Abstract: The Jewish calender has six fast days, which have different levels of stringency. On Yom Kippur and the Ninth of Av it is assumed that healthy women nursing healthy babies will undergo a 24 hour fast (no eating or drinking). Most mothers and babies will not experience negative consequences. However, the physician should convey any specific medical concerns to the woman and her halachic advisor so appropriate arrangements can be made. On the other fasts (which last only from dawn to nightfall), nursing women are not required to fast, although, according to some opinions, they may do so. If the physician feels a woman should not fast on the minor fast days, this should be indicated.
Discussion: The Jewish calendar includes six fast days, which have different levels of stringency.
- The strictest fast, and the only one required by biblical law, is Yom Kippur, the Day of Atonement for sins. On Yom Kippur both eating and drinking are absolutely prohibited for a little more than 24 hours, from sunset on one day until after nightfall the next day. Bathing, applying oil to the skin, sexual relations and leather footwear are also forbidden.
- Next in stringency is the Ninth of Av (Tisha B’Av), the fast that commemorates the destruction of the First and Second Temples in Jerusalem. This rabbinically-ordained fast is parallel to Yom Kippur in that eating and drinking are prohibited for a little over 24 hours and bathing, etc., are also proscribed.
- Three additional “minor” fasts (the 10th of Tevet, the 17th of Tammuz, and Tzom Gedaliah) mark other events connected with the destruction of the Temples. The Fast of Esther falls on the day before Purim and commemorates the fighting between the Jews and their enemies. These fasts are less rigorous: eating and drinking are prohibited but only from before dawn until nightfall, and bathing, etc., is permissible.
Healthy breastfeeding mothers of normal, healthy babies are obligated to fast on Yom Kippur and the Ninth of Av. On the other, minor fast days they are exempt. There is debate among halachic authorities as to whether they may voluntarily choose to fast on those days or not.
If there are any health or medical problems with the mother or baby, the mother should not fast on the minor fast days. In cases of actual danger to health, there may be room for leniency even on Yom Kippur and the Ninth of Av. In such cases, an individual question should be asked of a rabbi. The medical information should include the proven importance of breastmilk to health of children .
A woman’s fluid output is greater when she is breastfeeding than when she is not. Thus, there is a greater chance that she will develop symptoms of dehydration, such as headache, fatigue, etc. A woman can reduce her chances of developing such symptoms if she is careful to drink extra fluids before the fast, spends the fast day in a cool environment, and minimizes exertion. Fasting is more important than synagogue attendance. If symptoms are severe, an individual question should be asked of a rabbi, as health concerns take precedence in Jewish law.
There are anecdotal reports by women that fasting causes a decrease in milk production, and mothers are often concerned about the effect of the fast on their babies . This topic has not been adequately studied. A Medline search reveals no studies on the effects of fasting on milk production. One study, indicates that fasting causes a small, temporary decrease in milk production, which is easily reversed . Another study found statistically significant differences were found in calcium, phosphorus, lactose and protein when comparing milk before and immediately after the fast. These differences are not likely to be clinically significant when dealing will healthy full term infants. However, they may have an impact in the case of infants with health concerns. 
Please see the Case Study, Breastfeeding and Yom Kippur, for additional discussion of this topic.
Implications for Care: Health care providers should be aware of the presence of fast days in the Jewish calendar. The epidemiologic evidence suggests that healthy mothers of healthy infants can fast without negative consequences. For these women, the physician should give anticipatory guidance how to best manage the fast (e.g., drink extra fluids the two days in advance, minimize exertion during the fast, and be prepared for the possibility of some temporary difficulty the day after the fast).
When the physician is concerned about the effects of fasting on either the mother or baby, this information should be conveyed in advance of the fast to allow the woman to ask her rabbi whether she may eat. The information should include the importance of breastmilk to the health and development of children.
 American Academy of Pediatrics. Breastfeeding and the use of human milk. Published online Feb 27, 2012. www.pediatrics.org/cgi/doi/10.1542/peds.2011-3552.
 Ertem IO, Kaynak G, Kaynak D, Ulokol B, Glunar SB. Attitudes and practices of breastfeeding mothers regarding fasting in Ramadan. Child: Care, Health and Development 2001;27(6):545-54.
 Sheffi O. Tzom yom kippur v’hashpa’ato al nashim meinikot. Assia 5754;14:126-141. (Hebrew)
 Zimmerman DR, Goldstein L, Lahat E, Braunstein R, Stahi D, Bar-Haim A, Berkovitch M. Effect of a 24+ hour fast on breast milk composition. J Hum Lact. 2009 May;25(2):194-8.