Mormons fast the first Sunday of the month, called "Fast Sunday." They generally miss breakfast and lunch, so it is not a full 24 hour fast. But I remember as a teen, thinking I would die of hunger! Lol People calculate the money they saved by not eating, and then donate it to a particular church fund, to help the hungry in their area.
As I got older I developed respect for the practice of fasting, and now it is highly esteemed by the medical/nutritional community. I want to develop the discipline to do it once a week for a full 24 hours, for both physical and spiritual strengthening. As Christ said, "some demons only come out by much fasting and prayer!" And "demons" could be taken literally or symbolically, in terms of life challenges. I think fasting could be a key for me to make progress in my life and grow closer to God.
I have never fasted for 2-3 days, and I suppose that is usually done while drinking water, to keep hydrated. And for an even longer fast, a person could add something like natural fruit juices.
I find the Muslim practice during Ramadan, of only eating one meal after sundown, to be very interesting.