Done it for a week and also for 72 hours. I did no heavy workouts though. I do intermittent fasting since over 2 years (not super strikt, if I feel like I want breakfast and dinner, I do it, maybe once a week). My body had no problem adjusting to full fasting. Otherwise I would take it very slowly, since the feeling of hunger and focus on food in your thoughts might be very distracting. Oh, and I continued taking electrolytes during fasting.