Effects of low salt diet versus high salt diet on blood pressure: a randomized controlled crossover trial

High blood pressure is an important public health challenge and has become one of the leading causes of mortality in both the developed and the developing world. It has been widely believed that increase in dietary salt leads to elevation in BP. However, evidence regarding the effects of sodium loading and depletion on BP has been conflicting. We conducted a randomized, controlled, crossover community based trial in Karachi to: 1) assess the effect of low salt diet versus high salt diet on BP and 2) to estimate the prevalence of salt sensitivity (defined as mm Hg decline in SBP from the end of high to low salt phases of diet) among normotensive adults aged .-40 years in Karachi, Pakistan. We hypothesized that alteration in the dietary salt intake demonstrates significant changes in the systolic blood pressure. Two hundred normotensive, non-diabetic subjects randomly selected from six low income communities in Karachi were randomized to either low salt diet (20 mmol/day sodium - equivalent to about 1 gram/day of salt) or high salt diet (220 mmol/day sodium - equivalent to about 13 grams/day of salt) for one week, followed by one week of washout period, and then the reverse of initial week. The meals were specially prepared under the supervision of a nutritionist and delivered to the participants. The primary outcome was difference in systolic BP measured at the end of high salt diet phase compared to low salt diet phase measured by data collectors blinded to dietary assignment. Adherence to the diet was confirmed by measuring 24- hour urinary sodium excretion. The analysis was done on intention to treat basis using paired t-test. The overall mean age (± SD) of the subjects was 49.4 (± 10.0) years. The mean decline in 24-hour urinary sodium excretion [81.0 (± 70.7) mmol/24 hours equivalent to 4.7 grams/day of salt] between the two dietary phases was significant (p \u3c0.001). The mean decline in SBP in the overall study population was 1.2 (+ 11.4) mm Hg (p=0.167). The mean decline in SBP [5.7 (± 12.5) mm Hg] was significant in subjects with high normal baseline SBP ( 130 mm Hg) compared to those with normal baseline SBP (\u3c130 mm Hg) (p=0.002) and was an independent factor for change in SBP on reduction of dietary salt [5.7 mm Hg (95% CI: 1.8, 9.0)]. Similarly, subjects with positive family history of hypertension compared to those without a family history also had significant mean decline of 2.8 (± 11.9) mm Hg in SBP (p=0.034) [adjusted effect: 2.8 (95%CI: -0.4, 6.0)]. The subjects with normal BMI (\u3c23 Kg/m2) had significantly higher decline in SBP [4.2 mm Hg (0.9, 7.5)], compared to overweight and obese subjects. The overall prevalence (95% CI) of salt sensitivity was 26.6%, (95% CI: 20.2 — 33.0), and was significantly higher among subjects with high normal SBP (42.0%) as compared to those with normal SBP (20.9%) (p=0.004). Our findings suggest that reducing salt intake has no significant effect on lowering SBP in average healthy people with low normal SBP. However, salt reduction has significant effect on lowering SBP among individuals with high normal SBP, with normal BMI and among those with a family history of hypertension. Since blood pressure levels of the majority of Pakistani adults fall in the high normal SBP category or higher, salt restriction should be an integral component of hypertension prevention (and probably treatment programs) in Pakistan and in other Indo Asian populations worldwide