The Effect of Nutrient Warning vs. Traffic Light Labels and Taxation on Food Purchasing Behaviour Among UK Consumers: a Randomised Controlled Trial

The study will be a 2x2 (labelling and taxation) between-subjects randomised controlled trial in which participants will be recruited assigned to one of four conditions in the online survey platform "Qualtrics": (i) nutrient warning labelling and no taxation, (ii) nutrient warning labelling and taxation, (iii) current traffic light labelling and taxation or (iv) a control condition (current traffic light labelling and no taxation). In the nutrient warning labelling and no taxation condition, the corresponding warning labels will be placed on items that are high in saturated fat, salt, sugar, and/or calories. Items will be defined as 'high in' if they exceed >25% of the reference intake (RI) /100g or >30% RI/portion, consistent with the threshold for a red traffic light. RIs are established in European law (EU Regulation 1169/2011) and reflect the daily requirements of an average adult. In addition to these criteria, items will be defined as 'high in' calories if they exceed Public Health England (now UKHSA) guidelines for a meal (>600kcals/portion) or snack (>200kcals/portion). In the nutrient warning label and taxation condition, a 20% price increase (based on modelling work indicating efficacy at this level for reducing energy intake and obesity prevalence in a UK context) will also be applied to 'high in' items. In the traffic light and no taxation condition, all items will feature a traffic light label. This is broadly in line with current UK implementation (an estimated 75% of packaged products feature the label under voluntary implementation). In the traffic light label and taxation condition, a 20% price increase will also be applied to 'high in' items.

Participants will complete a food ordering task using a simulated online supermarket resembling a major UK retailer (Tesco). A subsample of items from three categories - Ready Meals for One, Starters and Sides, and Chilled Desserts - will be available for selection. All items will be from the "Tesco" brand. The subsample (8 items per category) was selected to be representative of the full sample in terms of (i) item range (i.e., cuisine type, including complementary combinations) and (ii) healthiness (i.e., similar proportions of items that are low, medium, and high across the nutrients of concern, and high/not high in energy). Ready meals that contained >1 portion, starters/sides that contained >2 portions, and desserts that contained more than >1 portion and portions that were not clearly designated (e.g., individual slices, packs) were excluded to reduce variability in item size within each category, as this could add noise to intake outcomes. The name, price, and image of the item will be displayed, consistent with the Tesco website. Item images can be clicked to display further information, such as the product description, ingredients, and nutritional information (including enlarged label), aligned with information that is presented on the Tesco website.

Participants will either be recruited via the online participant panel platform "Prolific" or via social media/a database maintained by researchers at the University of Liverpool. Participants will answer brief demographic questions, including age, sex, gender, highest education level, and annual post-tax household income (open response). Any participants completing the survey outside of Prolific will be asked to create a personal ID (first name initial, last name initial, last two digits of their phone number) which they will enter in both this survey and the follow-up survey, so that their responses can be linked. For Prolific participants, they will enter their unique Prolific ID. An attention check prompting participants to select a specific answer will be included here.

After answering demographic questions, participants will be shown instructions for the food ordering task: On the next page, a Tesco-style food selection interface will be displayed. Please choose the item(s) for yourself to be consumed for an evening meal within the next week. You are required to select one main, and can also optionally select a side, and/or a dessert. Bear in mind that you will be required to actually order, purchase, and consume these item(s).

Once participants have read the instructions, they will proceed to the Tesco-style food selection interface. Participants will be required to select one main, and, optionally, a side and/or dessert, indicated again by an on-screen "shopping list" that ticks off items as they are selected. Participants can add/remove items, navigate back-and-forth between main, side, and dessert options when making their selections, and see their selections populate a "basket" before checking out, in an interface that has been built to closely resemble the Tesco website.