Published: Apr 4, 2013
By Todd Neale , Senior Staff Writer, MedPage Today

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Action Points

The idea that reducing salt intake — and increasing potassium intake — can lower blood pressure and improve disease outcomes has received additional support from three systematic reviews.

In a Cochrane review, modestly reducing salt intake was associated with an average blood pressure reduction of 4.18/2.06 mm Hg (P<0.001), as reported by Feng He, MBBS, PhD, of Queen Mary University of London, and colleagues.

That finding was consistent with the results of another review conducted for the World Health Organization and reported by Nancy Aburto, PhD, of the United Nations World Food Program, and colleagues, which showed that reduced salt intake was associated with an average blood pressure reduction of 3.39/1.54 mm Hg, as well as lower risks of stroke and mortality from stroke and coronary heart disease.

And in a third review, also for the WHO by Aburto’s group, greater potassium intake was associated with an average blood pressure reduction of 3.49/1.96 mm Hg, though the relationship was significant only among hypertensive individuals.

All three reviews were published online in BMJ.

Salt’s Role in BP Reduction

Addressing the salt findings, He and colleagues wrote that the results “provide further strong support for a reduction in population salt intake, which will result in a lower population blood pressure and, thereby, a reduction in strokes, heart attacks, and heart failure.”

“The current recommendations to reduce salt intake from 9 to 12 to 5 to 6 grams/day will have a major effect on blood pressure,” they continued, “but a further reduction to 3 grams/day will have a greater effect and should become the long-term target for population salt intake.”

However, the assertion that reduced salt intake will have beneficial effects on disease outcomes contradicts the results of a 2011 meta-analysis, which failed to show significant relationships between reduced salt intake and mortality or cardiovascular outcomes.

But He and colleagues disputed those findings because of the inclusion of a trial of patients with heart failure who were “already severely salt and water depleted from aggressive treatment with diuretics.”

Excluding that study and combining the remaining studies of hypertensive and normotensive individuals, they added, revealed a significant 20% relative reduction in cardiovascular events (P<0.05) and a nonsignificant 5% to 7% reduction in all-cause mortality, even with a relatively small decrease in salt intake of 2.0 to 2.3 grams/day.

“These results show that salt reduction has a major impact on reducing strokes, heart attacks, and heart failure,” they wrote.

Mounting evidence supports an association between lower salt intake and reductions in blood pressure, but a recent meta-analysis suggested that some of that benefit could be canceled out by adverse effects on hormones and lipids.

He and colleagues explored the issue in a review of 34 randomized trials that compared a modest reduction in salt intake with usual salt intake — 22 in hypertensive individuals and 12 in normotensive individuals. All lasted at least 4 weeks.

On average, the reduction in 24-hour urinary sodium in the studies equated to consumption of 4.4 fewer grams of salt each day.

The reduction in blood pressure accompanying that drop occurred in both whites and blacks and in both sexes and was greater among individuals with hypertension (5.39/2.82 mm Hg) than among their normotensive counterparts (2.42/1.00 mm Hg).

The reduction in salt intake resulted in “only a small physiological increase” in plasma renin activity (0.26 ng/mL/hour), aldosterone (73.2 pmol/L), and norepinephrine (187 pmol/L). Because of the duration of only 4 to 6 weeks among the trials measuring those variables, however, “it is likely that such effects might attenuate over time,” according to the researchers.

Salt reduction was not associated with any changes in lipids.

WHO Studies Salt, Potassium

The findings of the review of salt reduction by Aburto and colleagues — conducted for the WHO Nutrition Guidance Expert Advisory Group Subgroup on Diet and Health — were largely similar and added information on clinical outcomes.

The review included 42 randomized controlled trials and 14 prospective cohort studies in adults and nine controlled trials and one cohort study in children.

In both adults and children, reducing salt intake was associated with a drop in blood pressure, although the magnitude of the reduction was greater in adults, particularly those with hypertension.

For all adults, consuming less than 2 grams of salt per day was associated with an average blood pressure reduction of 3.47/1.81 mm Hg compared with consuming 2 grams or more per day.

There were no adverse effects on blood lipids, catecholamine levels, or renal function observed with a reduction in salt intake.

The reductions in blood pressure appeared to have clinical benefits: increased salt intake was associated with significantly greater risks of stroke (risk ratio 1.24, 95% CI 1.08 to 1.43), stroke mortality (RR 1.63, 95% CI 1.27 to 2.10), and coronary heart disease mortality (RR 1.32, 95% CI 1.13 to 1.53).

There was not enough evidence from randomized trials to evaluate the relationships with mortality and morbidity, and in cohort studies there were no significant associations with all-cause mortality, cardiovascular disease, and coronary heart disease.

Nevertheless, Aburto and colleagues wrote, “almost all reductions in blood pressure are beneficial for health, and modest population-wide reductions in blood pressure can result in important reductions in mortality, substantial health benefits, and meaningful savings in healthcare costs.”

The other review by that group — which included data from 22 randomized controlled trials and 11 cohort studies — suggested similar benefits might be achieved by increasing potassium intake.

Systolic blood pressure dropped as potassium intake increased, particularly when individuals were ingesting 90 to 120 mmol of potassium per day (systolic blood pressure was 7.16 mm Hg lower, 95% CI 1.91 to 12.41). There was no evidence that consuming greater amounts yielded a larger benefit.

Potassium intake was not related to renal function or concentrations of lipids or catecholamines.

Increased intake was, however, associated with a lower risk of incident stroke (RR 0.76, 95% CI 0.66 to 0.89).

Supported by this review, the WHO released a guideline for potassium intake, “which states that adults and children without compromised renal handling of potassium should increase their potassium intake from food and that adults should consume more than 90 mmol potassium/day for beneficial effects on blood pressure and risk of related cardiovascular diseases,” the authors wrote.

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