Food-System Life Cycle Assessment

The functional unit is the common basis on which all impacts are expressed, so it decides what 'fair comparison' means. Comparing foods per kilogram, per calorie, and per gram of protein can yield different rankings, because a nutrient-dense food may look worse per kilogram but better per unit of nutrition delivered. If the functional unit does not reflect the role the food actually plays in the diet, the comparison can mislead. ISO 14040 makes defining the functional unit a formal first step precisely because it, together with the system boundary, determines whether two foods or diets are being judged on equal terms.

It provided the largest consolidated food-LCA dataset to date — around 38,700 farms across many countries and products — and drew two influential conclusions. First, impacts vary as much as fiftyfold among producers of the very same product, so how and where a food is produced matters enormously and single averages can mislead. Second, even the lowest-impact animal products typically have larger footprints than plant-based substitutes across several indicators, strengthening the evidence for dietary change. The work also illustrated how sensitive results are to allocation and method, reinforcing the need for transparent, multi-indicator, uncertainty-aware assessment.

Because foods trade off across environmental dimensions. A product can have low greenhouse-gas emissions yet demand large amounts of land or water, or cause significant eutrophication through nutrient runoff. Reporting only carbon would hide these burdens and could endorse a food that is harmful on other fronts. LCA therefore characterises the inventory into multiple categories — global warming potential, land use, water use, eutrophication, acidification — so decision-makers see the full set of trade-offs. Limiting attention to a single metric is one of the most common and consequential errors in food-footprint analysis.