The growing ubiquity of digital footprint data presents new opportunities for behavioural epidemiology and public health research. Among these, supermarket loyalty card data—passively collected records of consumer purchases—offer objective, high-frequency insights into health-related behaviours at both individual and population levels. This article explores the potential of loyalty card data to strengthen public health surveillance across four key behavioural risk domains: diet, alcohol, tobacco, and over-the-counter medication use. Drawing on recent empirical studies, we outline how these data can complement traditional epidemiological data sources by improving exposure assessment, enabling real-time trend monitoring, and supporting intervention evaluation. We also discuss critical methodological and ethical challenges, including issues of representativeness, data integration, and privacy, as well as the need for robust validation strategies. By synthesising the current evidence base and offering practical recommendations for researchers, this paper highlights how loyalty card data can be responsibly leveraged to advance behavioural risk monitoring and support the adaptation of epidemiological practice to contemporary digital data environments.

Synthetics created in the 20th century have become an evolutionary force, altering human biology and the web of life