Do you mean fasting from water or fasting from everything except water?
I believe you need to take into consideration any health issues that may be involved. And spiritual direction is very important as well…particularly when it involves something a bit more radical than usual.
I think he meant drinking only water- not just any liquid. I don’t recommend that- at least not for more than a day. Taking in nothing but water for a day will lower your blood sugar, and if you are not healthy, you have a very high metabolism, or you go more than a day (or even just a day, for some people), I imagine you could pass out. This is not the sort of thing a person should do without the permission of a good spiritual director, and without making sure you are in good health. God does not want you to kill yourself doing penance. Also remember that the body is not an evil thing- you have to control it with your mind and soul, rather than letting your body control those things, but it is also God’s creation. Do not fall into that early heresy that said that the body was evil.Don’t forget to consult your doctor as well as your spiritual director to check that you have no health problems.
If you’re sufficiently healthy, though, spending a day or even two or three on a liquid diet shouldn’t do too much harm at all.
You can get an awful lot of nourishment (not to mention calories) into a body in liquid form, after all - think of hearty soups, milkshakes, hot chocolate, fruit and vegetable juices … even alcohol if you so wish.
Ahh, my apologies. Yes, that is what he meant. And no, it’s not a good idea, even for a day, without seeking medical advice.