Does halacha allow letting a Jewish baby nurse from a non-Jewish woman (assuming Jewish wet nurses are available)?
The Gemara says that Baby Moses refused to nurse from a non-Jewish woman, saying, "Shall a mouth that will speak with the Divine Presence nurse impure milk?" [Sotah 12b] Does this teaching apply only to those destined to speak to God face-to-face, or to everyone?
(This is not really a chalav Yisrael issue. But on the other hand, the Ramban says, "You are what you eat.")
The Chayei Adam in 66:14 writes a two sentence Halacha:
According to the letter of the letter of the law, one is allowed to have a non-Jew nurse a Jewish child, but if possible, one should avoid allowing a non-Jew nurse a Jewish child since it 'taints the heart'.
Similarly, a Jewish woman who needs to eat forbidden foods for health purposes should hire another [Jewish] woman [to nurse the child during that time].
מדינא מותר לתינוק ישראל להניק מנכרית מ״מ אם אפש ר ע״י ישראלית לא יניחו להניקו מנכרית דמטמטם הלב ומוליד מזג רע וכן מינקת ישראל שצריכה לאכול מאכלי איסור לרפואה ל א יניחנה לינק אלא ישכיר אחרת
Interestingly, the first sentence is not found in most editions of the Chayei Adam (the quote above can be found in this edition). Most editions of the Chayei Adam just have the second sentence (that a Jewish woman who has to eat non-Kosher should not nurse her child). It has been speculated that the first half of the Siman was removed in later editions in order to placate the censors.
The Jewish Press explains:
The Gemara rules that a Jewish baby may nurse from a non-Jewish woman. It also rules (Yevamos 114a) that a Jewish baby may nurse from a non-kosher animal. As a result, Rabbeinu Chananel states that this is a case of Pikuach Nefesh. That is that a baby is always considered in a state of pikuach nefesh and would thus be allowed to have non-kosher food.
There are those who say that mothers' milk is never considered non-kosher (just as it is not considered dairy). However, the reason for avoiding the milk of a non-Jewish woman is for hashkafic reasons.
Rashi and Ritva explain that the milk of a woman who eats non-kosher food will affect the infant. Thus, a nonJewish woman who only eats kosher food (while she is nursing) will avoid this problem. A Jewish woman who must eat non-kosher food for medical reasons (so it is permitted) should still not nurse the infant.
Rashba and Meiri argue that the milk of a nonJewish woman will engender the traits of the goyim in the baby. Thus, even if the gentile only eats kosher food, the nursing should be avoided. One should nurse from a Jewish woman instead who will instill in the child the signature Jewish character traits of mercy (rachmanim), modesty (baishanim), and kindness (gomlei chasadim). Thus, they would allow a Jewish women who is forced to eat non-kosher food for health reasons to nursa the baby.
The Rama (Shulchan Aruch, Yoreh Deah 81:7) rules that a child should always avoid nursing from a non-Jewish woman when it is possible to nurse from a Jewish woman. The Rema quotes the Rashba’s reasoning that the nature of non-Jewish women will affect the child. The Rama also cites an opinion that a Jewish woman who is forced to eat non-kosher foods should refrain from nursing. Thus, he says to follow the stricter opinion of both possibilities.
