How Much Can You Reduce Blood Pressure By Eating Less Salt?

The world health organization recommends adults eat less than five grams of salt a day (the equivalent of two grams of sodium a day), and many do. Although numerous studies support and generally agree on the benefits of sodium reduction for blood pressure in the general population, data on the specific dose-response relationship between sodium reduction and blood pressure reduction, especially the long-term effects, are limited.

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A systematic review and meta-analysis based on data from 133 randomised trials involving more than 12,000 subjects, published online in the British medical journal, adds a jigsaw puzzle to our understanding of these questions. In the analysis, led by researchers at Queen Mary University of London, the team searched a database of randomized trials that compared sodium reduction with regular diets through January 2019. These trials all evaluated sodium intake through 24-hour sodium excretion, the ‘gold standard’ for assessing salt intake, to avoid errors in food measurements or deviations in some urine samples. The trials also measured the systolic or diastolic blood pressure of the subjects, who were in the general population and did not include children under the age of 18, pregnant women or people with chronic kidney disease, heart failure or other chronic diseases.

After screening, the team eventually included 136 trials, covering 12,197 subjects. On average, sodium excretion decreased by an average of 130 mmol (equivalent to about 3g of sodium and 7.6g of salt), and systolic and diastolic blood pressure decreased by an average of 4.26 mm Hg and 2.07 mm Hg, respectively.

After adjusting for intervention time, age, gender, race, and blood pressure levels before the study began, data showed that the magnitude of 24-hour sodium excretion was positively correlated with blood pressure changes. For every 50 mmol reduction in 24-hour sodium excretion (equivalent to about 1.15g of sodium and 2.9g of table salt), diastolic and systolic blood pressure decreased by 1.10 mm Hg and 0.33 mm Hg, respectively.

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Further analysis found that the effect was observed in both hypertensive and non-hypertensive populations. No matter how much salt you originally ate, reducing salt has significant benefits. However, there are some differences in the extent to which different populations benefit from the same 24-hour sodium excretion changes. Blood pressure declines more with age; People with high systolic blood pressure before the study also had a greater decrease in systolic blood pressure. The gains were also more pronounced among non-whites, including asians, and the authors’ team suggests that ethnic differences in the responsiveness of the renin-angiotensin system to sodium reduction may partially explain the racial differences.

The 136 trials varied in salt control duration, with 57% of the trials intervening within 14 days, 21% 15-30 days, 19%  intervening over 30 days but only 4% intervening over 6 months.
The same 50 mmol reduction in 24-hour sodium excretion resulted in twice the systolic blood pressure reduction (2.13 mm Hg vs. 1.05 mm Hg) for 14 or more days compared to short-term interventions. While overall, the duration of the intervention was not significantly associated with the magnitude of the change in blood pressure, the team noted that given the paeity of long-term studies, this suggests that reducing sodium intake in the diet may take several weeks to become fully apparent, and that short-term studies may underestimate the effect of sodium reduction on blood pressure.

The authors’ team concluded that the evidence suggests that reducing sodium intake has an effect on lowering blood pressure in a broad range of people and has a greater effect on high-risk groups. At the same time, there was a strong dose-response relationship between the reduction in sodium and the decrease in blood pressure. The long-term benefit of sodium reduction on blood pressure has yet to be definitively assessed by more studies.