Our food system is a major contributor to global warming while also contributing to land degradation and loss of biodiversity. Moreover, poor diets are responsible for one in seven preventable deaths annually in the UK. Addressing these issues requires dietary changes that not only reduce environmental impact but also improve public health. Therefore, there is an urgent need for effective interventions to promote both sustainable and healthier food choices for the benefit of both human and planetary health.

Online grocery shopping has become increasingly popular, a trend accelerated by the pandemic. However, there is currently little knowledge of how best to intervene in real-life online shopping environments to prompt not only healthier but more sustainable purchasing. We aim to test two interventions, eco-labels and price discounts, within the online grocery shopping environment to evaluate whether these interventions promote more sustainable and healthier food choices.

1. What is the impact of eco-labels and price discounts on more sustainable product alternatives, on the sustainability of groceries that are purchased, as measured by the average eco-score of all products in the shopping basket?
2. How do any changes in grocery purchases induced by each intervention affect the nutritional properties of purchased groceries, modelled health outcomes, and other co-benefits or unintended consequences?

1. Eco-labelling: providing participants with information on the environmental impact of their food purchases, graded from A (lowest impact, most sustainable) to G (highest impact, least sustainable).
2. Price discounts on more sustainable alternative products

The key outcome is the average eco-score of the basket of groceries purchased with a lower score meaning more sustainable purchases.  Using this outcome variable, we will estimate the effect size of introducing eco-labels (intervention 1) and the price-elasticity for sustainable alternative products (intervention 2), allowing us to calculate the willingness to pay for sustainable groceries. We aim to measure both short- and long-term responses to price discounts to see if trying a more sustainable alternative may shift demand persistently beyond a one-time purchase.

A pilot study was conducted from September to October 2023 to inform the design of the larger trial. The results of the pilot trial will be published soon (see Publications). Public involvement groups were consulted during the co-design phase, and before and after the pilot trial, to provide input on the design and implementation process of the trial, as well as quality control checks on the eco-labels. Sustained, the company that designed the browser extension is re-designing the extension to suit the trial's needs, such as including an information feature that, when clicked, will provide details on how the labels are constructed.

This will be a randomised controlled trial (RCT) implemented using an adaptive design. An adaptive design, rather than the traditional fixed sample size RCT design, was chosen because uncertainty regarding recruitment rates and attrition makes determining the sample size challenging. We will use a browser extension to implement the interventions on the online shopping website of a supermarket, accessed using the Google Chrome browser on a desktop or laptop computer.

Sales data will be obtained from electronic point-of-sale tills. The corresponding product data (product name, size, nutritional composition, etc.) will be collected from the retailer.

The process evaluation will assess the quality of implementation of the intervention and the barriers to and facilitators of implementation. The process evaluation will involve semi-structured interviews with store managers and customer-facing staff and interviews with key decision-makers.