Can anyone explain to me why the word über is used in this sentence:
Frauen reagieren eher als Männer auf neue Erkenntnisse über nützliche und schädliche Lebensmittel und orientieren sich gesünder.
"Über" is needed to inform the reader about which topic they gained new knowledge (in this case use and harmful food):
to gain knowledge about sth.
Erkenntnisse über etw. erlangen
"Über" has different meanings for many cases as you can see here (german source) or here (english source). The meaning depends on the context the word appears in.
