Dietary sodium intake has been considered a contributing factor to the elevation of blood pressure (BP). Nevertheless, the extent of the BP response towards dietary sodium intake can be inconsistent and affected by individual variability, thus hindering treatment outcomes.¹ To investigate the relationship between dietary sodium intake and within-individual BP responses, a crossover trial involving participants from the CARDIA study was conducted.¹                            

Excessive sodium intake has been a well-established cause of cardiovascular disease, being associated with the largest number of diet-related mortality (1.89 million/year).² In particular, the concept of “salt sensitivity” is thought to be correlated to the pathogenesis of hypertension.³ Previous studies suggested that salt-sensitive individuals possessed an impaired vascular function, which manifests as an abnormal vascular resistance in the renal circulation after salt intake, resulting in an increase in BP.³ Conversely, normal salt-resistant individuals are capable of facilitating a vasodilatory response after an increased salt intake, thus leading to the preservation of normal BP levels.³

While salt sensitivity has been identified as a dominant factor for hypertension, it may vary interpersonally, which limits the BP-lowering efficacy of personalized dietary sodium recommendations.¹ To obtain a more comprehensive understanding of the distribution of within-individual BP responses to dietary sodium intake and assess the impact of reducing dietary sodium intake on BP, the Coronary Artery Risk Development in Young Adults-Salt Sensitivity of BP (CARDIA-SSBP) study was initiated.¹

The CARDIA-SSBP study was a prospectively allocated crossover study that enrolled participants from 2 sources, where 72.8% of participants were from the CARDIA year 35 core examination and the rest of them were non-CARDIA participants from Chicago and Birmingham.¹ The study population (n=213) was randomized into receiving 1 week of high-sodium diet first, followed by 1 week of low-sodium diet (n=118); or vice versa (n=95).¹ The high-sodium diet consisted of the addition of 2 sodium bullion packets along with each participant’s usual diet, thereby increasing their daily sodium intake by 2,200mg.¹ Whereas low-sodium diet was comprised of standardized meals providing a daily sodium intake of 500mg.¹ The primary outcome was the overall estimate of salt sensitivity of BP (SSBP), calculated as the within-individual difference in 24-hour mean arterial pressure (MAP) between a high-sodium diet and a low-sodium diet.¹ A parallel-group analysis was also conducted to examine the contrast in BP between the 2 groups of participants.¹

For the primary outcome, the overall median SSBP for MAP was calculated to be 4mmHg (IQR: 0-8mmHg; p<0.001), with an overall 73.4% reduction induced by low-sodium diet, thereby indicating a significant association between dietary sodium intake and BP responses.¹ Furthermore, the SSBP for MAP remained consistent, regardless of age, gender, diet order, hypertension status or the use of antihypertensive medications.¹ These findings indicated a consistent MAP reduction associated with a low-sodium diet.¹

The results obtained from the parallel-group analysis demonstrated significant differences in BP between high-sodium and low-sodium diet groups.¹ By the end of first dietary intervention week, the low-sodium diet group possessed significantly lower systolic blood pressure (SBP) (MD=8mmHg; 95% CI: 4-11mmHg; p<0.001), diastolic blood pressure (DBP) (MD=3mmHg; 95% CI: 1-5mmHg; p=0.01), MAP (MD=5mmHg; 95% CI: 2-7mmHg; p=0.001) and pulse pressure (PP) (MD=5mmHg; 95% CI: 2-7mmHg; p=0.001) compared to their high-sodium intake counterparts. ¹ Such between-group difference for SBP was maintained by the end of the second week (MD=7mmHg; 95% CI: 3-11mmHg; p<0.001).¹  

In terms of safety, the incidence of adverse events (AE) was similar between interventions (8.0% for the low-sodium diet vs. 9.9% for the high-sodium diet), generally encompassing mild and self-remitted, involving cramping and weakness for the low-sodium diet group; or headache, vomiting and edema for the high-sodium diet group.¹

In summary, this study has demonstrated that a reduced dietary sodium intake can lead to significant reductions of BP parameters in older adults, with the effect observable as early as in the first week.¹ The decline in BP was consistent regardless of hypertension status and antihypertensive medication use.¹