Dietary Sodium Restriction, Blood Pressure and Cardiovascular Morbidity

A Cochrane review [Abstract] 1 included 195 studies comparing the effects of low sodium (65 mmol/day corresponding to the recommended level) versus high sodium intake (203 mmol/day corresponding to high usual level). Normotension: The effect of sodium reduction on mean difference (MD) of blood pressure in normotensive white people was SBP -1.14 mmHg (95% CI -1.65 to -0.63; 95 studies, n=5 982), DBP +0.01 mmHg (95% CI -0.37 to 0.39; 96 studies, n=6 276). The effect of sodium reduction in black people with normotension was SBP -4.02 mmHg (95% CI -7.37 to -0.68), DBP -2.01 mmHg (95% CI -4.37 to 0.35; 7 studies, n=253). The effect of sodium reduction in Asian people with normotension was SBP -1.50 mmHg (95% CI -3.09 to 0.10), DBP -1.06 mmHg (95% CI -2.53 to 0.41; 5 studies, n=950). Hypertension: The effect of sodium reduction in hypertensive white people was SBP -5.71 mmHg (95% CI -6.67 to -4.74; 88 studies, n=3 998), DBP -2.87 mmHg (95% CI -3.41 to -2.32; 89 studies, n=4 032). The effect of sodium reduction in hypertensive black people was SBP -6.64 mmHg (95% CI -9.00 to -4.27), DBP -2.91 mmHg (95% CI -4.52 to -1.30; 8 studies, n=398). The effect of sodium reduction in hypertensive Asian people was SBP -7.75 mmHg (95% CI -11.44 to -4.07), DBP -2.68 mmHg (95% CI -4.21 to -1.15; 8 studies, n=254).

In plasma or serum there was a significant increase in renin, aldosterone, noradrenaline, adrenaline, cholesterol and triglyceride with low sodium intake as compared with high sodium intake.

Another Cochrane review[Abstract] 2 on modest reduction in salt intake included 34 studies (duration of 4 or more weeks) with a total of 3 230 subjects. The mean change in urinary sodium (reduced salt vs usual salt) was -75 mmol/24-h (equivalent to a reduction of 4.4 g/d salt), the mean change in BP was -4.18 mmHg (95% CI -5.18 to -3.18, I² =75%) for systolic and -2.06 mmHg (95% CI -2.67 to -1.45, I² =68%) for diastolic BP. Meta-analysis by subgroup showed that, in hypertensives, the mean effect was -5.39 mmHg (95% CI -6.62 to -4.15, I² =61%) for systolic and -2.82 mmHg (95% CI -3.54 to -2.11, I² =52%) for diastolic BP. In normotensives, the mean effect was -2.42 mmHg (95% CI -3.56 to -1.29, I² =66%) for systolic and -1.00 mmHg (95% CI -1.85 to -0.15, I² =66%) for diastolic BP. Further subgroup analysis showed that the decrease in systolic BP was significant in both whites and blacks, men and women. There was a small physiological increase in plasma renin activity, aldosterone and noradrenaline. There was no significant change in lipid levels.

Third Cochrane review[Abstract] 3 included three trials in normotensives (n=2 326), five in untreated hypertensives (n=387) and three in treated hypertensives (n=801). This review of dietary salt restriction intervention studies of at least 6 months duration, found that intensive support and encouragement to reduce salt intake lowered blood pressure at 13 to 60 months but only by a small amount (systolic by 1.1 mmHg, 95% CI 1.8 to 0.4, diastolic by 0.6 mmHg, 95% CI 1.5 to -0.3). Deaths and cardiovascular events were inconsistently defined and reported; only 17 deaths equally distributed between intervention and control groups occurred.

Fourth Cochrane review[Abstract] 4 included 8 studies; 3 in normotensives (n = 3 518) and 5 in hypertensives or mixed populations of normo- and hypertensives (n = 3 766). End of trial follow-up ranged from 6 to 36 months and the longest observational follow-up (after trial end) was 12.7 years. The RR for all-cause mortality showed no significant reduction in normotensives (end of trial RR 0.67, 95% CI 0.40 to 1.12; longest follow-up RR 0.90, 95% CI 0.58 to 1.40; 3 studies, n=3 518) or in hypertensives (end of trial RR 1.00, 95% CI 0.86 to 1.15, 4 studies, n=3 085; longest follow-up RR 0.99, 95% CI 0.87 to 1.14, 5 studies, n=3 680). There was weak evidence of benefit for cardiovascular mortality (hypertensives end of trial RR 0.67, 95% CI 0.45 to 1.01, 3 studies, n=2 656) and for cardiovascular events (hypertensives end of trial RR 0.76, 95% CI 0.57 to 1.01, 4 studies, n = 3 397; normotensives at longest follow-up RR 0.71, 95% CI 0.42 to 1.20; hypertensives at longest follow-up RR 0.77, 95% CI 0.57 to 1.02; pooled analysis at longest follow-up RR 0.77, 95% CI 0.63 to 0.95, 6 studies, n = 5 912). These findings were driven by one study among retirement home residents that reduced salt intake in the kitchens of the homes, thereby not requiring individual behaviour change. Advice to reduce salt showed small reductions in SBP (MD -1.15 mmHg, 95% CI -2.32 to 0.02; n=2 079) and DBP (MD -0.80 mmHg, 95% CI -1.37 to -0.23 n=2 079) in normotensives and greater reductions in SBP in hypertensives (MD -4.14 mmHg, 95% CI -5.84 to -2.43 n=675), but no difference in diastolic blood pressure (MD -3.74 mmHg, 95% CI -8.41 to 0.93; n=675).

References

• Graudal NA, Hubeck-Graudal T, Jurgens G. Effects of low sodium diet versus high sodium diet on blood pressure, renin, aldosterone, catecholamines, cholesterol, and triglyceride. Cochrane Database Syst Rev 2020;(12):CD004022. [PubMed]
• He FJ, Li J, Macgregor GA. Effect of longer-term modest salt reduction on blood pressure. Cochrane Database Syst Rev 2013;(4):CD004937. [PubMed]
• Hooper L, Bartlett C, Davey SG et al. Advice to reduce dietary salt for prevention of cardiovascular disease. Cochrane Database Syst Rev 2004;(1):CD003656. [PubMed]
• Adler AJ, Taylor F, Martin N et al. Reduced dietary salt for the prevention of cardiovascular disease. Cochrane Database Syst Rev 2014;(12):CD009217. [PubMed]