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LETTER TO THE EDITOR |
Creighton University
601 North 30th Street
Suite 4841
Omaha, NE 68131
E-mail: rheaney{at}creighton.edu
Dear Sir:
In their article on calcium absorption in Nigerian children with rickets, Graff et al (1) report no difference in absorptive efficiency between children with and without rickets, despite a substantial difference in serum 25-hydroxyvitamin D concentrations [25(OH)D]. Both groups had fractional absorption values that the authors judged to be higher than would have been predicted for their age or for the calcium loads used to test absorption. They also noted that the measured absorption fraction did not correlate with dietary calcium, serum 25(OH)D, or serum 1,25-dihydroxyvitamin D concentrations. These latter inconsistencies are not surprising in themselves, because other studies have produced similar failures. However, the finding of no difference in absorptive efficiency leaves the authors with no other explanation for the rickets than the low absolute calcium content of the diets of these children. This does not, however, explain why one group with low calcium intakes had rickets and the other, with equally low calcium intakes, did not.
There is another, more likely, methodologic explanation for their finding of equivalent absorption in the 2 groups. Although the authors used the gold standard double-tracer method to measure the absorption fraction, their description of the labeling of the breakfast meal suggests that the food calcium was not well labeled with the extrinsically added tracer. Several studies have shown that extrinsic labeling of food calcium sources is usually incomplete (2-5), which leads to absorption values that are always spuriously high and often nutritionally misleading or uninterpretable. The reason is that the tracer is added in microgram amounts in a solubilized form, and as such would likely be readily absorbed, even in persons with limited absorptive capacity. In all such applications of the tracer methods, it is essential to show that the oral tracer is uniformly distributed through all of the moieties of the calcium source used to test absorption. I suggest that this did not occur in the study by Graff et al, and that as a result we actually know very little about absorptive efficiency in these Nigerian children (healthy or rachitic).
ACKNOWLEDGMENTS
There was no conflict of interest.
REFERENCES
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