Observational studies of salt and blood pressure.

The observational data relating salt and blood pressure (excluding INTERSALT) are reviewed. Important methodological difficulties and biases are inherent to both across- and within-population studies and confuse their interpretation. Across-population studies are positive but rely on data drawn from the international literature based on a variety of unstandardized field methods; they are prone to unmeasured (ecological) confounding. Within-population studies generally lack statistical power and are subject to major regression-dilution bias (because of considerable day-to-day variation in sodium intake), which could conceal true correlations between sodium and blood pressure. Nevertheless, an overview of reported studies that used 24-hour urine excretion to quantify intake shows positive and highly significant correlations between sodium and blood pressure for both men and women and for systolic and diastolic blood pressures. These results are consistent with the INTERSALT findings and those from trials of sodium restriction.