Just like in many other religions, in Hinduism also, fasting is a religious practice done by many. It is a very powerful Vedic remedy to remove sufferings from life. Due to many of your weaknesses, you may have done bad karmas in this life or in other past lives. As per the law of cause and effect (karma) you reap what you sow. And this is the reason why you undergo suffering. The more you do good and righteous deeds, the more your karma improves. Having done a bad karma in the past, if you genuinely and seriously repent and pay your penance, then Almighty is kind enough to forgive you.
Food is the primary requirement of a human body for survival. Without food, we cannot do work as it provides energy to our body. If you sacrifice food with the thought of paying penance for your bad deeds, the suffering it brings helps to eliminate your sins.
Vrata (Fasting) is generally prescribed on a specific day of the week or on a special date (tithi) according to vedic calendar. Amongst the 5 elements of time (panchangas), day of the week (vara) represents fire (agni) element which is responsible for your vitality. Tithi (vedic date) is jal tatwa (watery) by nature and rules our feelings, emotions and happiness.
When you sacrifice food and do fasting on a prescribed day of the week/date of month, you are invoking a fire (of suffering) in your stomach. When you tolerate this hunger for a prolonged duration you burn from inside, and this fire becomes so strong that it starts to destroy the 6 evils within you.
The fire acts as a cleanser and purifier (Vishva Vedha) and burns away your sins. When you have suffered from hunger for several hours and this fire is at its peak, if you pray at this time, the fire takes all your prayers directly to the Almighty.
Just like when you burn a candle or a lamp, the fire of the lamp takes your prayers to God, when you light the fire within you, it takes your prayers to God, with much more force than a lamps fire.
It is not easy to do fasting, and this is why this remedy is very powerful and gives quick results.
*make sure to consult your general physician before starting regular fasting.