I salute those who observe long fasts for eradication of karma. But Right Knowledge is essential before practicing fasts and other external austerities. Fast should be observed to attain stability in meditation and the aim should be eradication of karma, if it is not done for such goal then it is all futile.
So it is highly required to gain Right Knowledge from the right spiritual teacher about the Six Substances, Nine Fundamentals, Substance-Essence-Modifications, the Emergence-Destruction-Permanence nature of the substances and knowledge of difference between the self and non-self. The aim behind all penances and meditation should be to achieve non-alternative state of Upayoga, without which our path becomes just practical and further diverts to fame, physical welfare etc and that is not the path of liberation. If one has not known the true nature of the soul being knower-seer apart from the body then one’s practice becomes meaningless and it becomes responsible for the rise of pride, attachment, ego etc. The only solution is the practitioner has to move towards his/her real form, the pure soul.
Our entire practice should be to get rid of physical and mental attachments. The real path is the sum up of both Absolute (knowledge) and Practical (austerities). Practicing both helps the soul to eternal bliss, which is liberation. Fasts and such external austerities are important because it creates a base for the internal austerities. The high amount of karmic atoms eradicates by internal austerities only. That’s why the practitioner should do spiritual meditation during the periods of fasting and should try to be absorbed in his/her soul. Meditation is a must for self realization and the external austerities are practiced to create strong base for it. That is why the enlighten say, “the one practicing just external austerities eradicates the same amount of karma in billions of years which is eradicated in just few seconds by the person who is absorbed in spiritual meditation.” So it is essential to start austerities only after gaining right knowledge, not before that!