**Background:** High blood pressure is the leading cause of premature death worldwide, and dietary sodium reduction is a key strategy. In New Zealand, adults consume approximately 3373 mg sodium/day (40% above WHO recommendations), and 1 in 5 adults has hypertension. Two promising but understudied approaches are the SaltSwitch smartphone app (which provides real-time traffic-light nutrition labels and lower-salt alternatives when scanning packaged food barcodes) and reduced-sodium salts (RSS), which replace some sodium chloride with potassium chloride while maintaining similar taste. The Salt Alternatives Study (SALTS) aimed to determine whether a 12-week intervention combining both approaches could reduce estimated 24-hour urinary sodium excretion in adults with high blood pressure.
**Methods:** This was a 2-arm parallel randomized controlled trial conducted in Auckland and Wellington, New Zealand, between May 2019 and February 2021. Eligible participants were adults ≥18 years who owned a smartphone, had seated SBP ≥140 mm Hg or DBP ≥85 mm Hg, and did household grocery shopping. Exclusion criteria included SBP >200 mm Hg, DBP >120 mm Hg, contraindications to altering sodium/potassium intake, and use of furosemide, prednisone, or NSAIDs. After a 2-week baseline period, 168 participants were randomized 1:1 to intervention (SaltSwitch app + RSS) or control (generic heart-healthy eating advice). The RSS was a blend of 74.5% potassium chloride, 24.5% sodium chloride, and 1% silicon dioxide (75% less sodium than regular salt). The primary outcome was estimated 24-hour urinary sodium excretion at 12 weeks, estimated from spot urine samples using a standard urine volume of 1.99 L. Secondary outcomes included estimated 24-hour potassium excretion, SBP and DBP (measured via Wi-Fi-enabled monitors), sodium content of packaged food purchases (via barcode scanning), and intervention use/acceptability. Analyses were intention-to-treat using generalized linear regression adjusted for baseline outcome, age, and ethnicity, with multiple imputation for missing primary outcome data.
**Key Results:** Recruitment fell substantially short of the target of 326 participants; only 168 were randomized (84 per group) between June 2019 and February 2020. COVID-19 lockdowns prevented in-person recruitment and urine sample collection. At 12 weeks, valid urine samples were returned by only 45.2% (76/168) of participants. The adjusted mean difference between groups for estimated 24-hour urinary sodium excretion was 547 mg/day (95% CI −331 to 1424), favoring the control group (not significant). Sensitivity analyses using no imputation (670 mg/day, 95% CI −304 to 1645) and the INTERSALT formula (24 mg/day, 95% CI −254 to 302) were consistent. For secondary outcomes: estimated 24-hour potassium excretion adjusted mean difference was 132 mg/day (95% CI −1083 to 1347); SBP adjusted mean difference was −0.66 mm Hg (95% CI −3.48 to 2.16); DBP adjusted mean difference was −0.35 mm Hg (95% CI −2.20 to 1.50); sodium content of food purchases adjusted mean difference was 73 mg/100 g (95% CI −21 to 168). None were statistically significant. Among intervention participants providing data (64/84, 76%), 75% (48/64) reported using SaltSwitch, but only 31% (15/48) used it on more than half of shopping occasions; mean use was 6 shopping occasions over 12 weeks. For RSS, 94% (60/64) used it, with 69% (44/64) reporting it made up at least half of discretionary salt intake. Mean RSS consumption was approximately 37.2 g (6.5 tsp) per household over 12 weeks (~0.5 tsp/week). No serious adverse events were reported. Recruitment of Māori (14.3%) and Pacific (7.1%) participants was too low for subgroup analyses.
**Clinical Implications:** This trial found no evidence that a smartphone app plus reduced-sodium salt substitute reduces sodium intake, blood pressure, or sodium content of food purchases in adults with hypertension. The null findings may reflect low intervention dose (infrequent app use, minimal RSS consumption) and the trial being substantially underpowered (target 326, achieved 168). The results contrast with prior positive findings from pilot studies of SaltSwitch in motivated clinical populations and from meta-analyses of RSS trials, though those RSS trials were conducted in settings where discretionary salt contributes more to total intake. In countries like New Zealand where packaged foods contribute >50% of dietary sodium, RSS may be more impactful when used in food manufacturing rather than as a table salt replacement. The study highlights the difficulty of achieving meaningful behavior change through individual-level digital interventions alone and underscores the need for comprehensive population-level policies including food reformulation, labeling, and environmental changes. The low recruitment and engagement of Māori and Pacific participants also demonstrate that technology-based interventions may not be equitable without culturally tailored, community-led approaches.