I care more about my running than my health if the two conflict. However, I don't think it's really sustainable to run at a body fat percentage that actually is unhealthy for you (you might get faster temporarily, but FWIG you'll probably eventually break down and lose a lot in the long run).
But yeah, when I say optimal, I mean optimal for performance. This is why trying to hide the fact that I'd be faster with a leaner composition (I'm like 18%+ so I doubt I'm at my optimal composition) to protect my health doesn't do me any good. If you want me to stay healthy, show me how being unhealthy would impact my performance.
I know this might be a difficult mentality to understand, but to put it into perspective, if I could guarantee (ethically and legally by the rules of the sport) that I could train and get fast enough to run in the Olympics given that I'd die maybe 10, 20 years after that, I'd totally do it. I doubt I'm alone in feeling this way.