In Jain Dharma, fasting (upvās / tapa) is mainly for the soul, not for body‑shape or health. Its core spiritual benefits are:

1. Stops new karmas (Saṃvar)

- When you fast with awareness, you naturally speak less, move less, and control senses. - Attachment and anger reduce, so fewer new karmas bind to the soul. ( jainknowledge.com)

1. Sheds old karmas (Nirjarā)

- Upvās is a bāhya tapa (external austerity). - Bearing hunger and thirst calmly, without irritation, helps burn previously bound karmas, especially those linked with anger, pride, deceit, and greed. ( jainknowledge.com)

1. Control over desires (Saṃyam, Indriya‑nigrah)

- Tongue and taste are the strongest doors of attachment. - By saying “no” to taste for a time, we weaken this hold and strengthen self‑discipline in all areas of life (speech, money, pleasures, etc.). ( jainknowledge.com)

1. Detachment and simplicity (Vairāgya, Aparigraha)

- Fasting reminds us: “I am soul, not this body and its demands.” - Attachment to food, routine, and comfort slowly decreases; contentment and simplicity increase. ( jainknowledge.com)

1. Purity of mind and emotions

- With a lighter body, the mind becomes quieter. - Easier to do sāmāyik, pratikraman, svādhyāy (scripture study), and meditation. - Softens inner enemies – anger, ego, deceit, greed – and supports forgiveness and humility. ( jainknowledge.com)

1. Increase in spiritual merit (Punya)

- When done without show‑off, with right faith, ahimsā, and repentance, fasting brings punya that supports progress toward liberation. ( jainknowledge.com)

1. Support for right conduct and vows

- Regular tapa makes it easier to follow ahiṃsā, satya, brahmacharya, and aparigraha in daily life. - It strengthens all other small and big vows. ( jainknowledge.com)

Important Jain points

• Aim is inner purification and moksha, not health or material gain. Health benefits, if any, are secondary. ( jainknowledge.com)
• Must be done within one’s capacity, with proper pachchakkhān (vow) and guidance of guru/saṅgh. Hurting your own body out of ego is also a kind of hiṃsā. ( jainknowledge.com)
• Both Digambar and Shvetāmbara accept fasting as tapa for saṃvar–nirjarā. Rules about water, timings, etc., may differ, but the spiritual purpose is the same. ( jainknowledge.com)

You can read a simple explanation of Upvās and its purpose here