So, this guy wrote an article called "Women Should Not Run," and it's backed up by a ton of references from exercise/physiology journals. The gist of it is that an excess of "steady-state" cardio (not HIIT, but just you treadmilling all of the time) pushes your body into a sort of survival mode and pushes you into a form of hypothyroidism.
I thought that this was pretty compelling, but then again, I am not addicted to running as I have knee problems.
Just thought I'd throw this link out there, for those interested.
http://www.dangerouslyhardcore.com/5...hould-not-run/