07 Jun Protein quality as a nutritional functional unit: new insights and limitations
Life Cycle Assessment (LCA) is the current tool used to quantitatively measure environmental impacts resulting from human activities. Often, the output of an assessment is to calculate an environmental impact per functional unit. For example, an assessment for a food item might derive the kg of CO2-equivalents produced per 100g protein in the food.
To estimate the implication of diets on environmental impact, an added component of nutritional science has been included in LCA. This is known as nLCA, as covered in two previous Thought for Food articles (Simplifying Nutrition Metrics; Environmental Cost of Foods). While this incorporation provides a means to evaluate both the nutritional and environmental impact of different dietary patterns, there are many limitations; the most significant among these is the simplification of nutrition to a single value, thereby losing much information.
A recent paper in The International Journal of Life Cycle Assessment presented findings from a hypothetical case study using the Digestible Indispensable Amino Acid Score (DIAAS) in the functional unit to adjust for the bioavailable protein in different foods.
Results showed that when the correction was incorporated, some plant-sourced proteins had a greater environmental impact per quality-corrected protein unit than when simple protein content was used. Wheat, with its relatively low DIAAS score, had a 57% higher emissions and land use impact when corrected for protein quality. In comparison, dairy beef’s emissions reduced from 17 to 11.9kg CO2-eq/100g protein and its land use reduced from 22 to 15.4 m2*year/100g protein after the adjustment. Soy’s footprint also came down, due to its high DIAAS score.
Whether or not protein content should be assessed is dependent on the purpose of the assessment. This study makes the case that, if protein is an essential part of the comparison, then a protein-quality-adjusted metric should be utilised in the interpretation of results.
However, the use of DIAAS as a correction factor is not without limitations. For example, DIAAS is thought to be a less representative scoring mechanism for plant-sourced than for animal-sourced proteins, often based on data for protein isolates. Nonetheless, the study has found that adjusting nLCA for bioavailability has a significant effect on relative environmental impacts of food commodities, suggesting this approach as an important future direction.
This SNippet was written by Patricia Soh, a PhD Fellow in the SNi team.
Photo adapted from Ekaterina Novitskaya on Unsplash