Summary:

Environmental assessments of diets are most commonly based on average water and carbon footprint values, whereas considerably less attention has been paid to the variability of dietary patterns and the identification of the factors driving environmental pressure. Moreover, water and carbon footprints are often treated as interchangeable indicators of environmental impact, despite reflecting different mechanisms associated with food production and consumption. The aim of this study was to identify the main sources of environmental pressure arising from individual dietary choices through a comparative analysis of the water and carbon footprints of university students' diets and an assessment of their weekly variability. The study was based on seven-day food diaries. For each participant, the mean daily water and carbon footprints were calculated, along with two indicators of their weekly variability and concentration: the coefficient of variation (CV) and the Herfindahl-Hirschman Index (HHI). For both the water and carbon footprints, the average environmental footprint was found to be largely independent of its weekly variability (CV) and concentration (HHI). The CV and HHI showed strong positive correlations, suggesting that they describe related but distinct aspects of the weekly distribution of environmental pressure. Regression models identified different determinants of the two environmental indicators, demonstrating that imported food products contributed more strongly to the water footprint, whereas animal-based products were the primary determinants of the carbon footprint. These findings indicate that water and carbon footprints should not be considered interchangeable indicators, as they capture different sources of environmental pressure associated with dietary choices. Simultaneous consideration of both indicators, together with an assessment of weekly variability, provides a more comprehensive understanding of the factors shaping the environmental impacts of diets and may serve as a valuable tool for supporting the development of more sustainable dietary patterns.