I have two friends, one gay, one straight, that have been water/pool tested at 4% body fat. The former is a weight trainer, but has a distant background as a road cyclist/skier, and the other is a runner, and a real good one at that. Both have vascularity and incredible definition, without looking emaciated. In fact, the only way to get definition, in my opinion, is to have a body fat considerably under 10%.
As to what is healthy and what isn't, we'd need a medical professional to comment. Let's say you have a body fat of 5%, though, and you eat a healthy balanced diet. I don't see why there would be a health risk. And unless you are doing no cardio, and if you are leading an active life, you would not need to be on 1800 calories a day to maintain that. My friend Tim (the former) eats more than that per day...and he can be on a calendar (and, actually, was).