Timtum Haleiv

The Gemora cites a braisa (Kesuvos 60a): A child may be nursed until he is twenty-four months. From that age and onwards, he is to be regarded as one who is nursing an abominable thing (the Rabbis forbade it); these are the words of Rabbi Eliezer. Rabbi Yehoshua said: He may nurse even for four or five years. If, however, he ceased nursing after twenty-four months and started again, he is to be regarded as one who is nursing an abominable thing.

The Rashba asks: Something that is forbidden on a Biblical level, we are not permitted to provide for a child; however, there is no prohibition to give a child something that is only Rabbinically prohibited. Why would it be forbidden to let the child nurse?

Rav Elyashiv adds: Here, there is not even a Rabbinical prohibition!

He answers: We find that there is a halachic stringency not to allow a child to nurse from an idolater; this may result in timtum. So too, when a child is beyond the age where he should be nursing, if we allow him to nurse, it can lead to timtum.

There is a discussion amongst the Poskim if one eats a permitted item in a time that it is prohibited to eat (such as Yom Kippur, prior to davening or kiddush etc.); does that eating result in timtum or not. However, in our case, Rav Yitzchak Zilberstein asks: The milk is not a forbidden item at all! If we would extract the milk and place it into a utensil, it would be permitted for consumption; the prohibition is only when the child is nursing directly from the mother. Why should we assume that this may result in timtum?