Girls! I posted on this earlier today, but I can't remember where!
Anyway, my story is when I was a teenager I had bad acanthosis nigricans under my arms - I was healthy weight, probably more at the upper end of the healthy weight range, but not overweight, but I didn't eat that well for IR, and I didn't do a lot of exercise. As I got older, I got more health conscious and ate better (cooked my own food and that helped a lot I think) and also exercised more - particularly over the last 2 or so years. A couple of months ago when I was being 'investigated' for PCOS and read that a symptom can be acanthosis nigricans, I thought, yeah, I have that - and then I looked carefully in the mirror... and noticed that the dark, horrible skin patches didn't seem that obvious anymore!!
So from my experience I would say it is possible to make it go away, or at least be much less obvious. It seems it happened to me without my even realising it... before even being diagnosed, on just diet and exercise.
All the best to you both!
Michelle
Met helping with brown, patchy skin? 
