Mind the method! A systematic literature review of the methodological choices used to assess the environmental impacts of diets

Our food system is a major driver of environmental impacts, and a large proportion of these impacts are due to animal-based foods and food loss and waste. In recent years, many studies have been published looking at the environmental impact of our diets, often in the context of a shift towards a Planetary Health Diet or other environmentally friendly diets. Each study hereby relies on a number of methodological decisions that affect the calculations. We systematically reviewed the methodological decisions made for assessing the environmental performance of diets in literature. We searched the Web of Science database and identified 301 relevant studies published between 1992 and 2022. The aim was to analyse the comparability of methodological approaches, not to evaluate results of individual studies. The review highlights methodological challenges, as many different methodological choices can be made, not all of which can be correct and can often lead to over- or underestimation of the calculated impacts. For example, half of the studies considered only one environmental concern, mostly climate change, and only a third of the assessments considered all life cycle stages. We also found a lack of transparency, particularly when it comes to how food loss and waste are taken into account. Unless we get our methodologies right and aligned, we risk not measuring the right things and masking trade-offs between impacts in impact categories or life cycle stages. We therefore call for greater transparency and a more standardised and holistic assessment of the environmental performance of diets. • Methodological decisions in dietary impact calculations are often untransparent • This is particularly true for how food loss and waste (FLW) is considered • There is potential to under-/overestimate dietary impacts and ignore trade-offs • Importance of interplay between dietary data, system boundaries and FLW • Need for better awareness and a more harmonised approach