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LETTER TO THE EDITOR |
US Department of Agriculture
Agricultural Research Service
Grand Forks Human Nutrition Research Center
PO Box 9034
Grand Forks, ND 58202-9034
E-mail: curtiss.hunt{at}ars.usda.gov
Dear Sir:
Heaney questioned our use of a "balance"-based approach instead of a "maximal calcium retention"–based approach to estimate the calcium requirements for American adults aged 19–75 y (1). The maximal retention approach was not used by the Food and Nutrition Board (FNB) (2) for adults in their final report on the grounds that the proposed statistical model could not include 100% maximal retention, regardless of calcium intake. In addition, some of the estimates obtained with the use of this method are not biologically justifiable. In their final report, the FNB (2) used classic metabolic studies of calcium balance to obtain data on the relation between calcium intake and retention, from which a regression model was developed. From this relation, they derived an intake of calcium that would be adequate to attain a predetermined desirable calcium retention.
We believe that our balance-based approach expands and refines the approach used by the FNB in their final report. Our data, which covered a broad range of intakes, did not exhibit nonlinearity and, therefore, did not justify the use of a more complex nonlinear model such as the one used by the FNB. The FNB model was based on data either combined from multiple laboratories (for persons aged 18–30 y) or limited to 6 discrete intakes (for persons aged 
31 y). The FNB chose neutral calcium balance as desirable calcium retention for adults aged 31 y; we applied this criterion for all adults, which is consistent with our observation that calcium balance was unaffected by age in adults aged 19–75 y. For all age groups, the FNB made an adjustment for assumed significant whole-body surface calcium losses. Our data on whole-body surface mineral loss indicate that calcium surface losses are negligible.
Our data are consistent with the earlier observations by Heaney (3) that calcium has a threshold value for retention in bone such that further increases in calcium intake are simply excreted. Recently, Heaney (4) concluded from calcium balance studies with premenopausal women that a calcium intake in the range of 800–1000 mg/d is adequate to support bone health in mature women. Our values for a presumptive Estimated Average Requirement (741 mg/d) and Recommended Dietary Allowance (1035 mg/d) encompass that range.
The zone of safety between calcium balance and calcium toxicity may be relatively narrow. Our data also show a slow but inexorable increase in positive calcium balance with increasing calcium intake above the point of neutral balance. A new exhaustive meta-analysis of prospective cohort studies and randomized controlled trials (5) shows an increase in hip fracture risk with calcium supplementation. Furthermore, in a recent, large, 7-y study with 36 282 healthy postmenopausal women, supplementation with 1000 mg Ca/d and 400 IU vitamin D/d significantly increased the risk of kidney stones and did not significantly reduce hip fracture rates (6). Also, in a separate study with women with no history of kidney stones, intake of supplemental calcium was associated with an increased risk of kidney stones after adjustment for age (7). For renal stone patients, a regression equation predicted hypercalciuria when calcium intakes reach 1685 and 866 mg for men and women, respectively (2). Because there is mounting evidence that calcium supplementation may be detrimental to bone and kidney health, and there is a clear indication that menopausal bone loss cannot be substantially influenced by diet (4), we suggest a reevaluation of the rationale for advocating calcium intakes higher than those needed to ensure balance.
ACKNOWLEDGMENTS
Neither author had any financial or personal conflict of interest.
REFERENCES
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