Abstract

We appreciate the opportunity to respond to the points that Dr. Daniels made in his thoughtful letter. The 95% confidence interval for the main effect included 0. A narrower confidence interval would have provided more assurance that the point estimate (a -1.8 mm Hg difference in systolic blood pressure change between intervention and placebo groups) was close to the true magnitude of effect. This point estimate, however, remains the best estimate of the true effect. Because the upper limit of the confidence interval was 0.3, values exceeding 0 (i.e., showing a harmful rather than protective effect) were quite unlikely. Nonetheless, we agree that a study with a larger number of subjects would be helpful to address the question with more precision. In our article, we strived to convey the message that our results were suggestive but not conclusive. We too were intrigued by the trajectory of blood pressure levels in the placebo group during the course of the study. As Dr. Daniels notes, an increase of almost 3 mm Hg during a 12-week period cannot be a function of increasing age, especially when a decrease follows during the ensuing 6 weeks. Our best explanation, as we point out in the Discussion, is a seasonal effect. Among children of the ages of our subjects, we would expect systolic blood pressure to increase about 2 mm Hg per year, averaged over all four seasons.1Task Force on Blood Pressure Control in Children Report of the second task force on blood pressure control in children-1987.Pediatrics. 1987; 79: 1-25PubMed Google Scholar Within a particular year, however, there may be fluctuations from season to season. We followed these children's blood pressure levels for at most 5 months, so we could not directly estimate age-related increases in average blood pressure level in this study. We interpreted the fact that postintervention blood pressures were almost identical in treatment and placebo groups to mean that the effect of the calcium supplementation was no longer present 6 weeks after the end of the intervention period. Finally, the effect of calcium supplementation appeared strongest in children with habitually low intakes of dietary calcium. Although the stratum-specific estimates had wide confidence intervals, the trend of intervention effect over quartiles of baseline calcium intake was graded and strong. This result raises the intriguing possibility that achieving adequate calcium intake for blood pressure control is most important among children with low calcium intakes. Further studies with adequate numbers of such children will be necessary to address this question. 9/35/70189

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