In Jainism, "Athai Upvas" refers to undertaking eight consecutive days of complete fasting (upvas). During each day of upvas, the devotee refrains from consuming any food or water, practicing total abstinence. Therefore, according to Jain traditions, during Athai Upvas, no food or water is to be consumed for the entire period of eight days.

If, for health or other reasons, someone is unable to do a complete upvas, they may undertake alternative forms of fasting, such as Ekasana (eating once a day) or Biyasana (eating twice a day), but these are considered less austere than upvas. However, these alternatives are not considered a full Athai Upvas.

Summary: During Athai Upvas, you should not eat or drink anything. If you are observing alternative vows due to health or practical reasons, you may follow less strict forms of fasting, but this would not be called Athai Upvas in the strict sense.

Always consider your health and consult with knowledgeable elders or Jain monks before undertaking such rigorous fasts.