I have never heard that....many babies are actually allergic to cow's milk protein....while I was breast feeding exculsively  and my daughter was only a month or so old cow's milk protein caused my daughter to have (lots of) blood in her stool until we figured it out, really scary, and I had to completely cut it out of my diet.  However, she also ended up being allergic to soy, (I guess some babies that are allergic to milk are also allergic to soy)....she never grew out of the milk allergy (not that we would start to drink it or something), but she quickly grew out of the soy allergy.

Most people base the nutrition advice they fling out upon assumption and rumor... go with science, I say!! Some resources:

http://zenhabits.net/soy/

http://www.bryannaclarkgrogan.com/page/page/3476771.htm

http://www.veganhealth.org/articles/soymessina

And from http://www.google.com/search?hl=&q=messina+soy+breastfeeding&sourceid=navclient-ff&rlz=1B2RNFA_enUS215US226&ie=UTF-8 :

"13. Can I consume soy while breastfeeding?"

"It has been suggested that women should avoid soy foods while breastfeeding because their babies will be exposed to high levels of oestrogenic compounds (called isoflavones) from the soybeans.

While it is true that isoflavones can mimic the hormone oestrogen, they are much weaker in their effects. Paradoxically, isoflavones can also block oestrogens when the levels of oestrogen are too high in the body. In addition, isoflavones provide many other effects such as antioxidant benefits. An overwhelming number of studies suggest that a diet rich in isoflavones protects against many chronic diseases, such as heart disease and cancer1.

Breast milk is not a good source of isoflavones, even if a woman eats a lot of soy each day. Research has shown that breast milk from mothers who regularly consume soy contains only 0.015-0.03 mg per litre2 - the daily amount consumed by a four month old infant. These are miniscule amounts! To put this into some perspective, one glass of soy milk provides around 15 mg, depending on the brand. Even vegetarian and Asian women, despite regularly including soy foods in their diet, don't produce isoflavone-rich breast milk.

On the other hand, in the first few days of lactation a baby will be exposed to high levels of oestrogens from breast milk that have accumulated in the breast during pregnancy3. But it is the environmental oestrogens - such as pesticides - that we should be concerned about as these also find their way into breast milk.

In conclusion, breast milk is a poor source of isoflavones, even if a nursing mother eats soy foods on a regular basis. There is no evidence that eating soy during breastfeeding is unsafe for the mother or infant.
REFERENCES:

  1. Radd S & Setchell K, Eat to Live, Hodder Headline 2002.
  2. Franke AA, Yu MC, Maskarinec G et al., Phytoestrogens in human biomatrices including breast milk, Biochem Soc Trans 1999;27:308-318.
  3. Sahlberg BL & Axelson M, Identification and quantitation of free and conjugated steroids in milk from lactating women, J Steroid Biochem 1986;25:379-391."

Hope that's helpful!  :)

That doesn't really make any sense! I know it isn't exactly what you're talking about but when I was a baby I HAD to drink soy formula because I was born lactose intolerant. The soy didn't do anything harmful to me and in fact, I needed it. Tell her to mind her own business and come back to you when she has a medical degree specializing in pediatrics (or as an OBGYN).

This. I too grew up lactose intolerant. I eventually grew out of it, but it is not strange that human babies don't need cow's milk. That same person wouldn't be surprised to hear that kittens don't tolerate cow's milk, so why be so shocked about soy milk?

A baby being truly lactose intolerant is unusual, usually it is one of the proteins that is the issue with milk that young.  When I took an A&P class recently, we discussed that there are only 3 separate human populations where the gene to create the enzyme to digest lactose does "shut off" at about puberty.  Two of them are from the Sahara Desert, one from northern Europe.  In all three cases, historically, the climate in those places can regularly become severe enough to make milk the only readily available food source but this is definitely a geographic anomaly.  Most of the adult population of the world can not digest lactose.  It is just a matter of how severe and if they realize it or not.  I gave up liquid milk at about 13 but still ate cultured products until recently.  I did not realize how much it was still affecting me until I cut them out and felt so much better.  I would never have said I felt bad before except with liquid milk.  it is all a matter of what we get used to..

It isn't unusual for babies to be born lactose intolerant but they usually grow out of it as they get older.

Just smile sweetly and tell her you've done your research, and you have no fear of your uterus disintegrating or falling out anytime soon. And then enjoy the look on her face.
VegWeb Classic:
EVIL SOY

Crazy, all of the doctors I've talked to must be wrong then.

I wasn't sure if you were being sarcastic or not - I apologize if you were not and this is overhashing. Any doctor who told you that lactose intolerance was common in human babies was wrong. Just a few sources (first that came up in a cursory Google search):

http://www.kellymom.com/babyconcerns/lactose-intolerance.html (I think this is one of the best evidence-based breastfeeding sites on the web)
http://www.babycenter.com/0_lactose-intolerance_1201464.bc
http://genome.wellcome.ac.uk/doc_WTX038968.html
http://sxxz.blogspot.com/2005/01/are-humans-still-evolving.html

Here is a more scholarly source:
http://www.nature.com/ejhg/journal/v13/n3/full/5201297a.html

Google "lactose intolerance in babies" or "lactase gene" or the like for hundreds more.

On the other hand, it is quite common for infants/young children to be allergic to cow's milk proteins, and they do often outgrow it. Like I said, lactose is the carbohydrate in breastmilk (and I believe all mammal milk), it wouldn't make sense for infants to commonly be intolerant to it (of course it does happen, and these infants need to be on soy formula or a lactose free cow's milk formula).

Here is one more interesting link I found that talks about the production of soy and what it is used for:

http://www.epa.gov/agriculture/ag101/cropmajor.html

This was mentioned in the above link:

"Over 30 million tons of soybean meal are consumed as livestock feed in a year. Even the hulls are used as a component of cattle feed rations."

Soy is almost impossible to completely avoid.  So even those that argue about how unhealthy it is are either eating an entirely whole foods diet growing their own food in a controlled environment (in soil that does not have livestock manure mixed in or commercial fertilizer) or they are undoubtedly getting a good dose of soy themselves. 

Thanks!  I forgot to warn that if you venture further into that last link I provided be forewarned that it is upsetting as far as what is done to the animals (I just looked at the pork production link).  There is no mention anywhere about how cruel this operation is to the animals or how they suffer as the agricultural industry doesnt view animals as any more than food commodities.  Very sad.  I hate the way the industry tries to paint a nice neat picture of the food industry and how it "operates".  Ugh.  How can milk that comes from hormone induced and abused cattle be any healthier than soy milk?