Once you survive the first 24 hours of fasting (given you replenish electrolytes with salty water), i.e. some headache after 16-20 hours, then you can easily ad

Once you survive the first 24 hours of fasting (given you replenish electrolytes with salty water), i.e. some headache after 16-20 hours, then you can easily add another 48 hours, making it 3-in-a-row, and you can happily do high-intensity training as usually, without any effect on your performance. After 3 days you either start eating again or stop training. 3 days are recommended for "rebooting" the immune system; I try to do it once 1-2 months, not sure if it helps but I don't see any bad effects either, and it strengthens the will.