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Reply to TB Drüeke and B Lacour

Department of Foods and Nutrition Purdue University West Lafayette, IN 47907-2059
E-mail weavercm{at}purdue.edu

Dear Sir:

We thank Drüeke and Lacour for the opportunity to detail our analysis and expand on our methods. For each girl in our study, a daily diet composite was analyzed for calcium for almost all of the 42 d of the balance study. The mean (±SD) daily calcium intake was 815 ± 98 mg for the 78 analyzed diet composites. However, this mean calcium intake was not used in any of our calculations. For each girl, daily calcium absorption and retention were calculated on the basis of the analyzed daily calcium intake and fecal and urinary calcium excretion. Any food and beverages that were not consumed were analyzed, and the calcium from these foods was subtracted from the intake for that day for that subject. Daily calcium balances were calculated for each girl for the 42 d of the balance study. The range of daily calcium intakes averaged over 42 d for each girl was 729–862 mg/d. The same technique was used to calculate daily sodium excretion. This explains the discrepancies between Drüeke and Lacour's estimates of calcium absorption and retention and sodium excretion based on the mean values for the group and not on the daily values for the individual, which are provided in Table 2 of our article.

Drüeke and Lacour question why the 24-h dermal losses of calcium were not subtracted from the net retention values. Sweat calcium losses were measured on only one of the last days of each balance period. Because there were no significant effects of dietary salt intake or race on sweat calcium, as previously reported from this same study (1), we decided not to subtract the calcium sweat losses from a single 24-h period.

Although Drüeke and Lacour consider the 24-h sweat calcium losses of 51 and 54 mg/d to be high, we know of no studies other than our own that have estimated whole-body daily sweat calcium in adolescents. Estimation of sweat calcium in adults has been reported to vary from 0 to 149 mg/d under minimal sweating conditions (2). Using what is considered to be a more accurate method—whole-body ⁴⁷Ca retention minus urinary and fecal ⁴⁷Ca losses—Charles et al (3) calculated an average difference (assumed to be sweat calcium) of 63 mg/d in adults. The use of this method resulted in dermal calcium losses that were comparable with those we reported for adolescents; thus, we do not believe that our values are high.

ACKNOWLEDGMENTS

None of the authors had a personal or financial conflict of interest.

REFERENCES

1. Palacios C, Wigertz K, Martin BR, et al. Sodium retention in black and white female adolescents in response to salt intake. J Clin Endocrinol Metab 2004;84:1858–63.
2. Chu J-Y, Margen S, Calloway DH, Costa FM. Integumentary loss of calcium. Am J Clin Nutr 1979;32:1699–702.[Abstract/Free Full Text]
3. Charles P, Jensen FT, Mosekilde L, Hansen HH. Calcium metabolism evaluated by Ca-47 kinetics. Estimation of dermal calcium loss. Clin Sci 1983;65:415–22.

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