Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:We previously found that feeding a diet depleted of amino acids [protein-free diet (PFD)] to mice for 5 days decreases the total protein content and alters the protein pattern of the liver [14, 38, 42, 47, 48].
Category: