I read something on another forum on another site where someone stated that, no matter what kind of junk diet you ate, if you were breastfeeding then baby would still get high quality milk and everything they need. It was even asserted that fizzy drinks would not affect baby (but I was under the impression that it caused a problem with calcium). It just doesn't sound right to me: is it true?

Vitamin D supplementation during lactation to support infant and mother. Anyone interested in ensuring human breast milk is the best most complete food for babies needs to be aware that for breast milk to be replete with vitamin D3 the mother needs to maintain a 25(OH)D3 level above 58.8 ng/ml around 150nmol/l.
That level, at latitude 32N, requires 6400iu/daily D3 supplementation. Further north requires more.
That research also showed that DAILY supplementation was required to ensure an even supply of D3 to the fetus and baby.
When they just used 4000iu/daily babies were still being born with less D3 than required for optimum calcium absorption and Bone Mineral Density.

this report suggests that the mercury they found in almost all the sample of junk food tested came from HFCS and could be passed from the mother to the baby in breast milk.
Not So Sweet: Missing Mercury and High Fructose Corn Syrup

I also just read in Dr. Kwasniewski's book, Homo Optimus that the fat content can vary widely, and of course the lower end of fat content is in more malnourished mothers. If I get time, I will try to repost exactly what he stated.

No one can dispute that mother's milk is the ideal nutrition, as far as the biochemical composition is concerned. It contains three to 11 grams of fat per one gram of protein (0.4% unsaturated fat). The conclusion is obvious - if Nature included such a minute quantity of that constituent in such a wonderful food, then we should respect it. More at the above link.

I have always been curious about this. I have breastfed two children, DD(3) weaned herself at 3.5 months (bottles when I went back to work spoiled her.) DD (11 months) weaned around 6.5 months, I wish she still nursed. I never touched alcohol, avoided cigarette smoke and while I wont say I avoided all junk, I made sure to eat healthy nutrient rich foods especially when breastmilk was the only source of nutrition.
I believe breastmilk to be the best food on earth It is what babies are meant to start on, but that being said mothers aren't meant to drink soda and eat chips...
I will be honest..I have never been able to kick the soda habit, beit diet or regular so I am not condemning soda even during pregnancy and breastfeeding...I will not be a hypocrit, but I did eat/drink healthy for the most part.

I've always found this site to have a lot of good information:

http://kellymom.com/nutrition/mom/mom-diet.html

http://kellymom.com/nutrition/milk/change-milkfat.html

This is only my own personal experience...

DS #1, I ate healthy and avoided things that would possibly cause him gas or upset tummy. I felt good too! He was a great baby, rarely fussed, ate, slept, got nice and chubby!
DS#2, I ate crap. I ate junk and whatever was quick (ie: bowl of cereal for lunch). He was not a fun baby - fussy, projectile vomitting, woke up round the clock to eat for months... (he still got chubby and it's a good thing he was cute!)

So, do I think think "junk in, good out"? No. I say "junk in, junk out" You won't be doing yourself or your baby any favours by consuming junk, I know too many moms who complain about being sleep deprived and their nursing babies never sleep, but they drink 4 large caffeinated latte's a day... no brainer to me!

Personally, I wish I paid more attention (I was in survival mode and dealing with the loss of a family member). I later realized that DS#2 has a milk intolerance and what I was eating didn't help him while I was nursing... I have some guilt about it all...

I was always led to believe (by the nurses, LC's, docs...) that what you consume comes out in the breast milk.