I love to deep fry fresh sardines so that I can eat them bones and all. If I use lard or coconut oil, does deep frying still cause the food to be unhealthy?
No, but I wouldn't re-use the oil afterwards. Omega 3 fats (some of which are bound to leach out into your clean oil), while super nutritious and vital, are highly unstable PUFA's, though.
I'll re-use oil in my deep fryer 4 or 5 times for starches (mostly plantains and sweet 'taters), but once it's almost time to toss it, I defrost some US Wellness Meats chicken wings for the oil's final duty. I'd arrange the same for your sardines.