Salt and Blood Pressure: Conventional Wisdom Reconsidered
The "salt hypothesis" is that higher levels of salt in the diet lead to higher levels of blood pressure, with attendant risk of cardiovascular disease. Intersalt was designed to test the hypothesis, with a cross-sectional study of salt levels and blood pressures in 52 populations. The study is often cited to support the salt hypothesis, but the data are somewhat contradictory. Thus, four of the populations (Kenya, Papua, and two Indian tribes in Brazil) have very low levels of salt and blood pressure. Across the other 48 populations, however, blood pressures go down as salt levels go up-- contradicting the salt hypothesis. Regressions of blood pressure on age indicate that for young people, blood pressure is inversely related to salt intake-- another paradox.
This paper discusses the Intersalt data and study design, looking at some of the statistical issues and identifying respects in which the study failed to follow its own protocol. Also considered are human experiments bearing on the salt hypothesis. The effect of salt reduction is stronger for hypertensive subjects than normotensives. Even the effect of a large reduction in salt intake on blood pressure is modest, and publication bias is a concern.
To determine the health effects of salt reduction, a long-term intervention study would be needed, with endpoints defined in terms of morbidity and mortality; dietary interventions seem more promising. Funding agencies and medical journals have taken a stronger position favoring the salt hypothesis than is warranted by the evidence, raising questions about the interaction between the policy process and science.