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:Both in vitro (15, 51) and in vivo (33) studies have shown that fatty acids acutely enhance glucose-induced and tolbutamide-stimulated (27) insulin secretion and play an important role in the maintenance of basal insulin secretion in the fasting but not in the fed state (18, 19, 42, 43).
Category: