Dietary salt and blood pressure.

We have previously shown that the association between salt intake and blood pressure is large and of substantial public health importance. Analysis of the average blood pressure and average sodium intake in different communities yielded estimates of the decrease in blood pressure for a given reduction in dietary sodium according to age and blood pressure centile that successfully predicted both the association between sodium and blood pressure in observational studies and the reduction in blood pressure obtained in trials of dietary salt reduction. Here we consider the validity of criticisms of the appraisal. The fact that the observed blood pressure reductions in individual trials closely mirrored predicted values for individuals trials that varied widely (from 2.6 to 25.2 mmHg systolic according to age, existing blood pressure and the extent of salt restriction) renders bias implausible since it would be too great a coincidence for a source of error to reproduce such close matching. If the analysis is restricted to trials that avoided possible bias because (1) the high- and low-salt diets were otherwise identical, (2) the order of the diet avoided regression to the mean or (3) the trials were double-blind with diets that were otherwise identical and the order was random, the matching between observed and predicted blood pressure reductions is equally close. Our results are consistent with the Intersalt study. A reduction in dietary salt could be a simple change, yet the reduction in mortality would be large.