Salt warning labels in the out-of-home food sector: online and real-world randomised controlled trials in the UK

Abstract

Background

High salt intake increases the risk of cardiovascular disease. The salt content of many commonly consumed foods in the out-of-home food sector (eg, restaurants) is excessive, but there are few policy options to address this problem. In this study, we evaluated an emerging policy approach—high salt warning labels on packaged food and resturant menus—for which, to date, there is little supporting evidence from randomised controlled trials.

Methods

These randomised controlled trials (one online study and one trial conducted in a real-world setting) were conducted in the UK. For study 1, an online study, participants (stratified by age, sex, and education to be representative of the UK adult population) were eligible if they were a current UK resident, aged 18 years or older, fluent in English, purchased supermarket sandwiches and savoury snacks, and ate out at or ordered from restaurants at least monthly. Exclusion criteria included being pregnant or breastfeeding or having major dietary restrictions. Participants were randomly assigned (1:1:1:1:1) to one of four different salt warning label conditions or to a control condition (QR code). Participants assigned to each group completed three packaged food scenarios and three restaurant ordering scenarios, all online, followed by questionnaires about the labelling and their food choices. The primary outcome was the perceived message effectiveness of salt warning labels. In study 2, the inclusion criteria were similar, except that participants who ate an out-of-home meal at least once a month were recruited. Exclusion criteria were severe dietary allergies and veganism. As in study 1, participants were stratified by age, sex, and education. Participants were randomly assigned (block randomisation with block size ~50) to receive menus with or without salt warning labels, from which they purchased and consumed lunchtime meals in a real-world restaurant. Participants then completed questionnaires. Primary outcomes were perceived message effectiveness and salt awareness. In both studies, perceived message effectiveness was measured with adapted versions of the University of North Carolina Perceived Message Effectiveness Scale. Participants in both studies were paid and masked to the study aims. Study 2 is registered with ClinicalTrials.gov (NCT06458270) and is complete.

Findings

In study 1, 2549 participants were randomly assigned to one of four salt warning label groups (red triangle, n=512; black triangle, n=512; red octagon, n=509; and black octagon, n=510) or to the control group (n=506), with data collected between Feb 20, 2024, and April 2, 2024. 158 participants were excluded from analysis, resulting in a final analytic sample of 2391 (1205 [50%] female, 1181 [49%] male, and five [<1%] preferred not to say). All salt warning labels were perceived as significantly more effective at discouraging salt intake than the control, with mean perceived message effectiveness differences of 1·23 (95% CI 1·12–1·34; p <0·0001) for packaged food scenarios and 1·22 (95% CI 1·11–1·33; p <0·0001) for menu scenarios. In study 2, 465 eligible participants were randomly assigned to menus with red triangle salt warning labels next to high-salt items (n=240) or to the restaurant's standard menu (control group; n=225), with data collected between June 5, 2024, and Sept 14, 2024. Full data from 11 participants were excluded from analysis, resulting in a final analytic sample of 454 (246 [54%] female, 203 [45%] male, and five [<1%] missing). The labelled menu was rated as significantly more effective than the control menu in terms of perceived message effectiveness, with a mean difference of 1·00 (95% CI 0·79–1·18; p<0·0001). Participants assigned to the labelled menu condition were significantly more likely to think about the salt content of the meals when ordering than were participants assigned to the standard menu (odds ratio 19·50, 95% CI 8·24–46·16; p<0·0001).

Interpretation

Salt warning labels on restaurant menus are a promising policy option to discourage high salt intake in the out-of-home food sector. Further real-world studies are needed to optimise potential policy to reduce actual salt intake.

Funding

National Institute for Health and Care Research Oxford Health Biomedical Research Centre and European Research Council.