I did fast (no drink nor food) about 30 days in a row every year, for about 10 years or so. I usually fast from around 4 am to 6 pm. Sometimes it can be very exhausting as I had to cycle to school in the heat of the day. I never really got very sick because of it.
As I got older my religious view changed gradually and I no longer fast anymore... but now I sometime feel my digestion organs need a break and sometime I deliberately skip a meal if I don't feel really hungry. I only eat when I'm hungry and never eat too much.