Life cycle assessment (LCA) is a standard methodology for quantifying environmental impacts of a food product, operating primarily on a mass-based functional unit (FU) (e.g., impact per kilogram of product). However, this traditional approach neglects the primary function of food: providing essential nutrients, making it fails to reflect a product's dietary quality and health impact. This has spurred a growing interest in integrating nutrition and the environmental impacts in LCA of a food product, known as nutritional LCA (n-LCA).

The application of n-LCA is particularly important in the dairy sector. While livestock production is recognized for its significant contribution to greenhouse gas emissions, dairy products are also categorized as high nutrient dense. The core challenge of n-LCA is transitioning the FU from weight to nutritional output. However, such replacement causes several methodological issues and there is at present no uniform approach for integrating nutritional value into the FU of LCA.

The first refinement shifts the focus to a specific nutrient, such as protein. For example, the carbon footprint of milk production can be calculated per 100 g of protein. This allows for direct comparison between different protein sources (e.g., milk protein vs. soy protein). However, this approach can not reflect the impact of all other essential vitamins and minerals. Advanced methods use nutrient indexes to account for multiple nutrients. Key nutrients in milk- such as protein, calcium, potassium, and vitamins A and D can be combined into a single score, with considering the dietary recommendations and the needs of vulnerable population groups. For example, the product-group-specific nutrient index has been developed as a nutritional functional unit (nFU) for the LCA of protein-rich foods. The method has been applied in comparative nutritional LCA of milk and plant based milk substitutes (Advancing methods for comparative nutritional LCA of milk and plant-based milk substitutes). Results showed that while cow's milk had the highest climate impact per unit of mass, some unfortified plant-based drinks showed higher impacts than milk when compared using nFU.

Nutritional-LCA could provide a nuanced view by assessing the environmental costs of delivering a specific nutritional profile. A consistent methodology in n-LCA is highly needed that can be applied across various food contexts to aid policymakers and consumers in making informed decisions.