The question below was submitted to www.yoatzot.org, Nishmat's Women's Health and Halacha (Jewish Law) website for the lay public.
"I am exclusively breastfeeding my son, who is 5 and a half months old. I have heard a full range of opinions on whether or not I must fast on Yom Kippur. From what I understand, the main question is whether I should drink or eat in halachically acceptable small amounts (shiurim) a priori, or if I should wait if/until I feel dehydrated and then drink - risking that I will have to drink more at that point. I feed my son every 3-4 hours and I am his only source of food."
Physicians are commonly asked questions about fasting while breastfeeding. It is important to understand the halachic background to the question, and the dearth of medical information on the topic.
The Shulchan Aruch clearly states that breastfeeding women should fast on Yom Kippur in the same manner as others (Orach Chaim 617:1). As in all areas of halacha, if there is a medical risk, there is room for leniency in individual cases. A rabbi must be consulted in such situations. The physician can assist the woman by providing a clear explanation as to why fasting would create a particular risk in her situation.
Fasting by a breastfeeding woman raises questions as to the health of two people, the mother and the baby. Most breastfeeding women can fast without any risk to themselves. Many report, however, that fasting while breastfeeding is harder than it is at other times. A study by the medical supervisor of this site showed that 78.1% of 29 women reported symptoms while fasting; nevertheless, all but 2 completed the fast. Symptoms included headache (n=13), fatigue (n=17), nausea (n=5), dizziness (n=9), and a feeling of reduced milk supply (n=9). 
Based on physiologic logic, the physician should suggest that the woman make all efforts to drink additional liquids throughout the day (or even two days) before the fast. She should also be counseled to rest as much as possible during the fast. Women should be told not reduce the frequency of breastfeeding during the fast, as this is likely to lead to painful engorgement, reduction in milk supply, and/or mastitis.
As far as the effect on the nursing infant, there is almost no data on the effect of such a fast on human breastmilk. The only published article to date is by O. Shefi: "Tzom Yom Hakippurim v'Hashpa'ato al Nashim Menikot" (published in Hebrew), in Assia 14, Elul 5754. In her review article, the author found one human study that showed a slight difference in milk composition after an overnight fast. Studies in animals showed changes in composition and a small reversible decrease in milk supply the day following a 24-hour fast. The author reported anecdotal data from women she interviewed of reduced milk on the day after the fast. However, no statistical data are provided. In the study cited above, 42.4% of the women who completed the questionnaire reported symptoms in the baby. Symptoms included increased crying (n=5), increased sleepiness (n=4), and a need for supplemental feeding (n=6, all but one used expressed breastmilk).
Dr. Zimmerman also found statistically significant differences 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 with healthy full term infants. However, they may have an impact in the case of infants with health concerns. Based on the experience of the medical director of the site, as a pediatrician, lactation consultant, and breastfeeding mother, most babies can weather the fast with little more than slight fussiness. Mothers can help by being prepared with a bottle of expressed breastmilk from the previous day that can be administered at the end of the day (either by bottle, syringe, cup, or spoon, depending on the age and nursing history of the baby). Mothers should arrange to have a quiet day immediately after the fast when they can spend time with the baby and nurse more frequently to make up for any slight decrease in supply that might have resulted from fasting.
These are general guidelines. There are individual cases such as sick infants, difficult nursing situations, and past bad experiences where mothers should be told to drink in shiurim (small quantities). Any such case requires an individual medical assessment of the situation and an individual halachic ruling.
Please see the article Fasting and Breastfeeding for additional information on this topic.
 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.
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