in BMC (P < 0.01). Thiazide alone postponed the BMC fall for 6 months. After 2 years the thiazide group showed a BMC fall of 1.5% (P < 0.05), less than that of the placebo group (P < 0.05). BMC declined by 3.6%, 4.5%, 3.7% and 3.7% during the respective use of fluoride, D3, fluoride and D3 and 1 alpha D3. Nevertheless, the urinary calcium excretion during 1 alpha D3 and D3 treatment was 1--1.5 mmol/day higher than in the placebo groups. Apparently, there is no real alternative to oestrogen/gestagen in the prevention of postmenopausal osteoporosis.

Prevention of early postmenopausal bone loss: controlled 2-year study in 315 normal females.

Sixty postmenopausal women were placed in three groups--control, sex hormone-treated, and CaCO3-treated--and followed for 2 years. Skeletal mass decreased by 1.18%/year in the control group, 0.15%/year in the hormone group, and 0.22%/year in the CaCO3 group by radiogrammetry; and 2.88%/year in the control group, 0.73%/year in the hormone group, and 1.83%/year in the CaCO3 group by photon absorptiometry. The treatment groups differed significantly from the control group except for photon absorptiometry in the CaCO3 group. Bone accretion and resorption decreased in the treatment groups as measured by calcium tracer kinetics, resorption more so than accretion. We conclude that [1] these techniques are sufficiently sensitive to detect age-related bone loss; [2] postmenopausal sex-hormone replacement measurably decreases age-related bone loss by suppressing bone turnover, resorption more than accretion; and [3] calcium supplements produce the same effect but at the dose we used were slightly less effective.

B E C Nordin

Papers

Spinal osteoporosis and the menopause.

Some effects of ethinyl oestradiol on calcium and phosphorus metabolism in osteoporosis.