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INSERM Unit 507 and Nephrology Division
Necker Hospital
161 Rue de Sevres
F-75743 Paris
France
E-mail: drueke{at}necker.fr
Biochemistry Laboratory A
Necker Hospital
161 Rue de Sevres
F-75743 Paris
France
Dear Sir:
In a recent issue of the Journal, Wigertz et al (1) studied body calcium retention in black and white girls in response to changes in salt consumption. The authors used a randomized crossover design and tested a constant calcium intake of 815 mg/d (20 mmol/d) and 2 different intakes of dietary sodium: 1.30 g/d (57 mmol/d; low-sodium diet) and 3.86 g/d (168 mmol/d; high-sodium diet). They found that body calcium retention was significantly greater in black than in white girls and that body calcium retention was lower with the high-sodium diet than with the low-sodium diet. They suggested that a decrease in net intestinal calcium absorption was the main mechanism involved in the negative effect of the high salt load.
There is general agreement that high-sodium diets are associated with increased urinary calcium excretion, relative to low salt consumption (2). The white girls had the expected increase in urinary calcium excretion in response to a high-sodium diet, in line with previous reports. However, surprisingly, the black girls showed no such increase. The effect of changes in salt intake on intestinal calcium absorption and skeletal calcium uptake and release remains a controversial issue. In the present study, net intestinal calcium absorption decreased by 11% in response to a high-sodium diet in the black girls and by 3% in the white girls when measured with the fecal sampling method. In contrast, Breslau et al (3) found a 26% increase in fractional calcium absorption with the use of the isotope method.
Because of these discrepant findings, we examined carefully the calcium balance data for the young females studied by Wigertz et al (1; see Table 2). Calcium retention has been calculated as the difference between net calcium absorption (difference between calcium ingestion and fecal output) and urinary calcium excretion. As expected, the high-sodium diet induced an increase in urinary calcium excretion in the white girls but not in the black girls. However, sodium excretion was significantly higher in the white girls than in the black girls. In contrast, net intestinal calcium absorption decreased in the black girls but remained relatively constant in the white girls. When we recalculated calcium retention, based on the mean values indicated in Table 2 and a fixed calcium intake of 815 mg/d, we were surprised to end up with substantially different values for 3 of the 4 values in the bottom row of this table. Thus, we calculated a mean calcium retention of 431 mg/24 h (instead of 453 mg/24 h, as shown in Table 2) and of 250 mg/24 h (instead of 235 mg/24 h, as shown in Table 2), respectively, in the black and white girls who consumed the low-sodium diet and of 380 mg/24 h (instead of 359 mg/24 h, as shown in Table 2) and of 190 mg/24 h (instead of 189 mg/24 h, as shown in Table 2), respectively, in the black and white girls who consumed the high-sodium diet. Thus, the recalculation indicated a considerably lower than claimed difference in calcium retention in response to changes in dietary salt intake in the black girls (51 mg/24 h instead of the indicated 94 mg/24 h) and a slightly higher than claimed difference in the white girls (60 mg/24 h instead of the indicated 46 mg/24 h).
Finally, the finding by Wigertz et al (1) of a baseline sweat calcium excretion of 54 and 51 mg/24 h in the black and white girls, respectively, which is roughly the same amount as that excreted in urine under low-sodium conditions (50 and 53 mg/24 h, respectively), was unexpected as well. Although even higher calcium losses in sweat, namely 103 mg/d, were previously reported by this group of authors in healthy adult women (4), such high losses—to the best of our knowledge—have not been found by others. If correct, we wonder why the authors did not take into account sweat calcium excretion when they estimated body calcium retention and possible changes in response to changes in dietary salt intake in the black and white girls in their study (1), especially because changes in sodium load may lead to changes in sweat calcium loss.
ACKNOWLEDGMENTS
The authors acknowledge research support by the Comité des Salines de France.
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