Calcium supplementation does not augment bone gain in young women consuming diets moderately low in calcium.

In earlier observational work, the dietary calcium:protein ratio was directly related to bone accrual in healthy postadolescent women. In this study, we sought to test the hypothesis that augmented calcium intake would increase postadolescent skeletal consolidation, using a double-blind, randomized, placebo-controlled design. We recruited 152 healthy young women (age 23.1 +/- 2.7 y, BMI 22.5 +/- 3.0 kg/m2); their usual diets, as assessed by 7-d food diaries, were low in calcium (605 +/- 181 mg/d; 15.1 +/- 4.5 mmol/d) and in the calcium:protein ratio (10.1 +/- 2.0 mg/g). The subjects were randomly assigned to supplemental calcium [500 mg calcium (12.5 mmol) as the carbonate, 3 times/d, with meals] or placebo capsules identical in appearance; all participants also took a daily multivitamin, and they were followed for up to 36 mo with bone densitometry (dual energy X-ray absorptiometry; DXA) at 6-mo intervals. A total of 121 subjects remained in the study for at least 12 mo (median time in the study, 35 mo), with a mean compliance level (observed/expected tablet consumption) of 87.7%. DXA data for these 121 subjects indicated modest but significant mean rates of increase (i.e., 0.24 to 1.10%/y) in bone mineral content (BMC; total body, total hip, and lumbar spine) and in lumbar spine bone mineral density (BMD) but no change in total hip BMD. None of these rates of change differed by group, i.e., calcium supplementation did not have any measurable effect on bone mass accrual. By midstudy, the calcium content of the subjects' usual diets for both groups had risen by approximately 15%. The combined effect of improved intakes of dietary calcium and the small amount of calcium added by the multivitamin tablets resulted in a mean calcium intake for the control group > 800 mg (20 mmol)/d, possibly at or near the threshold beyond which additional calcium has no further effect on bone accrual.

Calcium absorptive efficiency is positively related to body size.

BACKGROUND: Calcium absorption efficiency is a more important determinant of calcium balance than calcium intake itself. The sources of variability in absorptive performance are only partly elucidated. PURPOSE: The aim of the study was to explore the relationship between body size and calcium absorption efficiency. DESIGN AND SETTING: Metabolic studies were performed on an inpatient metabolic unit in an academic health sciences center. SUBJECTS: One hundred seventy-eight women, with an average age of 50.2 yr, were studied from one to five times and yielded an aggregate data set containing 633 individual studies. METHODS: Calcium absorption fraction was measured by the dual-tracer method. Observed values were expressed as residuals from predicted values for each woman's actual calcium intake, using the previously published relationship between intake and absorption. RESULTS: Absorption residuals were significantly positively correlated with height, weight, and surface area, and after adjusting for estrogen status, these body size variables accounted for approximately 4% of the total variability. CONCLUSION: The magnitude of the effect is such that a woman 1.8 m in height would absorb 30+% more calcium from a given intake than a woman 1.4 m tall.

The elusive concept of bone quality.

This paper outlines information from recent publications that aid our understanding of bone quality in relation to osteoporosis. In practical terms, bone quality designates the properties of bone that contribute to strength but are not assessed by bone densitometry. While osteoporosis is still defined in terms of bone density, the limitations of this approach, long questioned, have become indisputable. In parallel, the results of treatment trials of antiresorptive agents demonstrate that bone density is a flawed surrogate for bone fragility and a weak indicator of antifracture efficacy. The case for emphasizing bone turnover in assessing fracture risk, has become increasingly strong, and a redefinition of osteoporosis on this basis may well occur. New technologies for studying bone microstructure and matrix composition, merging with sophisticated biomechanical assessments, are advancing our ideas regarding bone "damageability" and its effects over time.

Human serum 25-hydroxycholecalciferol response to extended oral dosing with cholecalciferol.

BACKGROUND: The cholecalciferol inputs required to achieve or maintain any given serum 25-hydroxycholecalciferol concentration are not known, particularly within ranges comparable to the probable physiologic supply of the vitamin. OBJECTIVES: The objectives were to establish the quantitative relation between steady state cholecalciferol input and the resulting serum 25-hydroxycholecalciferol concentration and to estimate the proportion of the daily requirement during winter that is met by cholecalciferol reserves in body tissue stores. DESIGN: Cholecalciferol was administered daily in controlled oral doses labeled at 0, 25, 125, and 250 micro g cholecalciferol for approximately 20 wk during the winter to 67 men living in Omaha (41.2 degrees N latitude). The time course of serum 25-hydroxycholecalciferol concentration was measured at intervals over the course of treatment. RESULTS: From a mean baseline value of 70.3 nmol/L, equilibrium concentrations of serum 25-hydroxycholecalciferol changed during the winter months in direct proportion to the dose, with a slope of approximately 0.70 nmol/L for each additional 1 micro g cholecalciferol input. The calculated oral input required to sustain the serum 25-hydroxycholecalciferol concentration present before the study (ie, in the autumn) was 12.5 micro g (500 IU)/d, whereas the total amount from all sources (supplement, food, tissue stores) needed to sustain the starting 25-hydroxycholecalciferol concentration was estimated at approximately 96 micro g (approximately 3800 IU)/d. By difference, the tissue stores provided approximately 78-82 micro g/d. CONCLUSIONS: Healthy men seem to use 3000-5000 IU cholecalciferol/d, apparently meeting > 80% of their winter cholecalciferol need with cutaneously synthesized accumulations from solar sources during the preceding summer months. Current recommended vitamin D inputs are inadequate to maintain serum 25-hydroxycholecalciferol concentration in the absence of substantial cutaneous production of vitamin D.

Bone microstructure in osteoporosis: transilial biopsy and histomorphometry.

The bone fragility of osteoporosis is not fully explained by a deficit in bone mass. Histomorphometric examination of transilial bone biopsies has identified microstructural defects that-in light of what is known about the mechanical properties of structural materials-further compromise bone strength. Histomorphometric measures describe the biopsy specimen, the configuration of its trabeculae in space, and the extent to which its trabecular lattice is intact. In postmenopausal women with established osteoporosis, a deficit of both cortical and cancellous bone is typical, i.e., both cortical thickness and cancellous bone volume tend to be substantially reduced. Much of the cancellous bone deficit can be attributed to loss of entire trabecular elements rather than to generalized thinning of trabeculae. Direct measures of trabecular connectivity confirm this impression: women with established osteoporosis have fewer trabecular nodes and more termini than healthy women, even at the same cancellous bone volume. Evidence for accumulated microdamage in transilial biopsies is circumstantial, and the phenomenon itself may well be localized to fracture sites. Histomorphometric data from transilial biopsies comprise a large body of information about the structural and functional character of osteoporosis and provide valuable information about the effects of new treatments on bone microstructure.

Effects of above average summer sun exposure on serum 25-hydroxyvitamin D and calcium absorption.

The purpose of this study was to examine the effects of summer sun exposure on serum 25-hydroxyvitamin D [25(OH)D], calcium absorption fraction, and urinary calcium excretion. Subjects were 30 healthy men who had just completed a summer season of extended outdoor activity (e.g. landscaping, construction work, farming, or recreation). Twenty-six subjects completed both visits: after summer sun exposure and again approximately 175 d later, after winter sun deprivation. We characterized each subject's sun exposure by locale, schedule, and usual attire. At both visits we measured serum 25(OH)D, fasting urinary calcium to creatinine ratio, and calcium absorption fraction. Median serum 25(OH)D decreased from 122 nmol/liter in late summer to 74 nmol/liter in late winter. The median seasonal difference of 49 nmol/liter (interquartile range, 29-67) was highly significant (P < 0.0001). However, we found only a trivial, nonsignificant seasonal difference in calcium absorption fraction and no change in fasting urinary calcium to creatinine ratio. Findings from earlier work indicate that our subjects' sun exposure was equivalent in 25(OH)D production to extended oral dosing with 70 micro g/d vitamin D(3) (interquartile range, 41-96) or, equivalently, 2800 IU/d (interquartile range, 1640-3840). Despite this input, at the late winter visit, 25(OH)D was less than 50 nmol/liter in 3 subjects and less than 75 nmol/liter in 15 subjects.

Calcium and weight: clinical studies.

Data from six observational studies and three controlled trials in which calcium intake was the independent variable (and either bone mass or blood pressure the original outcome variable) have been reanalyzed to evaluate the effect of calcium intake on body weight and body fat. Analysis reveals a consistent effect of higher calcium intakes, expressed as lower body fat and/or body weight, and reduced weight gain at midlife. Similarly, studies relating nutrient intake to body composition report negative associations between calcium intake and body weight at midlife and between calcium and body fat accumulation during childhood. There is a fairly consistent effect size, with each 300 mg increment in regular calcium intake associated with approximately 1 kg less body fat in children and 2.5-3.0 kg lower body weight in adults. Taken together these data suggest that increasing calcium intake by the equivalent of two dairy servings per day could reduce the risk of overweight substantially, perhaps by as much as 70 percent.