How to Prepare to Fast on Yom Kippur
The most important thing going into a fast is to try to hydrate as much as possible. If you can, start to drink extra water at least two days before the fast starts.
Many people also find caffeine withdrawal a problem while fasting. If you consume caffeine regularly, try to decrease your caffeine intake in the time leading up to Yom Kippur so that it will not be a huge shock to your system. If you have enough lead time, many people like to take the opportunity to wean themselves off of caffeine completely before Yom Kippur.
On erev Yom Kippur try to eat balanced meals. Go for proteins and complex carbohydrates; simple carbs and carb loading will make your blood sugar spike and then drop unpleasantly. Try to keep the salt content down so you don’t make yourself thirstier later. Don’t try to overstuff yourself.
During the fast, if you get dizzy or lightheaded try sitting down for a little while. If the feeling persists, or if you have other worrying symptoms, please drink some water immediately and eat a small amount of food. Judaism does not condone endangering your life in order to fast, even on Yom Kippur.
If you are ill or have a persistent medical condition, you might want to talk to your doctor before fasting to make sure it is safe. People who are pregnant or nursing are explicitly exempted by the rabbis, lest it harm them or the fetus/baby. Any medication that you take daily should also be taken on Yom Kippur. Again, fasting is not supposed to endanger your life or your health.
When you break your fast, drink first and then start eating slowly.
Everyone’s experience of fasting is different. The first time is the hardest, so if you don’t make it through the fast this year before you have to eat something, don’t give up! You will learn over time what your body needs.