When I was down around 8% body fat, and I'm just guessing based on vascularity and definition according to the pictorial, I was seriously struggling for energy at the gym. I was getting through the sessions, but it was taxing as all shit and I had nowhere near the workout intensity that I had before I started eating "clean" (read: lean meats and vegetables, no to very little junk food).
Also caught a lot more colds - that could just be coincidence, but since I'm now around 11-12%, I feel a whole lot healthier in general. Would probably feel even better with more...humans are meant to have fat, just not too much or too little.
So I don't think it's sustainable in the long term. Pro bodybuilders are seriously fatigued come comp day - add dehydration to the lean body sapped of energy and it's a wonder more don't just faint under the spot lights. But then, don't some professional basketball players hover around 6-7% or something ridiculous for a year round season? Guessing it makes a difference how much muscle mass you have too, as I can only imagine lots of muscle would starve your body of energy?