Our meta-analysis of sodium restriction trials in normotensive subjects1 predicted the termination blood pressure results of the Trials of Hypertension Prevention, Phase II (TOHP II),2 remarkably well, thus providing external validity for our estimates of sodium intervention effects. The TOHP II also provided an exceedingly wide range of effects of dietary sodium restriction on blood pressure, ranging from 1.2 to 2.9 mm Hg for systolic blood pressure and from 0.6 to 1.6 mm Hg for diastolic blood pressure. This extraordinary finding was not adequately explained in the report and did not appear to be related to poor dietary adherence, as there was only minor recidivism in the magnitude of the reduction in daily intake of sodium over the course of the study.
An inherent difficulty in clinical trials like the TOHP II, in which blinding of the intervention is impossible to achieve, is controlling for extraneous intervening factors
Logan AG, Greenwood CMT. Salt, Blood Pressure, and Cointervention. Arch Intern Med. 1997;157(22):2666-2667. doi:10.1001/archinte.1997.00440430148026