Recent salt research leads to misconceptions

A recent Cochrane systematic review/meta-analysis* by Dr. Rod Taylor and colleagues, "Reduced dietary salt for the prevention of cardiovascular disease", demonstrated mild, long-term (up to 3 years) blood pressure reduction (1 mmHg for normotensives; 4 mmHg for hypertensives) with advice or use of a "high potassium salt" compared with usual salt, yet a significant reduction in all-cause mortality was not seen. But sodium reductions in the food supply is one of the World Health Organizations "best buys" to improve population health in lead-up to the September 2011 United Nations High-Level Meeting on non-communicable diseases (NCDs). What explains this apparent discordance?

First, the review primarily included trials that evaluated the effects of advice on prevention of cardiovascular disease; only one of the seven trials evaluated the effect of a "high-potassium salt" compared with usual salt. But since more than 70% of sodium consumed in the US and other high-income countries is passive (and as an inexpensive flavor enhancer, sodium drives thirst for the beverage industry), individual-level efforts to actively reduce salt are often thwarted, particularly over long periods of time. In addition, the time spent providing advice went beyond the usual clinical encounter, yet still failed to demonstrate a significant reduction in cardiovascular disease events. The modest result of such a high-intensity intervention on blood pressure suggests that this approach would not be readily scalable.

Second, while Taylor's study did not demonstrate any difference in overall mortality (RR 0.67 [0.40, 1.12] for normotensives; RR 0.97 [0.83, 1.13] for hypertensives) nor cardiovascular morbidity (RR 0.71 [0.42, 1.20] for normotensives; RR 0.84 [0.51, 1.24] for hypertensives), both estimates were trending toward lower event rates, yet each estimate had wide confidence intervals. This lack of a significant difference may be due to the small sample sizes seen in the studies (5,808 individuals included in mortality analyses; 3,225 included in cardiovascular morbidity analyses) and the presumably modest, sustained decreases in sodium intake over the entire follow-up period. Sustained, population-level reductions in sodium may have similar or even greater effect sizes with tighter confidence intervals, leading to fewer cardiovascular events, including death.

Third, as expected, individuals with elevated blood pressure had the greatest reductions in blood pressure with sodium reduction. While some individuals do have greater blood pressure responses to sodium than others, there is no routine clinical or public health screening method that is practicable. Nor do such differences suggest that some salt-insensitive individuals will simply be spared from elevated blood pressure. Unfortunately, data from the Framingham Heart Study suggest that the lifetime risk for hypertension is >90%. In a sodium-soaked environment, we can all become salt sensitive.

Fourth, according to co-author Dr. Shah Ebrahim, none of the large, long-term studies evaluated other dietary constituents, which also impact blood pressure, as demonstrated in the landmark Dietary Approaches to Stop Hypertension (DASH) trial. In the DASH trial, researchers found that a Mediterranean-style diet led to a short-term reduction in blood pressure compared with an American-style diet, even with the same sodium content. Lower sodium diets in both groups led to lower blood pressures, and the lowest blood pressure group had a low-salt, Mediterranean diet, suggesting that other nutrients also play a role in moderating blood pressure. Most public health experts advocate for greater availability of fresh fruits and vegetables, legumes, nuts, whole grains, and low-fat dairy. While these long-term changes are being realized, rapid, substantial, and sustained reduction of sodium in the food supply remains a critical, widely recommended option to reduce blood pressure and prevent cardiovascular disease, based on the totality of evidence.

While Taylor and colleagues' article has stirred controversy and confusion, their findings that advice, while perhaps necessary, is insufficient to tackle a public health problem as pervasive as sodium should not come as much of a surprise. We agree with Scientific American's recent headline, "It's Time to Stop the Salt Wars" and look forward to substantial reductions of sodium in the food supply that result from subsequent treaties. To revert to such stale debates as we count down to the UN High-Level Meeting would be a failed opportunity with the world watching.

Mark D. Huffman Northwestern University Feinberg School of Medicine Chicago, IL

Sandeep P. Kishore Weill Cornell / The Rockefeller University / Sloan-Kettering Institute New York, NY

Sanjay Basu University of California San Francisco San Francisco, CA

Matthew R. Price Harvard School of Public Health Boston, MA

Aria I. Ahmad University of Toronto Toronto, Ontario

All of the authors are members of the Young Professionals' Chronic Disease Network (http://www.ypchronic.org), whose vision is to "battle the neglected epidemic of global NCDs by harnessing the talents and energies of the next generation of health leaders to collaborate with established stakeholders in the field."

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*Editor's note:

"Reduced dietary salt for the prevention of Cardiovascular Disease"
American Journal of Hypertension 2011; 24:843-853
http://bit.ly/p981ts

The review on the Cochrane Collaboration's website: http://bit.ly/qAyEV8

A Reuters article discussing the review: http://reut.rs/qwCPRB

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