Srimad Bhagavatam — Saptama Skandha, Shloka 23
Brahmacarya and Vānaprastha Duties; Gradual Dissolution of Bodily Identity
यदाकल्प: स्वक्रियायां व्याधिभिर्जरयाथवा । आन्वीक्षिक्यां वा विद्यायां कुर्यादनशनादिकम् ॥ २३ ॥
yadākalpaḥ sva-kriyāyāṁ vyādhibhir jarayāthavā ānvīkṣikyāṁ vā vidyāyāṁ kuryād anaśanādikam
当因疾病或年老而不能履行规定的本分、亦不能研习吠陀以求灵性增上时，应修持断食等行，亦即持斋不食。
FAQs
This verse states that when one becomes unable to perform prescribed duties due to illness or old age, or when one has matured in spiritual knowledge, one should adopt austerities such as fasting as a supportive practice for spiritual focus.
In the context of describing varṇāśrama-dharma and the path of gradual purification, Śukadeva explains that when active duties can no longer be sustained—or when discernment is mature—one should shift toward renunciation-based disciplines like fasting.
As responsibilities change with health or age, one can simplify life, reduce sense-indulgence, and adopt moderate, medically appropriate austerities (like occasional fasting and restraint) to deepen prayer, study, and devotion.