Nursing while pg?

DD2 is 19 months now. We just found out we're expecting yet another. I
just found out the other day, so it's still real early.
DD2 is still nursing, and still nursing strong. She nurses usually 3-4
times a day (not including if and when she's REAL tired or manages to hurt
herself or something) 3-4 good nursing sessions per day.
I have one friend that isn't really big on breastfeeding after 12 months,
although we've always maintained to each their own and it's never been an
issue at all. She did say something that did make me wonder, however... She
said something along the lines of how DD2 *has* to quit the nursing now that
I'm pg again. I asked why I would do that, seeing as she still is young and
as far as *I* am concerned, she's not too big or old (again, in my eyes) to
nurse, nor does she show any signs at all of wanting to wean.
Friend then said something about how DD2 nursing will take away from this
next one and (oh the horrors) how it won't get anything it really needs to
develop and grow because DD2 will be taking all of that with nursing.
I have another friend that is big on the breastfeeding. She nurses her 2
youngest (they're about 2 and 4 now) and she's pregnant - due late June or
early July. I am going to ask her about it, as I do feel her knowledge on
the whole subject is pretty good (she runs a breasfeeding support group in
my city) but I really haven't been talking to her very much lately as
everyone always so busy.
Should I change my diet to allow for DD2's breastfeeding plus the pregnancy?
Is there anything I should add to my diet or anything? Or does nursing
really not have too much of an affect while pregnant?
Anyone familiar with this kind of stuff?