Effect of a salt-restricted traditional Japanese diet (washoku) on blood pressure in young Japanese adults: protocol for a three-arm pilot randomized controlled trial

Background Hypertension is a major public health concern. Although dietary patterns such as the Dietary Approaches to Stop Hypertension and Mediterranean dietary patterns have demonstrated beneficial effects on blood pressure (BP), evidence regarding the provision of salt-restricted traditional Japanese (washoku) diet is limited. This pilot trial aims to estimate the effect on BP of providing salt-restricted washoku meals as part of the usual diets among young Japanese adults. Methods This is an open-label, three-arm, parallel-group pilot randomized controlled trial recruited 240 community-dwelling young adults aged 20–39 years with a clinic systolic BP (SBP) < 160 mmHg who are not receiving antihypertensive treatment. Participants are randomly assigned in a 1:1:1 ratio to receive salt-restricted washoku meals, regular washoku meals, or salt-restricted regular meals. Each participant receives one assigned study meal per day, five days a week, for four weeks, while otherwise maintaining their usual diet. The primary outcome is the change in clinic SBP from baseline to the post-intervention assessment, conducted within one week after completion of the four-week intervention. Mean changes and 95% confidence intervals will be estimated for each group and compared using analysis of covariance, with age as a covariate. Discussion This trial will provide preliminary estimates of the effects on BP of supplying salt-restricted washoku meals within the usual diets among young adults. Its findings may inform the design of a larger, adequately powered trial; however, the short intervention period and limited control over dietary intake outside the provided meals preclude conclusions about the long-term effects of the intervention. Trial registration Japan Registry of Clinical Trials (jRCT), jRCT1050240271; registered on February 17, 2025.