The Salt Swap intervention to reduce salt intake in people with high blood pressure: protocol for a feasibility randomised controlled trial

High salt intake is a risk factor for hypertension and cardiovascular disease. Reducing salt intake has been shown to reduce blood pressure. Despite population-level interventions, including product reformulation and public awareness campaigns, adult salt consumption in the UK still exceeds recommendations; this is primarily due to salt consumed in processed and pre-packaged foods. Moderate or high-intensity dietary advice to encourage individuals to reduce their salt intake has been shown to be effective at reducing blood pressure, but evidence of the effectiveness of interventions that are suitable for delivery at scale in routine primary care is scarce. This feasibility trial investigates a complex behavioural change intervention to reduce dietary salt intake and blood pressure by encouraging individuals to purchase lower-salt foods when grocery shopping.

This randomised controlled trial will test the feasibility of a novel intervention to reduce salt intake, and the trial procedures to assess its effectiveness. We will recruit participants through UK general practices and randomise 40 participants with high blood pressure, in a 2:1 allocation to receive either the Salt Swap intervention or a control information leaflet. The primary outcomes relate to the criteria for progression to a large-scale trial. These include follow-up rates at 6 weeks, fidelity of intervention delivery and use of the intervention mobile app. Secondary outcomes include the effect of the intervention on the salt content of purchased foods (grams per 100 g), urinary sodium excretion assessed through 24-hour urine samples and blood pressure. Trial process measures will be collected and qualitative assessment will provide insights into participant engagement with the intervention content and perceived barriers to and facilitators of salt reduction dietary behavioural change.

If the outcomes indicate the trial is feasible and there is evidence that behavioural change may result in salt reduction, we will proceed to a definitive trial to test the effectiveness of the intervention to lower blood pressure. If successful, this intervention approach could be applied not only to people with high blood pressure, but also to the wider population with normal blood pressure in whom dietary salt intake exceeds recommendations.

ISRCTN, 20910962. Registered on 5 April 2017.