(Courtesy of Puah) “Pregnant and nursing mothers must fast the entire fast on Yom Kippur” (Shulchan Aruch. Orach Chaim 717:1).
Most nursing mothers find it difficult to fast, due to the increased loss of fluids. Nevertheless, they are obligated to fast on Yom Kippur as long as it doesn’t pose a threat to the life of the mother or the baby.
What constitutes a life threat? For most healthy babies, the fact that the milk supply will become lower is not considered life-threatening. Even if the baby drinks less milk than usual, as long as he is receiving enough to survive, the mother is still obligated to fast (Har Tzvi, Orach Chaim 1:301). There are rare circumstances when a doctor may determine that the baby will indeed be in danger if the mother’s fasting causes a significant dwindling of her milk supply, and then—only with the instruction of a doctor who is also a yarei shamayim—she should drink small, halachically measured amounts (“shiurim”).
Where does baby formula come into play? Nowadays, there are many organic infant formula available, and it would seem that these can easily supplement the baby’s feedings on a fast day. However, medical experts in the field have long promulgated the clear advantages of mother’s milk, both in its nutritional and immunological values, and there is no comparison to formula substitutes. Therefore, many poskim feel that there’s no need to take the availability of baby formula into account, at least when dealing with a baby who is exclusively nursing, when determining whether the mother’s reduced milk supply is a reason for her to drink in shiurim (Rav Shlomo Zalman Auerbach, Halichot Shlomo).
However, practically speaking, in almost all cases there should be no need for a woman to use this heter. Even the Israeli Association of Lactation Consultants supports the idea of a nursing woman fasting, by publicizing important tips for getting through the fast well—such as drinking large quantities of water the day before the fast, and resting as much as possible. It’s also a good idea to pump breastmilk in advance, and store the milk for use during the fast.
It’s important to remember that, on Yom Kippur, fasting takes precedence to davening in shul. This means that if a woman needs her husband to help her take care of her young children so that she can make it through the fast, then the husband must do so, and do whatever he can to make her as comfortable as possible.
Even for an exclusively nursing mother, having formula on hand is a good idea, even only for its psychological value: If the mother knows that she has a backup available, it can give her the peace of mind she needs to get through the day—much like in the days of the Beit HaMikdash, when they would offer bread and water to the man whose job it was to lead the sacrificial goat out into the desert. He never actually needed to break his fast—but just knowing that the drink and food was available was enough to enable him to make the journey into the desert.
May Hashem accept our fast lovingly, and forgive us all our sins.