Here are Swami Sivananda’s words on fasting and Ekadasi:
Vaikunta Ekadashi falls in the month of Marga-seersha (December-January). This is observed with all solemnity in the temples of Lord Vishnu. Fasting is prescribed on all Ekadashis, that is, the 11th day of the lunar fortnight, twice a month.
In this Kali Yuga, even if just one Ekadashi is observed with dispassion, faith and devotion, and if the mind is wholly fixed on Hari, one is freed from the rounds of birth and death. There is no doubt about this. The scriptures give us their assurance on this point.
Devotees fast on this day, observe vigil the whole night and do Japa, Hari Kirtan and meditation. Some do not take even a drop of water. Those who are unable to fast completely can take some light fruit and milk.
No rice should be taken on Ekadashi days. This is very important. The sweat that fell down from the head of Brahma assumed the form of a demon and said to the Lord, “O Lord! now give me an abode to dwell.”
Brahma replied, “O demon! go and dwell in the rice particles eaten by men on Ekadashi day and become worms in their stomach.”
For this reason rice is prohibited on Ekadashi. If one observes the Ekadashi fast regularly, Lord Hari is propitiated. All sins are destroyed. The mind is purified. Devotion gradually develops. Love for God becomes intense. Orthodox people in South India observe complete fasting and vigil even on ordinary Ekadashi days. For the devotees of Lord Vishnu, every Ekadashi is a very sacred day.
Fasting controls passion. It checks the emotions. It controls the senses also. It is a great penance. It purifies the mind and the heart. It destroys a multitude of sins. Fasting controls the tongue in particular which is the deadliest enemy of man. Fasting overhauls the respiratory, circulatory, digestive and urinary systems. It destroys all the impurities of the body and all sorts of poisons. It eliminates uric acid deposits. Just as impure gold is rendered pure by melting it in the crucible again and again, so also this impure mind is rendered purer by repeated fasting.
Fasting is one of the ten canons of Yoga. However, avoid excessive fasting. It will produce weakness. Use your common sense. If you cannot fast for the full twenty-four hours, at least fast for 10-12 hours and then take some milk and fruit. Gradually increase your fast to 15 hours and then up to 24 hours. Fasting makes a man strong, both spiritually and mentally.