Prune juice

Prune juice was an Earth beverage, made from the juice of dried plums. It is noted for being rich in fiber.

Admiral Luther Whitetree offered prune juice to Klingon Captain Krenn while visiting the Federation, though he quickly thought better of it, noting that the Medical Corps would object. 

Guinan introduced Worf to prune juice in 2366. He pronounced it "a warrior's drink," and it thereafter became a favorite drink of his. (, et. al.)