I think 13 is possibly maintainable (for me at least). I was at 13 last year and I could have still gone down more if I wanted to - Trouble is we moved city's and it just messed up my whole routine. Im back in the swing of things again looking to get back to 13 once again
I only have to go below 20% or train very heavy and I lose mine. I have to take the BCP now because not having periods was causing me to lose bone density.i initially lost them because i was underweight but have never been able to regain them. am now a bit worried about osteoporosis.just be careful!