Will fasting while I'm breastfeeding harm my baby?

Your baby will not be harmed, because you will be able to keep making breastmilk while you are fasting. Reducing how many calories you have for this period of time should make no difference to the amount of milk you produce.

Your body adapts by changing the way it uses the available calories. It appears to make up for the lack of food or fluid by becoming better at releasing energy and stepping up milk production.

In fact, you could eat nothing for 24 hours without it affecting either the quantity or the nutritional value of your breastmilk. You would feel the effects of fasting, and probably need to stop, before it affected how much milk you produced.