(covers information from several alternate timelines)
A calorie was a measurement of the energy derived from foods.
In 2365, so as not to embarrass Worf, Doctor Katherine Pulaski chalked up his fainting spell to him taking part in a Klingon ritual involving fasting, saying he had overexerted himself beyond what his caloric intake could handle. In reality, he was suffering from rop'ngor, a Klingon childhood disease. (TNG: "Up The Long Ladder")
In 2370, Jadzia Dax devoured a piece of Andorian tuber root Benjamin Sisko had left on his plate, claiming she would need the extra calories to keep up with the hardworking Gideon Seyetik. (DS9: "Second Sight")
In an alternate 2379, Neelix offered Kes Jiballian fudge cake at her ninth birthday party, while acknowledging that she should have been watching her calories. (VOY: "Before and After")