Massive publicity accompanied (even preceded) British Medical Journal publication on April 20 of the fifteenth observational study of health outcomes of lower salt diets. A team of experts led and funded by the National Heart, Lung and Blood Institute, all prominent in their previous advocacy of salt-restricted diets, reported on "Long terms effects of dietary sodium reduction on cardiovascular disease outcomes." The researchers used data from the Trials of Hypertension Prevention whose outcome was blood pressure, not cardiovascular risk, but they compared the groups that were on the low-sodium diets with the control group -- adjusting for "demographic information" and sodium excretion -- and concluded that low-salt diets confer a 25% benefit in reduced heart attacks. This was big news and received massive publicity -- more than the earlier 14 studies of the same question -- do lower-sodium diets reduce the risk of cardiovascular events and mortality? None of the earlier studies had found improved health outcomes in populations similar to Americans. Three found additional risks associated with low-salt diets.
The findings muddy the waters of the underlying question, but, paradoxically clarify the course of action. The record is clear -- observational studies are divided confirming the Salt Institute's long-standing conclusion that we need a controlled trial of the question to resolve whether public policy should advocate salt reduction. Moreover, by choosing the TOHP study, these anti-salt researchers embrace the very study design that the Salt Institute has advocated to the US Department of Health and Human Services. SI has suggested the TOHP model for the controlled trial.
Critics of the new study have observed that the findings depend entirely on five deaths among the 3,000-plus subjects and that the reduction in cardiovascular events failed to reach significance when revascularization (surgical intervention) procedures were excluded, and the change in total mortality failed to reach significance also. These are serious criticisms, but no observational study can hope to resolve these matters. Let's not get bogged down on the methods or conclusions of this single study.
The significance of this study is not its authors' conclusion that lower-salt diets will reduce cardviovascular events, but, rather, that it represents a powerful argument for a controlled trial of the question. We cannot make policy on observational studies nor on the latest single study. While the first 14 "health outcomes" studies offers no support for salt reduction in a US-type population, this study virtually demands that the question be resolved to prevent the public being confused by "dueling studies."
