On five categories of physical needs from which we customarily abstain; fasting (not eating or drinking) is the most familiar. Although everyone responds differently to fasting, these suggestions may help ensure that even as you focus on your spiritual needs during Yom Kippur, you do so in a way that is safe for your body., we put aside our physical needs to concentrate on our spiritual needs through prayer, repentance, and self-improvement. To do so, there are
1. Hydrate, hydrate, hydrate.
Experienced fasters agree that hydrating as much as possible before a fast is vitally important. If you can, start to drink extra water at least two days before the fast begins.
2. Cut back on caffeine.
Many people also find the lack of caffeine to be a problem while fasting. If you consume caffeine regularly, try to decrease your caffeine intake in the days leading up to Yom Kippur – perhaps beginning on Rosh HaShanah – so the lack of caffeine will have as little effect as possible on your body during the fast. With enough lead time, some people wean themselves off caffeine completely before Yom Kippur.
3. Eat thoughtfully before your fast begins.
On Erev Yom Kippur (during the day before the holiday begins) try to eat balanced meals – and continue to drink lots of water. Choose foods high in protein and complex carbohydrates; simple carbs and carb-loading will result in blood sugar spikes and drops. Be aware of the salt content in the foods you choose; consuming a lot of salt means you’ll be thirsty later. Lastly, don’t overeat – it’s not healthy and it won’t help to ward off hunger pangs on Yom Kippur.
4. While fasting, pay attention to your body's needs.
During the fast, if you feel dizzy or lightheaded, try sitting down for a little while. If the feeling persists, or if you have other symptoms that concern you, it’s important to drink some water immediately and eat a small amount of food. Judaism does not condone endangering your life to fast, even on Yom Kippur.
5. Do not fast if you are physically unable to do so.
If you are ill or have a chronic 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 from fasting, lest it harm them or the fetus/baby. Any medications you take daily should also be taken on Yom Kippur. Again, fasting is not supposed to endanger your life or your health.
For a meaningful alternative, see this "Meditation Before Yom Kippur for One Who Cannot Fast."
6. Break your fast slowly.
When you break your fast, drink first and then eat, starting slowly. Here, again, it’s important not to overeat. As before the fast, it’s not healthy, and it won’t compensate for the meals you skipped during the day.
Everyone’s experience of fasting is different. The first time can be the most difficult, so if you don’t make it all the way through the day before you have to eat something, it’s fine. You will learn over time what your body needs.