Salt Intake and Blood Pressure Control in Adults With Essential Hypertension

Hypertension is a major public health concern globally, contributing to cardiovascular disease and premature mortality. According to the World Health Organization (2023), one in three adults worldwide suffers from hypertension. In Singapore, the 2022 National Population Health Survey reported that over 35% of residents aged 18 to 74 years had hypertension, with nearly 65% of those under medical follow-up exhibiting poor blood pressure control. One of the most significant and modifiable risk factors for hypertension is diet-specifically, excessive sodium (salt) intake and insufficient potassium intake.

Excess sodium elevates blood pressure by causing fluid retention and adversely affecting vascular function. Conversely, potassium intake helps regulate blood pressure through mechanisms like promoting sodium excretion and reducing vascular resistance. The benefits of dietary sodium reduction and increasing potassium intake are well-established in both clinical trials and population studies, particularly in individuals with hypertension.

Despite this, Singaporeans continue to consume well above the recommended daily intake for sodium and below the optimal levels for potassium. The average sodium intake is approximately 3,620 mg/day (exceeding the WHO recommendation of 2,000 mg/day), while potassium intake averages 2,500 mg/day (below the recommended 3,510 mg/day). Public health initiatives like the "War on Salt" campaign (launched in 2011) have had limited long-term success in addressing these dietary gaps.

Previous studies in Singapore and neighboring countries have identified significant gaps in knowledge, attitudes, and behaviors related to dietary sodium and potassium. However, there has been limited research focused on practical, context-specific educational interventions aimed at improving dietary behaviors related to salt and potassium intake, especially within the hypertensive population in the primary care setting.

Study Objectives and Hypothesis This study is designed in three parts to systematically develop, refine, and evaluate a novel educational tool-the Decreased Salt High Potassium (DSHP) Diet Tool-for hypertensive patients. The tool aims to support patients in reducing dietary sodium and increasing potassium intake, thereby improving blood pressure control and overall cardiovascular risk profiles.

Primary Hypothesis: A targeted educational tool, developed based on patient and healthcare provider input, will be feasible and effective in supporting reductions in sodium intake and increases in potassium intake among hypertensive patients, leading to improved blood pressure outcomes.

Objectives:

Part 1:

• Identify perceived barriers, facilitators, and attitudes towards reduced sodium and increased potassium diets among hypertensive patients.
• Understand healthcare providers' (HCPs) perspectives on the barriers in educating patients about reducing sodium intake and increasing potassium intake.

Part 2:

• Develop a prototype DSHP Diet Tool based on findings from Part 1.
• Alpha-test the tool with both patients and HCPs to assess usability and acceptability.
• Refine the tool through multiple feedback cycles using qualitative interviews and usability surveys (System Usability Scale - SUS).

Part 3:

• Conduct a pilot randomized controlled trial (RCT) to evaluate the feasibility, usability, and preliminary effectiveness of the DSHP Diet Tool in real-world clinical settings.

Study Design Overview

The study will be conducted in three distinct but interrelated parts:

Part 1: Qualitative study

Design: Qualitative study using in-depth interviews and/or focus group discussions guided by the Theoretical Domains Framework (TDF). Population: 40 hypertensive patients and 20 healthcare providers from two SingHealth Polyclinics (Punggol and Pasir Ris). Sampling: Purposive sampling to ensure diversity in age, gender, ethnicity in patients Data Collection:

• Audio-recorded, semi-structured interviews will be carried out Data Analysis: Thematic analysis using the TDF as a guiding framework. Coding will be conducted independently by multiple researchers to ensure reliability. Data collection will continue until saturation is achieved.

Part 2: Development and Alpha Testing of DSHP Diet Tool

Design: Mixed-methods developmental study. Tool Development: Content developed using findings from Part 1 and validated through collaboration with dietitians, nurses, content experts, and HCPs. Components include:

• General education on sodium and potassium.
• Sodium intake calculator and tracker.
• Food lists and risk matrix.
• Behavior change strategies and goal setting.
• Progress tracking and support resources.

Alpha Testing:

• 20 hypertensive patients and 20 HCPs will test the tool over four iterative cycles (5 patients and 5 HCPs per round).
• Participants will use the DSHP Tool for two weeks.
• Feedback collected via System Usability Scale (SUS) and semi-structured interviews.
• DSHP Tool will be revised based on feedback from each cycle.

Outcome Measures:

• SUS scores (target usability score ≥ 68).
• Feedback on usability and feasibility of the DSHP Tool

Part 3: Pilot Randomized Controlled Trial (RCT) Design: Parallel, two-arm, 1:1 allocation pilot RCT with mixed-method evaluation. Sites: SingHealth Polyclinic - Punggol and SingHealth Polyclinic - Pasir Ris. Sample Size: 80 patients with hypertension (40 per arm), with 25% over-recruitment to account for attrition. Randomization: Using sealed, sequentially numbered opaque envelopes (SNOEs) prepared by an independent researcher. Duration: 24-week intervention period.

Arms:

• Intervention Group: DSHP Tool + Patient Information Leaflet (PIL).
• Control Group: Standard care + PIL.

Assessments (Baseline and 24 weeks):

• Blood pressure (systolic/diastolic)
• Body weight and height
• Serum sodium, potassium, creatinine
• 24-hour urinary sodium and potassium
• Salt literacy and DASH adherence scores

Qualitative Component: At the end of the RCT, a subset of patients from the intervention group will participate in interviews to provide in-depth feedback on acceptability, usability of the DSHP Tool.

Statistical Methods

Quantitative Analysis:

• Independent t-tests or Wilcoxon rank-sum tests for group comparisons.
• Paired t-tests or Wilcoxon signed-rank tests for within-group comparisons.
• Categorical variables analyzed using chi-square tests.
• Intention-to-treat (ITT) principles will be used.
• Missing data managed via multiple imputation or last observation carried forward.

Qualitative Analysis:

• Framework analysis based on TDF.
• Coding verified by multiple investigators.
• Triangulation with quantitative data for integrated interpretation.

Ethical Considerations and Safety

• The study has obtained approval from the SingHealth Centralised Institutional Review Board (CIRB).
• Written informed consent will be obtained from all participants.
• Participants may withdraw at any time without penalty.
• All data will be de-identified and stored securely.
• Risks include potential discomfort during interviews or dietary changes, and rare risks such as hyperkalemia or hyponatremia. Patients at risk of hyperkalemia or hyponatremia are excluded from participation.

Expected Impact and Significance

This study aims to address critical gaps in dietary management of hypertension in Singapore by:

• Developing a practical, theory-informed, user-centered dietary tool.
• Empowering patients to self-manage their hypertension through improved dietary habits.
• Supporting HCPs with an effective educational resource.