Comments on a meta-analysis of the relation between dietary calcium intake and blood pressure

The role of dietary calcium in the etiology of hypertension is controversial. In 1995, Cappuccio et al. (American Journal of Epidemiology,1995;142:935-45) examined this issue in a meta-analysis of observational studies published between 1983 and 1993. The author of the present paper reviewed the original studies underlying this meta-analysis and discovered that data from one study had been inappropriately extracted and converted, leading to an understatement of the calcium-blood pressure relation by a factor of about 30. This review also raised questions about the extraction and conversion of data from several other studies and about the statistical methods used. The author repeated the meta-analyses and discovered an unadjusted regression slope between dietary calcium and systolic blood pressure of -0.34 mmHg/100 mg per day (95% confidence interval (CI) -0.46 to -0.22) for men, -0.15 mmHg/100 mg per day (95% CI -0.19 to -0.11) for women, and -0.39 mmHg/100 mg per day (95% CI -0.47 to -0.31) for men and women. For diastolic blood pressure, the pooled regression slope for men was -0.22 mmHg/100 mg per day (95% CI -0.32 to -0.13), while for women it was -0.051 mmHg/100 mg per day (95% CI -0.090 to -0.012); for men and women it was -0.35 mmHg/100 mg per day (95% CI -0.67 to -0.02). These slopes are still modest but are larger than those reported in the original analysis. However, since all of these analyses were based on zero-order correlations or regressions, extreme caution must be exercised in interpreting the results.