e-Library of Evidence for Nutrition Actions (eLENA)

Effect of longer-term modest salt reduction on blood pressure

Systematic review summary

This document has been produced by the World Health Organization. It is a summary of findings and some data from the systematic review may therefore not be included. Please refer to the original publication for a complete review of findings.

Original publication
He FJ, Li J, MacGregor GA. Effect of longer-term modest salt reduction on blood pressure. Cochrane Database of Systematic Reviews 2013, Issue 4. Art. No.: CD004937. DOI: 10.1002/14651858.CD004937.pub2.

Key findings

  • Most of the data in this review come from high-income country settings, and participants of white, black, and South Asian origin were included
  • In adults ≥18 years of age, a modest reduction in salt intake for ≥4 weeks led to a reduction of 4 mmHg in systolic blood pressure and 2 mmHg in diastolic blood pressure
  • The fall in blood pressure was observed across all ethnic groups and in both men and women, and was greater in hypertensives (5/3 mmHg)
  • A dose-response association between a reduction in salt intake and a fall in systolic blood pressure was observed, where a greater reduction in salt intake led to a larger fall in systolic blood pressure

1. Objectives

To determine the effect of a longer-term, modest reduction in salt intake on blood pressure in both hypertensive and normotensive individuals and to assess whether a dose-response relationship exits between salt reduction and change in blood pressure. Additional objectives included to investigate the effects of a reduction in salt on plasma renin activity, aldosterone, noradrenaline, adrenaline, cholesterol, low-density lipoprotein (LDL), high-density lipoprotein (HDL) and triglycerides, and to investigate whether there are differential effects of salt reduction by sex and ethnic group.

2. How studies were identified

The following electronic databases were searched to November 2012:

  • The Cochrane Hypertension Group Specialised Register
  • CENTRAL (The Cochrane Library, Issue 11, 2012)

  • MEDLINE
  • EMBASE

Handsearching of relevant reviews and articles was also conducted.

3. Criteria for including studies in the review

3.1 Study type

Randomized controlled trials

3.2 Study participants

Adults ≥18 years with normal or raised blood pressure

(Trials in children, pregnant women, or patients with diseases other than hypertension, such as diabetes or heart failure, were excluded)

3.3 Interventions

A modest reduction in salt intake for a minimum of four weeks was compared to usual salt intake

(This review was explanatory rather than pragmatic in that only studies in which the treatment group achieved a reduction in 24-hour urinary sodium of 40 to 120 mmol (2.3 to 7.0 g/day less salt intake) were included)

(Studies including concomitant interventions, e.g., antihypertensive medications, were excluded, and in factorial studies, arms using concomitant interventions were excluded)

3.4 Primary outcomes
  • Change in systolic blood pressure
  • Change in diastolic blood pressure
  • 24-hour urinary sodium excretion (mmol)
3.5 Secondary outcomes

Secondary outcomes included plasma renin activity, aldosterone, noradrenaline, adrenaline, cholesterol, LDL, HDL and triglycerides

4. Main results

4.1 Included studies

Thirty trials enrolling 3230 adults were included in this review.

  • Nine-hundred and ninety participants were hypertensive and 2240 participants were normotensive; median blood pressure on usual salt intake was 141/86 mmHg
  • Study duration ranged from four weeks to 36 months, with a median of four weeks; two-thirds of studies were crossover trials and one-third were parallel trials
  • Twenty-four-hour urinary sodium excretion was reduced in the treatment group by 40 to 118 mmol, with a median reduction of 75 mmol, equivalent to a reduction in salt intake of 4.4 g/day. Median 24-hour urinary sodium on usual salt intake was 160 mmol (9.4 g/day of salt)
  • Sample sizes ranged from 12 to 1029 participants and subjects were predominantly white and male, however studies also included black participants, women, and in one study in the United Kingdom, South Asians also participated. Age ranged from 20 to 80 years, with the median being 50 years
4.2 Study settings
  • Countries and study settings of the 30 included trials were not specified in the review, but included high income countries covering a wide geographic area including Australia, Europe, the United Kingdom and the United States of America
4.3 Effect of intervention on primary outcomes

4.3.1 How the data were analysed

The effect of salt reduction was compared with that of usual salt intake. Separate analyses were conducted by blood pressure status:

  • Overall (both hypertensive and normotensive individuals)
  • Hypertensive individuals
  • Normotensive individuals.

The four studies that enrolled both hypertensive and normotensive individuals were treated as separate trials in analyses, giving a total of 34 rather than 30 trials in overall analyses. For crossover trials, the treatment effect was the difference in outcomes between the end of reduced salt period and the end of the usual salt intake (control) period. For parallel trials, the treatment effect was the difference between the two treatment groups in the change in outcomes from baseline to the end of follow-up. Data were pooled by the inverse variance method in random effects meta-analysis, generating mean differences (MD) and corresponding 95% confidence intervals (CI). The I2 test was used to detect the presence of heterogeneity. Multiple meta-regression analyses including the variables 24-hour urinary sodium (continuous), ethnicity (black, white), age (continuous), and sex (male, female) were performed to explore potential sources of heterogeneity. Meta-regression analysis was also used to examine whether there was a dose-response relationship between the change in 24-hour urinary sodium and the change in blood pressure. Pre-specified subgroup meta-analyses to further investigate heterogeneity included:

  • By ethnic group: white, black
  • By sex: male, female

4.3.2 Summary of effects

Overall (both hypertensive and normotensive individuals)
Blood pressure
In meta-analysis, systolic blood pressure was lowered by 4.18 mmHg (95% CI [-5.18 to -3.18], p<0.00001; I2=75%), and diastolic blood pressure was lowered by 2.06 mmHg (95% CI [-2.67 to -1.45], p<0.00001; I2=68%; 34 trials/3230 individuals) with a modest reduction in salt intake. In multiple meta-regression analysis adjusting for age, ethnicity and blood pressure status, a 100 mmol reduction in 24-hour urinary sodium (equivalent to a reduction in salt intake of 6 g/d) was associated with a decrease of 5.8 mmHg (95% CI [-9.2 to -2.5], p=0.001) in systolic blood pressure. Furthermore, each additional year of age was associated with a 0.06 mmHg (95% CI [-0.12 to -0.01], p=0.03) greater fall in systolic blood pressure with salt reduction, being hypertensive was associated with a greater reduction in systolic blood pressure compared with being normotensive (p=0.042), and a larger proportion of the study population being white was associated with a smaller decrease in systolic blood pressure (p=0.001). Ethnic group was also statistically significantly associated with the change in diastolic blood pressure (p=0.021). Sex was not associated with change in systolic or diastolic blood pressure.

Secondary outcomes
Change in plasma renin activity was statistically significantly different between salt reduction and usual salt intake (MD 0.26 ng/ml/hr, 95% CI [0.17 to 0.36], p<0.00001; 14 trials), as was change in plasma aldosterone concentration (MD 73.20 pmol/l, 95% CI [44.92 to 101.48], p<0.00001; 9 trials) and change in plasma noradrenaline (MD 31.67 pg/ml, 95% CI [6.57 to 56.77], p=0.01; 6 trials). Changes in adrenaline, total cholesterol, LDL cholesterol, HDL cholesterol, and triglycerides were not statistically significantly different between salt reduction and usual salt intake.

Hypertensive individuals
Blood pressure
A modest reduction in salt intake resulted in a lowering of systolic blood pressure by 5.39 mmHg (95% CI [-6.62 to -4.15], p<0.00001; I2=61%) and a lowering of diastolic blood pressure by 2.82 mmHg (95% CI [-3.54 to -2.11], p<0.00001; I2=52%; 22 trials/990 individuals). In subgroup meta-analyses, the fall in systolic blood pressure remained statistically significant in whites, blacks, and South Asians, and in men and women. The decrease in diastolic blood pressure was also significant in most subgroup analyses. In meta-regression analysis adjusting for age and ethnicity, a 100 mmol reduction in 24-hour urinary sodium (equivalent to a reduction in salt intake of 6 g/day) was associated with a drop in systolic blood pressure of 10.8 mmHg (95% CI [-18.2 to -3.5], p<0.01). Age, ethnicity and 24-hour urinary sodium were not associated with diastolic blood pressure change in meta-regression analysis.

Normotensive individuals
Blood pressure
Pooled analysis revealed a fall of 2.42 mmHg (95% CI [-3.56 to -1.29], p<0.0001; I2=66%) in systolic blood pressure and a fall of 1.00 mmHg (95% CI [-1.85 to -0.15], p=0.02; I2=66%; 12 trials/2240 individuals) in diastolic blood pressure with salt reduction. In subgroup meta-analyses, there was a statistically significant fall in systolic blood pressure in whites and blacks, and men and women, and the fall in diastolic blood pressure also remained statistically significant in most analyses. In meta-regression analysis adjusting for age and ethnicity, a 100 mmol reduction in 24-hour urinary sodium (equivalent to a reduction in salt intake of 6 g/day) was associated with a decrease in systolic blood pressure of 4.3 mmHg (95% CI [-8.5 to -0.1], p<0.05), while age and ethnicity were not found to be associated. Ethnic group was statistically significantly associated with change in diastolic blood pressure (p=0.042).

5. Additional author observations*

In general, the methodological quality of the included trials was good, with 26 trials judged to be at low risk of bias for allocation concealment, and there was a low rate of attrition overall (6.7%). While studies were conducted in high-income country settings, results remained statistically significant regardless of ethnicity.

Inclusion criteria of the review were explanatory rather than pragmatic, meaning that trials in which the intervention failed to achieve the desired salt reduction (as measured by 24-hour urinary sodium excretion) were excluded. This allowed the reviewers to examine what effect reducing salt intake at an environmental level, for example via public policy to reformulate food with less salt, would have on average blood pressure in the population. The mean reduction in salt intake of 4.4 g/day in the included studies is similar to that of public health recommendations, which advise a reduction from current levels of 9 to 12 g/day to 5 to 6 g/day. The pooled results demonstrated that a longer-term modest reduction in salt intake of 4.4 g/day leads to a statistically significant and, at the population level, important fall in blood pressure by 5/3 mmHg in hypertensives and 2/1 mmHg in normotensives. In addition, meta-regression analyses demonstrated a clear dose-response association between a decrease in salt intake and a reduction in systolic blood pressure, leading the reviewers to suggest that a further reduction to 3 g/day would have a greater effect on blood pressure, and indeed on strokes, heart attacks, and heart failure. Of note, NICE has recommended a reduction in salt intake to 3 g/day by 2025 for the UK adult population.

Further studies investigating the mechanisms underlying the effect of plasma sodium on blood pressure are warranted, as increasing evidence suggests that plasma sodium may have an effect on blood pressure independent to its effect on extracellular volume.

*The authors of the systematic review alone are responsible for the views expressed in this section.