170 lbs is a ridiculously low number for a lot of men. I am 6'1", and do not have a heavy frame. Currently I weigh in at 211. Four years ago, when I was really into a rigorous diet and exercise routine (4 times a week for 2-1/2 hrs each session), I got down to 195. At that point, people started telling me that I looked like a late-stage AIDS victim (even my own mother).
I think if I ever got down to 170, I would probably be rushed to the emergency room and fed intravenously.