When straighteners first came out, I remember people saying "don't use them more than once a week", then it was a couple of times a week, now it's supposedly ok to use them every other day...soon people will say 2-3 times a day is alright as well! I personally think 2 times week is the most you should use straighteners.
Basically, once your hair is out of your head, it's dead, so eating healthily won't make much difference, it'll only help the hair that hasn't grown through yet. Similarly, you can't give vitamins to something which is already dead anyway! Although a rich conditioner will definitely help protect against any future damage.