If you are fasting tomorrow and you feel yourself getting sick, eat something.
If you are fasting tomorrow and you are so light-headed you can’t stand up: drink some water.
If you are fasting tomorrow and you take meds regularly, take them and the water you need to wash them down.
Yom Kippur is about reflection, atoning, and becoming a better person. Fasting is meant to take away distractions from that, and to keep us mindful for the rest of the year. It’s not meant for you to get to the point where you can’t make it through the day.
Like a lot of mizvot, fasting isn’t the destination — it’s the guide rail. Repentance is the observance of the day, not fasting. And to keep yourself healthy and alive and functional is the most important commandment, taking precedence over any other. There are many ways to reflect, to stay mindful, to atone, to filter the past year and approach the next; if you aren’t able to do it with your body, you can still do so with your heart.
Shana tova, all. May you be inscribed in the Book of Life.