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:Although, MPOD is often positively associated with dietary and serum lutein and zeaxanthin concentrations [26], conflicting results have been reported on the influence of sex [22], age [8], BMI [2,31], and other postprandial or environmental factors (e.
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