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The cardiovascular benefits of salt restriction remain unproven on the basis of currently available evidence, authors of a systematic review concluded.

The accumulation of clinical-trial data on 6,500 participants failed to produce a statistically significant outcome for hypertensive or normotensive individuals with respect to overall mortality or cardiovascular morbidity.

Moreover, salt restriction was associated with an increased mortality risk in patients with congestive heart failure (CHF), according to a report in the Cochrane Database of Systematic Reviews.

"There is still insufficient power to exclude clinically important effects of reduced dietary salt on mortality or cardiovascular morbidity in normotensive or hypertensive populations," Rod S. Taylor, PhD, of Peninsula College of Medicine and Dentistry in Exeter, England, and co-authors wrote in conclusion.

"Further randomized clinical trial evidence is needed to confirm whether restriction of sodium is harmful for people with heart failure. Our estimates of benefits from dietary salt restriction are consistent with the predicted small effects on clinical events attributable to the small blood pressure reduction achieved."

The report was published online simultaneously in the American Journal of Hypertension. Michael Alderman, MD, who serves as editor of that journal has a relationship with the Salt Institute.

Observational studies have identified high dietary salt intake as a risk factor for cardiovascular disease, and most developed nations have public health recommendations that include advice to reduce salt intake by about 50%.

The results came as no surprise to Salim Yusuf, DPhil, of McMaster University in Hamilton, Ontario.

"I had long been concerned that the bold and strident public health recommendations of trying to reduce salt intake in Western societies were not based on robust data and may be premature, Yusuf wrote in an email to MedPage Today and ABC News.

"While undoubtedly increased sodium [salt] intake is associated with higher blood pressure, the epidemiologic relationship of high sodium intake in Western societies related to stroke or cardiovascular disease (CVD) is weak and inconsistent."

"The current paper from the Cochrane review group indicates the possibility of important reductions in CVD, but the data are not robust and so they also are consistent with little or no benefit," Yusuf added.

"Therefore, the collective evidence from epidemiologic studies, and from the randomized trials, calls for the generation of more robust epidemiologic data in Western populations, examining the relationship of sodium to CVD events and mortality, and at the same time trying to generate more robust data from large and long-term trials," he concluded.

Evidence in support of restricted salt intake to prevent cardiovascular morbidity and mortality has come, in large part, from the relationship of salt intake to blood pressure, Taylor and co-authors noted in the background of their report.

Meta-analyses of randomized trials of salt restriction have generally shown a positive effect on blood pressure, but the magnitude of reduction has tended to decline over time, the authors continued.

A previous Cochrane review of studies with a duration of at least six months showed that an intensive salt-restriction strategy led to only modest reductions in blood pressure, averaging 1.1/0.6 mm Hg (Cochrane Database Syst Rev 2004; DOI:10.1022/14651858.CD003656.pub2).

The modest effects of salt restriction contrast with evidence suggesting that sustained blood pressure reductions of 2mm to 3 mm Hg are needed to achieve clinically important reductions in cardiovascular events, Taylor and co-authors noted.

The earlier Cochrane review was based on a small number of clinical events and deaths. Given the longer follow-up and accumulation of additional studies and participants, the authors of the updated review focused on confirming whether interventions to reduce dietary salt intake resulted in fewer deaths and cardiovascular events.

The seven studies included in the review consisted of three involving 3,518 normotensive participants; two involving 758 hypertensive participants; one with a mixed population of 1,981 participants; and one involving 232 patients with heart failure.

End-of-trial follow-up ranged from seven to 36 months and the longest period of observational follow-up was 12.7 years.

An analysis of normotensive participants showed that salt restriction was associated with a 33% reduction in the risk of death during the trial, declining to 10% at the end of observation. Both outcomes were associated with overlapping confidence intervals that precluded statistical significance.

The analysis of hypertensive participants showed even smaller effects of salt restriction on mortality: a 3% reduction at the end of the trial and 4% at end of observation.

Salt restriction was associated with a 29% lower risk of cardiovascular events among normotensive individuals and a 16% reduction in hypertensive participants. Neither difference achieved statistical significance.

In the trial involving patients with heart failure, salt restriction significantly increased the mortality risk more than twofold as compared with the control group (RR 2.59, 95% CI 1.04 to 6.44).

Also in response to the study, the American Heart Association issued a statement pointing out limitations of the analysis and imploring the public to limit daily salt intake to 1,500 mg, as recommended in the current consensus statement from AHA and the American College of Cardiology.

"Nine out of 10 Americans will develop high blood pressure in their lifetime," AHA officials said in the statement. "Reducing sodium now - even for people who currently have normal blood pressure - can reap enormous long-term benefits by reducing the risk for developing high blood pressure and helping those with high blood pressure manage their condition more effectively."

Limitations of the analysis cited by the AHA included low representation of African Americans and older people, two groups at high risk for hypertension.

The AHA also questioned the applicability of the findings to a North American population, the adequacy of the follow-up duration, and use of food diaries instead of measuring urinary salt excretion to determine salt consumption.

This article was developed in collaboration with ABC News.

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