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Reply to RP Heaney

USDA/ARS Children's Nutrition Research Center
Department of Pediatrics
Baylor College of Medicine and Texas Children's Hospital
Houston, TX 77030

Tom D Thacher

Department of Family Medicine
Jos University Teaching Hospital
PMB 2076
Jos Nigeria

Philip R Fischer

Department of Pediatric and Adolescent Medicine
Mayo Clinic
299 First Street, SW
Rochester, MN 55905
E-mail: fischer.phil{at}mayo.edu

Mariaelisa Graff

Division of Foods and Nutrition
Emory University
771-2 Houston Mill Road
Atlanta, GA 30329

Diane D Stadler

Division of Endocrinology, Diabetes, and Clinical Nutrition
Department of Medicine
Oregon Health Sciences University School of Medicine
Portland, OR 97201

Sunday D Pam

Department of Paediatrics
University of Jos
PMB 2084
Jos Nigeria

John M Pettifor

MRC Mineral Metabolism Research Unit
Department of Paediatrics
University of the Witwatersrand and Chris Hani Baragwanath Hospital
PO Bertsham 2013
South Africa

Dear Sir:

The letter from Heaney raises one of the major issues concerning the methodology for measuring intestinal calcium absorption. We recognize that there are no perfect methods for evaluating usual dietary absorption of calcium or any isolated nutrient within the context of whole diets as typically consumed. Balance methods often involve the use of nonrepresentative diets and do not isolate nutrient sources. Furthermore, tracer methods can only trace, as Heaney notes, certain aspects of the diet. Therefore, such studies must evaluate either the bioavailability of calcium from a particular food, which requires either intrinsic labeling or assurance of full distribution of the extrinsic label, or a component of absorptive capacity by providing the label as part of a meal, which may not completely mix with all of the components. The former method does not necessarily assess whole dietary absorption and is most suitable for supplement and fortification studies, such as those referenced by Heaney. The second method, although it may overestimate food-based absorption, provides substantial information regarding absorptive capacity.

In our study, a large amount of phytates in the diet, which are known inhibitors of calcium absorption, might have been expected to inhibit the absorption of calcium from the test meal. In fact, this did not occur in children with or without rickets, undoubtedly because of the very low calcium content of the test meals. At low calcium intakes, vitamin D-dependent active transport is most critical for calcium absorption (1). It is reasonable to conclude from our data that there is no evidence that Nigerian children with rickets have a fundamental inability to absorb calcium; thus, vitamin D deficiency is excluded as a factor in the causation of the disease. Our results are also consistent with those of early calcium balance studies conducted in South African children with dietary calcium-deficiency rickets (2). After nutritional rehabilitation, calcium absorption increased to a very high level in children who previously had rickets. This finding is inconsistent with Heaney's hypothesis that the tracer was absorbed independent of nutritional status and suggests that children with rickets may appropriately respond to overall nutritional rehabilitation with bone growth and remineralization.

It should be noted that we recently found fractional calcium absorption to be similar in rachitic children and in pubertal children with low calcium intakes (3), in whom calcium was traced from milk (in which there has been shown to be appropriate interchange between the tracer and natural calcium). Although the children in that study and in another study involving pregnancy (4) and very high absorption levels were in physiologic states of high absorption, these data indicate that a direct comparison of our values with those of adults may be erroneous and that the absorption values we found are probably not spuriously elevated.

ACKNOWLEDGMENTS

None of the authors had a conflict of interest.

REFERENCES

1. Bronner F, Pansu D. Nutritional aspects of calcium absorption. J Nutr 1999;129:9–12.[Abstract/Free Full Text]
2. Pettifor JM, Ross P, Wang J, Moodley G, Couper-Smith J. Rickets in children of rural origin in South Africa: is low dietary calcium a factor? J Pediatr 1978;92:320–4.[Medline]
3. Abrams SA, Griffin IJ, Hicks PD, Gunn SK. Pubertal girls only partially adapt to low dietary calcium intakes. J Bone Miner Res 2004;19:759–63.[Medline]
4. Vargas Zapata CL, Donangelo CM, Woodhouse LH, Abrams SA, Spencer M, King JC. Calcium homeostasis during pregnancy and lactation in Brazilian women with low calcium intakes: a longitudinal study. Am J Clin Nutr 2004;80:417–22.[Abstract/Free Full Text]

This Article Full Text (PDF) Purchase Article View Shopping Cart Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Abrams, S. A Articles by Pettifor, J. M Search for Related Content PubMed Articles by Abrams, S. A Articles by Pettifor, J. M Agricola Articles by Abrams, S. A Articles by Pettifor, J. M