Protein quality in life cycle assessment

Life cycle assessment (LCA) of food often uses mass of food as a functional unit, i.e. environmental impacts are reported per kg of food. Another common functional unit is weight of protein contained in the food, which is important for comparing different sources of protein, notably meat and its alternatives. This paper goes one step further and examines how protein quality, as opposed to protein quantity, can be accounted for in food LCAs, by using a scoring system to reflect the availability and digestibility of indispensable (i.e. essential) amino acids.

The paper illustrates its method for four animal-sourced foods (beef from dairy herds, cheese, eggs and pork) and four plant-sourced foods (nuts, peas, tofu and wheat). As shown in the figure below, both the carbon footprint (GWP100) and land use of the four animal-sourced products appear lower when calculated with the new method (the orange bars - i.e. when accounting for amino acid content) than when calculated according to total grams of protein (the blue bars). This is because the four foods score highly for amino acids (shown in Table 2 of the original paper). 

Similarly, the moderately high amino acid score for tofu means that impacts appear slightly lower for tofu under the new scoring system than for total protein. The other three plant-based foods, having lower amino acid scores, see an increase in their apparent impacts under the new scoring system. Nevertheless, the carbon footprints of all four plant-based foods remain lower than those of all four animal-sourced foods. With land use, the new method means that there is more overlap in the scores of the two food categories than before - for example, the land use of pork falls below that of nuts, and the land use of wheat rises above that of eggs and pork.

Image: Figure 1, McAuliffe et al. Differences in A global warming potential (GWP100; kg CO2-eq / 100 g protein) and B land use (LU; m.2*year) / 100 g protein) per product according to internationally weighted averages calculated by Poore and Nemecek (2018a) when products are either uncorrected for protein quality (No DIAAS) or corrected using untruncated DIAAS (as labelled in both graphs). DIAAS = Digestible Indispensable Amino Acid Scores.

The authors note that in future, the amino acid scoring system should be used to assess the impacts of food at the level of an entire meal rather than at the level of single food items. This is particularly important where a meal contains multiple sources of protein with different amino acid profiles. Indeed, plant-based meals commonly combine multiple different protein sources (e.g. rice and beans, or bread and peanut butter), to provide all of the essential amino acids in one meal. The author’s point is therefore particularly important for assessing the protein quality of plant-based meals, since in such meals, it is not necessary that all of the essential amino acids come from one high-quality protein source.

Abstract

Goal and theoretical commentary

A number of recent life cycle assessment (LCA) studies have concluded that animal-sourced foods should be restricted—or even avoided—within the human diet due to their relatively high environmental impacts (particularly those from ruminants) compared with other protein-rich foods (mainly protein-rich plant foods). From a nutritional point of view, however, issues such as broad nutrient bioavailability, amino acid balances, digestibility and even non-protein nutrient density (e.g., micronutrients) need to be accounted for before making such recommendations to the global population. This is especially important given the contribution of animal sourced foods to nutrient adequacy in the global South and vulnerable populations of high-income countries (e.g., children, women of reproductive age and elderly). Often, however, LCAs simplify this reality by using ‘protein’ as a functional unit in their models and basing their analyses on generic nutritional requirements. Even if a ‘nutritional functional unit’ (nFU) is utilised, it is unlikely to consider the complexities of amino acid composition and subsequent protein accretion. The discussion herein focuses on nutritional LCA (nLCA), particularly on the usefulness of nFUs such as ‘protein,’ and whether protein quality should be considered when adopting the nutrient as an (n)FU. Further, a novel and informative case study is provided to demonstrate the strengths and weaknesses of protein-quality adjustment.

Case study methods

To complement current discussions, we present an exploratory virtual experiment to determine how Digestible Indispensable Amino Acid Scores (DIAAS) might play a role in nLCA development by correcting for amino acid quality and digestibility. DIAAS is a scoring mechanism which considers the limiting indispensable amino acids (IAAs) within an IAA balance of a given food (or meal) and provides a percentage contribution relative to recommended daily intakes for IAA and subsequent protein anabolism; for clarity, we focus only on single food items (4 × animal-based products and 4 × plant-based products) in the current case exemplar. Further, we take beef as a sensitivity analysis example (which we particularly recommend when considering IAA complementarity at the meal-level) to elucidate how various cuts of the same intermediary product could affect the interpretation of nLCA results of the end-product(s).

Recommendations

First, we provide a list of suggestions which are intended to (a) assist with deciding whether protein-quality correction is necessary for a specific research question and (b) acknowledge additional uncertainties by providing mitigating opportunities to avoid misinterpretation (or worse, dis-interpretation) of protein-focused nLCA studies. We conclude that as relevant (primary) data availability from supply chain ‘gatekeepers’ (e.g., international agri-food distributors and processors) becomes more prevalent, detailed consideration of IAA provision of contrasting protein sources needs to be acknowledged—ideally quantitatively with DIAAS being one example—in nLCA studies utilising protein as a nFU. We also contend that future nLCA studies should discuss the complementarity of amino acid balances at the meal-level, as a minimum, rather than the product level when assessing protein metabolic responses of consumers. Additionally, a broader set of nutrients should ideally be included when evaluating “protein-rich foods” which provide nutrients that extend beyond amino acids, which is of particular importance when exploring dietary-level nLCA.

Reference

McAuliffe, G.A., Takahashi, T., Beal, T., Huppertz, T., Leroy, F., Buttriss, J., Collins, A.L., Drewnowski, A., McLaren, S.J., Ortenzi, F. and van der Pols, J.C., 2022. Protein quality as a complementary functional unit in life cycle assessment (LCA). The International Journal of Life Cycle Assessment, pp.1-10.

Read the full paper here. See also the TABLE report Primed for power: a short cultural history of protein as well as the blog summary Where does protein get its power?