An estimated 26 million patients worldwide experience heart failure, a syndrome associated with major symptoms and a significant increase in mortality. Treatment involves a variety of drugs (ACE inhibitors, angiotensin receptor antagonists, etc.), and dietary interventions generally include salt restriction. However, the evidence that salt restriction is beneficial for individuals at risk of cardiovascular disease is inconsistent and inadequate. A 2018 systematic review surveyed randomized controlled trials (RCTs) testing the impact of salt intake reduction in adults with heart failure and found insufficient research and trial results to support this oft-prescribed intervention.
The review surveyed several bibliographical databases (such as the Cochrane Central Register of Controlled Trials, MEDLINE, Embase, and CINAHL) and considered the methodological quality of the studies, as well as the relevant outcomes. The authors found only nine trials meriting inclusion in the review, involving a total of 479 patients. Out of these nine studies, one was published as an abstract only, and no single study involved more than 100 patients. The reviewers found that none of the trials under consideration provided sufficient data on mortality outcomes (whether overall or pertaining to heart disease mortality, specifically) to assess whether salt restriction mitigates mortality. Six of the eight trials providing data on signs and symptoms of heart failure showed no difference between salt-restricted and control groups, while two trials favored the intervention. One trial reported a significant decrease in mean blood pressure, but two additional trials showed no significant difference between groups. None of the trials under consideration were thought to be at low risk for bias.
In a recent article in The New York Times, Aaron Carroll summarizes the findings accordingly:
Despite a number of studies questioning the usefulness of very low-salt diets in the last few years, most major medical organizations continue to recommend them. We would assume that they do so from a strong base of evidence. But with respect to heart failure, there is a shockingly small amount of evidence. … Recently, researchers searched for randomized controlled trials that evaluated the use of reduced sodium intake to treat heart failure. In all the literature, they found nine studies that involved 479 patients. … There were no data that showed that salt restriction reduced mortality or cardiac disease; affected whether someone was admitted to the hospital; or influenced how long they had to stay if admitted. …This is a minuscule amount of data on which to base strong recommendations.
The authors of the review conclude:
Despite broad advocacy, uncertainty remains about the robustness of advice to reduce salt intake in patients with heart failure. In this systematic review, we found limited evidence for modest improvement in clinical symptoms from reducing salt intake in outpatients. A paucity of robust high-quality evidence was available to support or refute current guidance. Well-designed, well-described and adequately powered studies are needed to reduce uncertainty about the use of this intervention.