Population advice on salt restriction: the social issues.

The scientific evidence that underlies public health advice depends upon critical integration of information from several sources. The most informative evidence relating to the effects of population reduction in salt intake comes from systematic reviews of clinical trials. Recent rigorous reviews of salt restriction trials in normal subjects show extremely small effects ranging from 1 to 2 mm Hg for systolic blood pressure and 0.1 to 1.0 mm Hg for diastolic pressure. These are the result of much greater reductions in sodium intake than can be achieved by population advice, and may be further amplified by publication bias and effects of changes in other dietary components. There is little trial evidence to enable possible benefits and adverse effects to be balanced. Reviews biased by the inclusion of nonrandomized studies exaggerate the apparent blood pressure fall 5- to 50-fold and create spurious apparent progressive falls in blood pressure. Nevertheless, citation analysis shows that they are quoted much more frequently than rigorous reviews reaching more negative conclusions. This appears to be the result of an attempt to create an impression of scientific consensus. The salt debate has important implications for social policy.