A fairer unit of comparison

A kilogram of food is not a useful like-for-like comparison when foods contain very different amounts of protein. The more meaningful question is what resources and emissions are associated with delivering a defined amount of protein.

This guide uses a functional unit of 100 grams of protein as a starting point. It helps compare the protein contribution of different foods, but it does not make every food nutritionally interchangeable. Amino-acid profile, digestibility, micronutrients, portion size and the rest of a diet still matter.

Three footprints, three questions

Climate impact is usually reported as greenhouse gases expressed as carbon-dioxide equivalents. Land use asks how much land is occupied or required. Water metrics need special care: water withdrawn is not the same as water consumed, and a litre used in a water-scarce basin has a different local consequence from a litre used where water is abundant.

There is no single “green” score that can hide these trade-offs. A route can look favourable on one measure and less favourable on another, and the answer can change with location and production method.

A comparison is only as good as its boundary

Life-cycle assessments make choices about what to include: farming or factory energy, land-use change, processing, transport, co-products and the point at which a product is counted. Studies using different boundaries can produce results that should not be placed side by side without explanation.

For newer protein technologies, independent data are often thinner than for established foods. Where evidence is not comparable, we will say so rather than draw a precise-looking bar from incompatible studies.

What “greener” means here

It means asking what is measured, what is being replaced and how certain the evidence is. It does not mean assuming every new technology is better, or that livestock has no place in a lower-impact food system.

The aim is to make the comparison understandable and honest: protein first, context always, and a source beside every important claim.