Verifying the dietary authenticity of grass-fed meat or milk

How can we verify that meat or milk with the label "grass fed" actually comes from grass-fed animals? Here, we present the results of a literature review that examined current methods for verifying the dietary origins of grass-fed meat or milk from cattle or sheep. We discuss the results that have been obtained using different methods, which have been applied either in contrasting dietary situations or in more complex dietary situations (e.g., when dietary regimes are alternated or when grass represents just one part of the ration). We also discuss the utility of these methods, their specific challenges (e.g., variability in dietary conditions or in animal responses), their limitations, and the benefits derived from combining them. This work has highlighted new perspectives, but it is necessary to increase its breadth and the robustness.