Background: Modifying dietary sodium intake is a cornerstone of diet advice for lowering blood pressure (BP) under the assumption that it is protective against cardiovascular disease. Previous meta-analyses of normotensive participants have not excluded all studies that recruited participants with systolic blood pressure (SBP) > 140 mmHg, which greatly hinders generalization to the wider normotensive population.
Objectives: The objective of this review was to identify the effectiveness of reducing or increasing sodium intake on BP in normotensive participants with SBP ≤ 140 mmHg.
Inclusion criteria types of participants: This review considered studies on adult participants (≥18 years) with SBP ≤ 140 mmHg. Studies on pregnant women or patients prescribed antihypertensive or vasoactive medications were excluded.
Types of interventions: Interventions that quantitatively evaluated dietary sodium intake for equal to or greater than four weeks duration were considered. Only studies that included two study arms comprising different levels of sodium intake were included.
Types of outcomes: Studies that reported SBP, diastolic blood pressure (DBP), pulse wave velocity (PWV), pulse wave analysis or flow mediated dilatation were considered.
Types of studies: Experimental study designs including randomized controlled trials and non-randomized controlled trials were considered.
Search strategy: An initial search strategy was conducted on databases MEDLINE and CINAHL before an extensive search of all relevant published and gray literature databases, and clinical trial registries were searched.
Methodological quality: Potential papers were assessed for methodological validity using the standardized critical appraisal instrument from the Joanna Briggs Institute Meta-Analysis of Statistics Assessment and Review Instrument (JBI-MAStARI).
Data extraction: Quantitative data were extracted from papers using the standardized data extraction tool from JBI-MAStARI.
Data synthesis: Quantitative data were pooled in statistical meta-analysis. Effect sizes were expressed as weighted mean differences and 95% confidence intervals. Meta-analysis was conducted using a random-effect model, and heterogeneity assessed statistically using the standard Chi-square test and the I index. A priori sub-group analysis was undertaken on studies achieving ≥40 mmol versus <40 mmol in urinary sodium excretion and post hoc on studies with a mean body mass index (BMI) ≥ 30 versus less than 30.
Results: Five trials were included with a total of 1214 participants. The overall reduction in SBP was -0.71 mmHg (95% CI: -2.62, 1.20, P = 0.47) and DBP -0.57 mmHg (95% CI: -1.26, 0.12, P = 0.10). There was no significant change in PWV following reduction of dietary sodium over a four to six-week period. Sub-group analysis did not find a significant effect of urinary sodium excretion or BMI on outcomes; however, a trend toward a greater reduction in BP was observed in those with a higher BMI (MD -2.41, 95% CI -5.72, +0.91, P = 0.16).
Conclusion: Blood pressure in normotensive participants was not significantly affected by sodium modification and was controlled to within 1% of baseline values. Reducing dietary sodium in normotensive participants may still be of importance for cardiovascular risk management; however, good quality interventional research is limited.