As the urgency to tackle climate change and public health issues intensifies, so does the need to encourage people to make sustainable and healthy food choices. Grocery shopping is a frequent, routine activity that shapes diets, and therefore the health of both people and the planet. The SALIENT Sustained field trial, led by researchers at the University of Warwick and Queen Mary University of London, will evaluate practical ways to guide consumers toward sustainable food choices when shopping online. The trial uses a real-world online grocery shopping environment to test two consumer-focused strategies: eco-labels and price discounts for environmentally friendly food options. Given that many eco-friendly choices align with healthier eating patterns, the interventions could promote both environmental sustainability and healthier eating.

Why Online Grocery Shopping?

Online grocery shopping, now representing 12% of the UK grocery market, presents a valuable platform for intervention. In contrast to traditional, in-store shopping, the online environment allows researchers to quickly and cheaply implement interventions, and monitor consumer purchasing, making it an ideal testing ground.

Most previous studies on sustainable grocery shopping have been limited to simulated environments, in which shoppers often make different choices than they would in their daily lives. We use a browser extension, developed in collaboration with tech company Sustained, that enables us to run the trial on the website of a major UK supermarket. Consumers participate in the trial by shopping as they usually would, creating an accurate picture of how real consumers respond to our interventions.

The Interventions: Eco-Labels and Price Discounts

The study tests two key interventions:

1. Eco-Labels: One group of participants will see eco-scores on a range of products, rated from A (best) to G (worst) based on their environmental impact. These ratings are grounded in comprehensive life-cycle assessments, which measure the overall ecological footprint of each product. Eco-labels provide shoppers with clear, at-a-glance information about the sustainability of their choices. Providing information alone is often weak at changing behaviour. However, France has already mandated climate impact labels on grocery products, and we hope this study will offer the UK government valuable insights into how a similar policy might influence British consumers.
2. Price Discounts on Sustainable Alternatives: All participants will be offered recommendations for sustainable alternatives to some of the items in their shopping basket. For instance, if someone adds a traditional yogurt to their basket, the system might suggest a plant-based yogurt alternative. Participants in the treatment arms of the trial will be offered discounts of £0.50 or £1.00 on these sustainable swaps. Members of the public consulted about this research frequently mentioned that cost is a main factor they consider when buying groceries, so we expect this to have a larger effect than the eco-labels. By reducing the cost barrier to sustainable choices by different degrees for different consumers, we aim to estimate the willingness to pay for more sustainable groceries, and gain insight into how important affordability is as a barrier for consumers to try new, eco-friendly products.

How are we planning to implement the trial?

Participants in the trial are recruited from Prolific, a UK-based platform for online research. Eligible participants must regularly shop online and be the primary grocery shopper in their household. Once enrolled, they install the browser extension, which applies the assigned interventions over the course of an eight-week shopping period. During this time, the extension tracks their activity on the supermarket website, including the products they purchase. These data will allow us to analyse the impact of eco-labels and discounts on purchases and diets.

The study uses an adaptive design model, a flexible research approach that enables us to adjust the sample size across different recruitment waves based on early findings. This means that if initial results indicate significant effects, the sample size may be optimized accordingly, maximizing resource efficiency.

Study outcomes

This study seeks to answer two main questions:

1. Will eco-labels and/or price discounts encourage shoppers to choose more sustainable products?
2. What impact might these interventions have on the nutritional quality of grocery baskets?

Policy implications: Supporting Health and Sustainability

The Sustained trial is funded as part of the SALIENT research programme, led by the Universities of Oxford and Cambridge. This research programme, which was announced in the government’s response to Henry Dimbleby’s independent review of the food system, contributes to the evidence base to support government policy to promote healthier diets and reduce the environmental impact of the food system. This research is co-funded by several government departments, including the Department for Environment, Food and Rural Affairs (DEFRA) and the Food Standards Agency (FSA), through the ESRC.

The clearest lessons from this work will be for online grocery shopping, but it seems likely that some of the findings would carry over to policies that extend into in-store grocery shopping. If the interventions are found to be effective, scaled up versions implemented by the government have the potential to contribute to a healthier population and a more resilient planet.

Broader implications and further research

We believe that this field trial is more than another study into specific interventions in the food system. We provide a blueprint for how researchers can explore food system interventions involving large and powerful actors in the food system without requiring their collaboration or even permission. Our browser extension allows us to “look over the shoulder” of our participants while they go about their day-to-day shopping. Since these shoppers own these data just as much as the supermarket does, we do not rely on the retailer making the data available to us. It is difficult to imagine transformative change in the food system that does not involve powerful actors like major retailers, and we therefore think that this study is just the start of an impactful research agenda contributing to a healthier, more sustainable world.