रसस्य मनसश्चैव चञ्चलत्वं स्वभावतः ।
रसो बद्धो मनो बद्धं किं न सिद्ध्यति भूतले ||
– Hatha Yoga Pradipika
By nature, रस (अन्नमय कोष + प्राणमय कोष) and mind (मनोमय कोष) are unsteady: there is nothing in the world which cannot be accomplished when these are made steady.
And so उपवास, ब्रह्मचर्य are essential tools for self-realization.
From initiation at age 6 (it varies for different persons) to the end of Grihasthi, first absolute ब्रह्मचर्य, followed by Grihasthi’s उपवास & ब्रह्मचर्य, helps in making रस & मन stable. Helping in self-realization by performing own duties towards the world.
Think about it.
Most fasting vows and व्रतs in local culture of this land are for householders (गृहस्थी). Very few days of the year when elderly and children observe fast.
Why?
Married couples are backbones of the healthy Nation and society. Middle age (25-50) is the age of performing duties. Married couple in this age has several responsibilities. Taking care of children, taking care of elderly, taking care of society and local community, taking care of mother nature, earning livelihood etc. All these activities are energy intensive. On the other hand, excessive eating, mindless indulgence in entertainment etc are doubly energy consuming activities with zero productive inputs towards duties. Duties are for spiritual progress of an individual (And so marriage is important fabric of the society).
If major chunk of energy source is consumed in eating activity, all other duties suffer due to lack of agility and liveliness. Full dedication is not possible. And so to conserve vital energy to perform duties towards self, family, society, various restoration activities are prescribed in term of vows, व्रतs and उपवास.
Also, this is the age when body’s general tendency is Pitta. Fasting helps in balancing excessive pitta. So we can have healthy, fresh, sickness-free days to dedicate for family, society and nation.
There are spiritual benefits but I prefer not to share them here 
🙂. They better should be experienced.
Prolonged Fasting Reduces IGF-1/PKA to Promote Hematopoietic-Stem-Cell-Based Regeneration and Reverse Immunosuppression
Highlights
- •Prolonged fasting downregulates a IGF-1/PKA pathway in stem cells
- •Prolong fasting protects hematopoietic cells from chemotoxicity
- •Prolonged fasting cycles promote HSC self-renewal to reverse immunosuppression
- •Inhibition of IGF-1 or PKA signaling mimics the effects of prolonged fasting
Summary
Immune system defects are at the center of aging and a range of diseases. Here, we show that prolonged fasting reduces circulating IGF-1 levels and PKA activity in various cell populations, leading to signal transduction changes in long-term hematopoietic stem cells (LT-HSCs) and niche cells that promote stress resistance, self-renewal, and lineage-balanced regeneration. Multiple cycles of fasting abated the immunosuppression and mortality caused by chemotherapy and reversed age-dependent myeloid-bias in mice, in agreement with preliminary data on the protection of lymphocytes from chemotoxicity in fasting patients. The proregenerative effects of fasting on stem cells were recapitulated by deficiencies in either IGF-1 or PKA and blunted by exogenous IGF-1. These findings link the reduced levels of IGF-1 caused by fasting to PKA signaling and establish their crucial role in regulating hematopoietic stem cell protection, self-renewal, and regeneration.
