High Salt Intake Boosts Stroke, CVD Risk

Susan Jeffrey

November 24, 2009

November 24, 2009 — A new meta-analysis confirms that high salt intake is associated with increased risks of stroke and total cardiovascular disease (CVD).

The pooled relative risk showed that an average difference in intake of approximately 5 g/day of salt was associated with a 23% increased risk of stroke, the researchers report, and a 17% increase in CVD risk. The average habitual salt intake in most Western countries is 10 g/day, double the level currently recommended by the World Health Organization.

"Given that the case-fatality rate for stroke is estimated at 1 in 3, and [the rate] for total cardiovascular disease at 1 in 5, a 23% reduction in the rate of stroke and a 17% overall reduction in the rate of cardiovascular disease attributable to a reduction in population salt intake could avert some one and a quarter million deaths from stroke and almost 3 million deaths from cardiovascular disease each year," the researchers, with lead author Pasquale Strazzullo, MD, from "Federico II" ESH Excellence Centre of Hypertension at the University of Naples Medical School, Naples, Italy, conclude.

Moreover, because of some imprecision in salt intake measurements in these cohort studies, the actual effects are likely to be underestimated, they add.

The study is published online November 25 in the British Medical Journal.

Salt of the Earth

During the past century, evidence of the risks associated with high salt intake has become "compelling," the authors write. The link between salt intake and blood pressure, for example, has been shown through a variety of study types, including randomized trials. Previous studies have indicated that a reduction of about 6 g/day of salt intake translates to a 7/4-mm Hg reduction in blood pressure in those with hypertension and a 4/2-mm Hg reduction in normotensive individuals, changes that at a population level would be expected to reduce stroke by about 24% and coronary heart disease by 18%, they write.

In the absence of a long-term randomized trial, which is not likely to ever be performed because of the practical difficulties and long duration that would be required, prospective cohort studies can provide indirect evidence, the researchers note. In this study, Dr. Strazzullo and colleagues performed a systematic review and meta-analysis of prospective studies published between 1966 and 2008, identified from a variety of databases.

Their analysis examined 19 cohort samples in 13 eligible studies, including 177,025 participants who had more than 11,000 vascular events during 3.5 to 19 years of follow-up. For each study included, relative risks and 95% confidence intervals were extracted and pooled using a random-effects model, and heterogeneity, publication bias, subgroup, and meta-regression analyses were carried out.

Higher salt intake was associated with a significantly higher risk of stroke and a higher risk of CVD that was of borderline significance, the authors report. However, when 1 study was excluded as an outlier, the relationship between higher salt intake and CVD was also significant.

Table. Risk of Stroke and CVD Associated With Higher Salt Intake

Event Pooled Relative Risk (95% CI) P Value
Stroke 1.23 (1.06 – 1.43) .007
Total CVD 1.17 (1.02 – 1.34) .02

CI = confidence interval; CVD = cardiovascular disease

Efforts to reduce salt intake on a populationwide basis have resulted in some progress, but most countries still have consumption high above recommended levels. "One barrier to a more effective implementation of public health policies has been the historical opposition of the food industry, based on the arguments that the available evidence does not show significant benefits on hard endpoints at a population level," the authors write. "Our study now clearly addresses those doubts."

Voluntary changes by the food industry in cooperation with governments and public health sectors have reduced salt intake by about 10% in the United Kingdom during approximately 4 years, for example, but overall levels remain high. "While the voluntary approach is the preferred choice for many governments, the 'regulatory' approach has advantages, sometimes being the most efficient, effective, and cost-effective way of achieving public health targets," they conclude.

A Welcome Addition

In an editorial accompanying the publication, Lawrence J. Appel, MD, from the Welch Center for Prevention, Epidemiology and Clinical Research at Johns Hopkins University, Baltimore, Maryland, calls the report by Dr. Strazzullo and colleagues, "a useful and welcome addition to the medical literature."

Salt reduction lowers blood pressure, and lowering blood pressure reduces cardiovascular events, Dr. Appel points out. However, this line of reasoning is still largely indirect because of the methodological challenges of conducting such studies. What evidence there is, however, supports the indirect evidence of benefit from salt reduction, he notes.

Results from observational studies have been the most problematic, largely because of problems measuring salt intake and large variations day to day in salt consumption. This problem was also seen in the current study as well, where "the disparate and often poor quality of measurements of dietary salt probably contributed to the significant heterogeneity in the study results seen by Strazzullo and colleagues," Dr. Appel writes.

"Policy makers have previously dismissed the results from prospective observational studies in favour of the considerably more robust body of evidence that links salt intake with blood pressure," he points out. "At a minimum, Strazzullo and colleagues' analyses should dispel any residual belief that salt reduction might be harmful (a canard resulting from misinterpretation of studies, often with flawed analyses).

"The case for population-wide salt reduction is now stronger," Dr. Appel concludes. "A reduced intake of salt not only lowers blood pressure but also prevents its major sequelae — stroke and other cardiovascular diseases."

The study was funded in part by an EC Grant. The authors and Dr. Appel have disclosed no relevant financial relationships.

BMJ. Published online November 25, 2009.

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