I've done water-only fasts before, and juice fasts (as a Protestant - tends to be among those that fast that way).
It actually IS very possible to go without food. I'm on a medically directed diet right now - Ketogenic, which I have moved into the "fat-adapted phase". That means the mitochondria of the cells are adapted to running on fat rather than forms of glucose, so once you get there, you have energy and no longer get hungry as long as your body has fat stores. I still eat but not because I'm hungry - only to prevent metabolizing of muscle tissue at this point.
But that's not "fasting" as a spiritual discipline. I'm actually rather unhappy not have been able to follow the usual Lenten fast, because I've learned how incredibly spiritually beneficial it is. Fasting for spiritual benefit is about discipline, denying the flesh, resisting the passions, feeding yourself spiritually instead. The body/flesh grows weaker - the spirit grows stronger. We turn to God instead of physical comfort.
What I'm doing is not fasting in the same way. Not really. It does require a lot of discipline up front but no more than I've been challenged with before. Less in fact. And the body isn't weakened - it has MORE energy and is healthier now.
Which is good and has its place in the scheme of things. But if you're doing it for a healthier body, it's not fasting - it's a diet of some sort (not limited to weight loss).
Fasting without the spiritual component may be good for the body, but is much less beneficial for the spirit.