During atthai (8‑day fast), blood glucose can drop quite a lot, so health has to come first.

1. What happens to blood glucose in atthai?

Traditional atthai upvās = no food at all for 8 days, usually only boiled water in daylight (some even do nirjal: no water). ( jainknowledge.com)

In simple terms:

• First 24 hours – body uses stored glucose (glycogen) from liver → blood sugar starts to fall.
• After 1–3 days – body shifts to using fat for energy; blood sugar usually stays on the lower side.
• After several days – if you are weak, under‑weight, or on diabetes medicines/insulin, there is real risk of dangerous low sugar (hypoglycemia), dizziness, fainting, confusion, etc.

So:

• A healthy person with no medical issue may tolerate it (under guidance).
• A person with diabetes, heart, kidney, pregnancy, elderly, teens still growing, or any serious illness should not do strict atthai without medical clearance. It can be unsafe.

2. If you have diabetes or any health issue

From a Jain dharmic point of view:

• Ahiṁsā starts with not harming your own body.
• Jain ācāryas allow lighter tapas when health is weak – e.g.:

- Āyambil atthai (1 very simple meal per day), - Ekāsan / Byāsan with strong dhyān, svādhyāy, pratikraman, etc. ( jainknowledge.com)

Practical points (especially for diabetics):

• Do NOT start atthai without talking to your doctor.
• Ask which medicines/insulin doses must be changed or stopped if you fast.
• If you feel:

- sweating, shaking, strong hunger, dizziness, confusion, blurred vision → immediately treat as low sugar (take sugar/glucose and break the fast; life is more important than the vrata).

• It is not paap to break or soften the fast to protect life; stubbornness that harms the body goes against Jain spirit.

3. Dharmic alternative if strict atthai is unsafe

You can still do strong tapa with safer blood sugar levels, for example:

• Āyambil for 8 days (Atthai of Āyambil),
• Ekāsan with very simple food and limited quantity,
• Extra pratikraman, svādhyāy, samayik, and seva in those 8 days.

Bhāv (inner intention) + non‑himsā are more important than the “record” of 8 literal nirjal days.

4. Learn more about atthai itself

For a simple explanation of what Atthai is and its rules, you can read here:

• “In Athai Upvas we can eat?”
• “How to fast for 8 days” (options incl. Āyambil atthai)

Summary: Atthai strongly lowers and stresses blood glucose regulation. If there is any medical issue (especially diabetes), choose a milder, medically‑safe form of tapa with guidance from both doctor and Jain guru/elder. That is fully in line with Jain Dharma.