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Background: Variation in absorption efficiency explains more of the variability in calcium balance than does actual calcium intake. Several investigators have suggested that the relatively high phosphorus and protein intakes of the diets of industrialized nations reduce calcium absorption and thereby aggravate the problem of calcium deficiency. Objective: My objective was to test whether variation in phosphorus and protein intakes is associated with variation in calcium absorption. Design: One hundred ninety-one Roman Catholic nuns aged 48.7 ± 7.0 y were studied 3 times each over a >20-y period with a full metabolic balance regimen; controlled, chemically analyzed diets; and double-tracer measurement of calcium absorption. Results: Although the expected associations with absorption were found for age, calcium intake, and estrogen status, no association was observed for intakes of either phosphorus or protein. Conclusion: Phosphorus and protein intakes do not contribute to the wide variability in calcium absorption efficiency.

To achieve adequate dietary calcium intake, several choices are available that accommodate a variety of lifestyles and tastes. Liberal consumption of dairy products in the diet is the approach of most Americans. Some plants provide absorbable calcium, but the quantity of vegetables required to reach sufficient calcium intake make an exclusively plant-based diet impractical for most individuals unless fortified foods or supplements are included. Also, dietary constituents that decrease calcium retention, such as salt, protein, and caffeine, can be high in the vegetarian diet. Although it is possible to obtain calcium balance from a plant-based diet in a Western lifestyle, it may be more convenient to achieve calcium balance by increasing calcium consumption than by limiting other dietary factors.

Calcium balances performed on 519 individuals from birth to 30 years of age, derived from 34 published reports, were pooled by age group and the relationship between intake and balance for each group was compared at both extremes of the intake range. At all ages, from infancy through the young adult years, balance values tended to flatten and become constant at higher intakes, while at lower intakes balance was invariably highly correlated with intake. Threshold intakes and balances were estimated by fitting the data to a two-component, split, linear-regression model. The threshold values at which balance no longer rose with intake exceeded the US recommended dietary allowances for calcium for all the age groups concerned.

BACKGROUND Low intakes of calcium are associated with an increased risk of both osteoporosis and cardiovascular disease. OBJECTIVE To provide new estimates of the average calcium requirement for men and women, we determined the dietary calcium intake required to maintain neutral calcium balance. DESIGN Calcium balance data [calcium intake -(fecal calcium + urinary calcium)] were collected from 155 subjects [women: n = 73; weight: 77.1 +/- 18.5 kg; age: 47.0 +/- 18.5 y (range: 20-75 y); men: n = 82; weight: 76.6 +/- 12.5 kg; age: 28.2 +/- 7.7 y (range: 19-64 y)] who participated in 19 feeding studies conducted in a metabolic unit. Balance data from the final 6-12 d of each dietary period (minimum length:18 d) of each study (1-9 observations per subject) were analyzed. Data were excluded if individual intakes of magnesium, copper, iron, phosphorus, or zinc fell below the estimated average requirements or exceeded the 99 th percentile of usual intakes from the 1994 Continuing Survey of Food Intakes by Individuals (for iron, above the upper limit). Daily intakes of calcium ranged between 415 and 1740 mg. The relation between intake and output was examined by fitting random coefficient models. Coefficients were included to test for sex and age differences. RESULTS The models predicted a neutral calcium balance [defined as calcium output (Y) equal to calcium intake (C)] at intakes of 741 mg/d [95% prediction interval (PI): 507, 1035; Y = 148.29 + 0.80C], 9.4 mg kg body wt(-1) d(-1) [95% PI: 6.4, 12.9; Y = 1.44 + 0.85C], or 0.28 mg kcal(-1) d(-1) [95% PI: 0.19, 0.38; Y = 0.051 + 0.816C]. Neither age nor sex affected the estimates when calcium intakes were expressed as mg/d or as mg kg body wt(-1) d(-1). CONCLUSION The findings suggest that the calcium requirement for men and women is lower than previously estimated.

Background: Additional calcium is required during lactation, and several calcium regulatory factors are involved in calcium balance. In lactating rural women who have marginal nutrition and consume a high-fiber diet, negative calcium balance may be expected. Objective: We evaluated calcium balance and its association with potential calcium regulatory factors in lactating, rural Mexican women who had marginal nutrition and consumed a high-fiber diet. Design: This cross-sectional study included women at 1, 3, 6, and 12 mo of lactation (L1, L3, L6, and L12 groups) and women who had weaned their infants (W group). Age-matched, nonlactating women (NL group) were also included. Calcium balance and concentrations of calcium regulatory factors were determined. Correlation analysis was performed by using data from all of the lactating women. Results: Calcium balance in the L1, L3, and L6 groups was negative and was significantly different (P < 0.05) from that in the W and NL groups. Serum parathyroid hormone (PTH) and 1,25-dihydroxyvitamin D [1,25-(OH) 2 D] concentrations were significantly higher (P < 0.05) in the W group than in the L and NL groups. Calcium balance was positively associated with serum estradiol concentrations (r = 0.58, P < 0.05) and negatively associated with serum 1,25-(OH) 2 D concentrations (r = -0.52, P < 0.05). Breast-milk calcium concentrations correlated positively with serum PTH-related peptide (PTHrP) concentrations (r = 0.51, P < 0.05) and negatively with serum estradiol concentrations (r = -0.57, P < 0.05). Conclusions: Negative calcium balance was observed during lactation in rural Mexican women who consumed a high-fiber diet. Furthermore, the data suggest that the hormones estradiol and PTHrP are involved in the regulation of calcium balance and of the calcium content of milk during lactation.

The accepted practice ofcalculating the dietary calcium requirement from the relationship between calcium bal- ance and intake was recently criticized on mathematical grounds. To try to determine the appropriate calcium intake in patients with osteoporosis, I examined the relationships between calcium intake, output, and balance in a group of such patients. Calcium- balance studies were performed on 25 patients with untreated osteoporosis: intake(i± SEM) was 30.34 ± 3.02 mmok/d, output was 30.59 ± 2.69 mmol/d, and calcium balance was -0.25 ± 0.64 mmoh/d. There was a significant correlation between in- take and both output and balance (r = 0.98, P < 0.001 and r = 0.61, P < 0.01, respectively). Intake and output became equal at 31.7 (95% CI 27.9, 35.5) mmol/d. This value is the same as that obtained when the conventional method of calculation is used. Furthermore, if random values for calcium output are as- signed to the observed values of intake it can be demonstrated that the same values will always be obtained by both methods. The only difference is that the calculation based on balance and intake results in a narrower confidence interval. Am J Clin Nutr 1994;60:944-8.

We suggested that calcium may be an important determinant of peak bone mass. For further elucidation, calcium balances in adolescent females with different calcium intakes (270-1637 mg/d), and a 2-y intervention study of calcium supplementation were performed. Hereditary influences on bone status were also evaluated by comparing subjects' and parents' bone mass. The main determinant of calcium balance was calcium intake; net calcium absorption increased with intake and urinary calcium did not change. Adolescent females retained 200-500 mg Ca/d, suggesting that inadequate calcium intake may translate into inadequate calcium retention and a reduction in peak bone mass. There was a more pronounced increase in bone mass over time in the calcium-supplemented group (1640 mg Ca/d) than in the control group (750 mg Ca/d), but the differences between bone mass measurements were not statistically significant, possibly because of a type II error. By the age of 16 y daughters had accumulated 90-97% of the bone mass of their premenopausal mothers.

Calcium balance performance was evaluated in 130 normal perimenopausal women ages 35 to 50, studied on their usual, self-selected dietary calcium intakes. Two distinct balance methods were used, one based on customary intake and output measurements, and the other based on absorption and clearance of two simultaneously administered calcium isotopes. Both methods yielded essentially identical results. Under study conditions as closely approximating normal living as possible, these women averaged -24 and -31 mg Ca/day by the two balance methods (exclusive of unmeasured dermal losses). More significantly, there was a positive correlation between intake and balance, with women on higher self-selected intakes exhibiting more positive balance than women selecting lower intakes. The calcium intake predicted from our data which would be required to produce an average balance of zero was 1.241 g/day. This is significantly in excess of the current recommended dietary allowance for calcium. The implications and significance of this discrepancy are explored in detail.

Distribution of true fractional absorption of calcium in estrogen-deprived, nonos- teoporotic, middle-aged women was examined in 273 person-studies, using a double-tracer method. Fractional absorption averaged 0.266 (±0.096). Calcium intake averaged 0.802 g/day (±0.419) and urine calcium, 0.144 g/day (±0.062). Fractional absorption was inversely correlated with intake. Using parameters ofthe regression equation for the two variables, each absorption value was adjusted to the mean intake ofthe group so that absorption could be evaluated apart from the effect of intake. The distribution of such diet-adjusted values revealed that 55% of these estrogen-deprived women had insufficient absorption to maintain calcium balance at an intake equal to the 1980 RDA. Nearly one-fourth would still be in negative balance at an intake of 1.5 g/day. Since urine calcium was positively correlated with absorbed calcium (r = 0.521), we conclude that defective absorption and not bone breakdown is the basis for low absorptive performance in these women. Am J C/in Nutr l986;43:299-305.

Integumentary calcium loss was studied in 16 healthy young men. The daily loss by the 16 ambulatory but relatively sedentary young men in 52 determinations of 6-day periods each was 8.7 ± 1.9 mgJm2 per day (average 15.8 mg/man per day). The amount lost was not influenced by calcium intake (0. 1 to 2.3 g/day). In contrast to urinary calcium excretion, which is directly related to protein intake, there was no significant change in integumentary calcium loss with varying protein intakes ( I to 96 g nitrogen per day). No compensatory relationship between urinary and integumentary calcium excretion was noted. During strenuous exercise calcium loss increased to an average of 25 mg in 40 mm. There was no compensatory decrease in urinary excretion on the day of strenuous exercise. It was also noted that integumentary calcium loss was not affected by general calcium balance. Am. J. Clin. Nutr. 32: 1699-1702, 1979.

T HE HYPOTHESIS that osteoporosis is a disorder of the protein matrix of bone has resulted in a lack of interest in calcium balance tests as a means of studying the pathogenesis and therapy of this disorder. It is true that the work of Albright and his associates, which was reviewed in their monograph published in 1948, ‘ included many careful balance studies which demonstrated the effect of estrogens and androgens on calcium balance. It was implicit in their work, however, and explicitly stated by Moldawertm that feeding large doses of calcium per se would not produce the positive balance which could be induced by hormone therapy, and other investigators have been slow to challenge this assumption.