Fasting is called upavas in Sanskrit, or vrata, fasting being one of the strongest, most intense spiritual travels provided in Hinduism. Derived from Hindu metaphysics that looks into the relationship between the body, mind, and transcendent reality, fasting serves as the heart of the discipline.

The Metaphysical Basis of Fasting

According to Hindu philosophy, the human being is beyond the physical body. According to the Taittiriya Upanishad, five interconnected layers, called "koshas", actually constitute a person: the physical body (annamaya kosha), the vital energy (pranamaya kosha), the mind (manomaya kosha), the intellect (vijnanamaya kosha), and bliss (anandamaya kosha). Fasting acts upon these levels, starting with the physical but finally reaching consciousness itself.

In the essence, the Bhagavad Gita (6.16-17) states: "Yoga is not for one who eats too much or too little, nor for one who sleeps too much or too little. For one who is moderate in eating and recreation, in his efforts in actions, and in sleep and wakefulness, yoga becomes a destroyer of suffering."

Different Ways of Fasting in Hindu Tradition

1. Ekadashi Vrata-The 11th day of every lunar fortnight is dedicated for fasting for Lord Vishnu. 
2. Shivratri Upavasa-that is on 14th night of the dark fortnight in Phalguna month 3. Nine days of Navratri fasting in honor of the Divine Mother 4. Karva Chauth-for married women for longevity of their husbands 5. Purnima Vrata-Fasting on full moon days

"Fast in Ekadashi, and you will surely absolve every sin and attain the Supreme Lord Vishnu" (Skanda Purana, Vaishnava Khanda, 4.75.15-16). 

Purposes of Fasting

Physical Purification 

The mind purifies itself when food is pure; the mind purifies memory performance when it is pure. All these give the idea of the purification of the body. In that, sleeping was compared with periods of fasting: "Cleanliness in the body through fasting."

This is where the central idea of agni or digestion comes as he burns the food eaten by the body. According to Charaka Samhita, fast burns everything in impurities in the body and subtle karmic residue; thus, middle fire-agnis-reference.

Mental Discipline

Willpower and concentration are developed through fasting. The immediate gratification is voluntarily denied by the fast to improve the mental strength to go forward to a "higher" goal namely spirituality.

"The purification includes the separation of the impurities of the body, so that it can be purified by the holding of the highest merits, thus gives rise to exceptional faculties of the sense organs" (Yoga Sutras of Patanjali, 2.43). 

Spiritual Elevation

This perhaps may be the most critical category, as it causes the aware movement from gross to subtle, meaning that when bodily demands are suspended, then the consciousness comes closer to flying high.

Thus is characterized as "sattvic tapas" (Bhagavad Gita 17.5-6): "Worship of the gods, the twice-born, teachers and the wise; purity, straightforwardness, celibacy, and non-violence-these are said to be the austerity of the body" 

Adapting this Modernized Hindu Fasting, 

You can start integrating practices of Hindu fast into your daily life: 
1. Partial fasts-of fruits or some foods only. 
2. The fast has a higher spiritual significance beyond just practice. 
3. Use fasting time for meditation, prayer or scripture study. 
4. Identify the purpose behind every fast. 

About fast: "Fasting purifies a man's heart; from the pure heart, knowledge and detachment grow spontaneously." (Bhagavata Purana 11.18.18) .

Conclusion 

Hindu fasting is far more than abstaining from food; it has a complete and multilevel approach to purification within existence itself. Fasting accounts not only for cleansing the physical body but also the mind and upward advances toward the ultimate reality through elevating the consciousness. 

That rising is the symbol of the final aspect of Hindu spirituality: the pathway from the gross to the subtle, from the limited to the limitless, and from the time-bound to the eternal.l., thus goes the line: "Fast, and high rises the vital air of a man." (Chandogya Upanishad 6.7.1). 

In one simple and yet deep entry into this thousand-year-old tradition of self-realization, fasting would probably be one of the most immediate ways to approach Hindu practices for those who wish to radically adopt them.