Foods to avoid during nursing:

1. Non-Kosher foods:[1]

A nursing mother is to especially avoid eating non-Kosher foods.[2] Thus, one is not to give their child to be nursed by a woman who does not keep Kosher, even if she is a Jew. Even if a woman has a Halachic allowance to eat non-Kosher food due to medical reasons, nevertheless, she should not nurse her child after eating such foods. Rather, she should hire a wet nurse who keeps Kosher to nurse her child.[3]

The child:[4] Certainly, the child himself is not to be given non-Kosher food to eat, as such food will damage the child when he gets older. [This applies even to food that is only Rabbinically forbidden, and hence permitted from the letter of the law to be fed to a child according to some opinions[5], nonetheless, one should not do so, and should protest his child from eating such food, due to the spiritual damage that it can cause his mind and heart, creating bad traits within him.[6] Furthermore, due to this reason, one should only feed their child the most Mehadrin of foods, and not rely on various Halachic leniencies for permitting the food even if the food contains a nutritional benefit for the child, so long as it is not absolutely necessary for his general health.[7] Thus even snacks and treats, such as candies and cookies and chips should be of the highest levels of Kashrus.[8]] 

1. Foods to avoid during nursing:[10]

A nursing mother should only eat foods that are beneficial for her milk [and healthy for the spiritual growth of the child].[11] Thus, she should avoid the following foods:

1. Nursing women are to especially avoid eating the hearts of animals.[12]
2. Nursing women are to especially avoid eating liver.[13]
3. Some sources record that nursing women are to avoid eating squash of various types.[14]
4. Some sources record that nursing women are to avoid eating melons of various types.[15]
5. Some sources record that nursing women are to avoid eating onions.[16]
6. Some sources record that nursing women are to avoid eating Garlic.

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[1] See Shevach Habris 3:2; Toras Hayoledes 46

[2] Rama Y.D. 81:7; Taz Y.D. 81:12; Shach Y.D. 81:24; Arugas Habosem O.C. 138; Sefer Shemiras Haguf Vihanefesh [Lerner] 142:1; 147:2

[3] Shach Y.D. 81:24; P”M 81 M”Z 12; Sefer Shemiras Haguf Vihanefesh [Lerner] 147:3

[4] Rama Y.D. 81:7; Levush 81:7; Shevach Habris 3:3

[5] See Shach 81:26; Admur 343:5-6;

Background: If there is a necessity for the child, for his behalf, to eat Rabbinically forbidden foods then it is disputed amongst Poskim whether one may or may not feed him the food. Some Poskim rule that it is forbidden for any person to give even a Rabbinically forbidden item to a child with his hands, even if the child is completely below the age of comprehension, and even if the child is in need for it. This applies even if the child is sick, so long as he is not in danger. Thus regarding feeding a child a prohibition they have the same status as an adult irrelevant of the child’s age. [First Stam Opinion in Admur and only opinion in Michaber/Rama 343; Rambam] Other Poskim rule it is permitted to feed a child, even above the age of education, any Rabbinical prohibition so long as it is being done for the need of the child, such as for the child to eat and drink, and there are not other foods available. [Second Opinion in Admur ibid; Ran; Rashba; Shach 81:26] Practically, one may rely on the lenient opinion to feed a child a Rabbinical prohibition [even if the child is above the age of education] if the child needs it and no gentile is available to feed the child himself.  However if a gentile is available to feed the child the prohibition then it is forbidden for a Jew to feed it to him, even for the need of the child, even if the child is sick. It is always forbidden to feed the child a Biblical prohibition. [Admur ibid]

[6] Shach Y.D. 81:26

[7] Igros Kodesh 18:109; Shulchan Menachem 4:8

[8] See Koveitz Likkutei Dinim [Greenglass] p. 67; Shevach Habris footnote 3

[9] One who is slightly ill and is in need for milk products may be lenient to recite an after blessing, wash out the mouth, and wait one hour between milk and meat: Chochmas Adam 40:11; Igros Kodesh Rashab Halacha 35; Aruch Hashulchan 89:6; Amudei Hashulchan on Kitzur SHU”A 4; Ben Ish Chaiy Shelach 11; Zivcheiy Tzedek 89:11; Darkei Teshuvah 89:20; Kaf Hachaim 87:7 and 21 and 26; So rule regarding Meiy Chalav: Chasam Sofer Y.D. 73; Pischeiy Teshuvah 87:3; Nitei Gavriel 66:33; Toras Hayoledes 69:4

[10] See Segulas Yisrael Mareches Mem 59; Keser Torah Eitz Chaim 9:24; Sefer Shemiras Haguf Vihanefesh [Lerner] 11 and 147:5 footnote 10

[11] Pela Yoeitz Erech Yonkei Shadayim

[12] Sefer Zechira Inyanei Zivug; Ben Ish Chaiy Acharei 2:11

[13] Ben Ish Chaiy Acharei 2:11; Sefer Zechira Inyanei Netilas Yadayim

[14] Rashi on Bamidbar 11:5

[15] Rashi on Bamidbar 11:5

[16] Sefer Hanhagas Yesharos of Arizal; Keser Torah Eitz Chaim 9:24; Segulos Yisrael ibid