Fasting and Hinduism - some facts
Fasting or Vratham, as it is known commonly in  India,  is  nothing but not taking food of any kind, for a whole day or half  a day or during  particular period of time. In Sanskrit  it is called upavaasa. Upa means "near"; vaasa means "to stay" meaning gaining close mental and physical  proximity with the Lord. Some people do not even drink water during the period of fasting. Fasting has  been an integral part of the Indian culture and tradition  for centuries and it has more religious and spiritual  connotation than ethical. The period of fasting also varies, depending on the situation and festivals. Commonly fasting could be part of a day or  whole day  - just 12 or 24 hours. Fasting is being commonly observed in all religions. Among the Muslims Ramzan fasting is an important religious event. In  Ayurveda or Naturopathy, fasting is used to treat some ailments.
In Hinduism, much emphasis is given to fasting as a way to cleanse our  body and mind to get spiritual or religious ecstasy. Various Indian scriptures mention about the importance of fasting and, it is believed, that it
helps  establish a harmonious relationship between the body and the soul and the realization of the Absolute. It  also plays a key role as  part of  religious worship.  Hindus observe  religious fasting once or twice a week, concentrating their mind on God by way of silently chanting appropriate Mantra for that particular occasion. They, on certain days,  skip the meal and after fasting,  take light food before retiring to bed or  tak food only in the morning  and continue the fasting till night. They normally  just take a glass of juice or soup or some fruits before going to bed.  

Besides its spiritual  and religious significance,  fasting, it is widely accepted, will improve one's self-discipline and determination that are essential ingredients to lead a successful life. Apart from it, it gives you amazingly enduring power that helps you mellow down in a tense situation in life. It is needless to say  that you  cultivate the art of controlling your senses and be in peace with the mind. Invariably, fasting is done for religious purpose mainly to fulfill our prayer to the deity. People also undertake fasting once their prayer is answered to express their gratitude to the deity - a way of thanks giving.

Fasting is part and parcel of the lives of Pundits or priests who are in charge of the presiding deities in the Sri Kovil or Garbagraha (Sanctum Sanctorum). For example, the Nambudri Brahmin Pundits  of Kerala Hindu temples  follow the fasting strictly if they happen to be the chief Tantris at temples. From 3 AM in the morning till noon, the Tantris at Guruvayur Sri Krishna temple, Ayyappan temple on Sabarimalai, etc., never take a drop of water while engaged in temple worship protocols. Further, they do prayer  and worship in Sri Kovil with their dress fully drenched. To undertake such a tough religious worship in temples of Kerala, the Pundits need firm determination and self-discipline besides good training and  intense devotion to God.

As far as Hinduism is concerned,  during certain major Hindu festivals pious people go on a fast. The following are some of the festivals associated with fasting: Shivaratri (Lord Shiva), Chaturthi (Lord Ganesa), Ekadashi (Goddess), Kandha Shasti (Lord Subramanya or Karthkaya), Gokulastami (Lord Sri Krishna), Rama Navami (Lord Sri Rama), etc., As for the week days, some deities are associated with certain days: Monday (lord Shiva), Tuesday and Friday (Goddess such as Lakshmi, Durga, Parvathi or Mariamman), Thursday (Lord Dattaraya), Saturday (Lord Hanuman)  The practice of fasting is quite  common among the elderly people. 

However, there are many others who fast solely for maintaining good health. Thus fasting has both religious and medical significance in India. Every part of our body works day in and day out without a break and it needs rest and overhauling. Fasting provides an opportunity for our inner parts to take the required respite and get re energized.

Some facts of Fasting:

01. In the Indian society fasting  by devout  people has been there for several centuries. Fasting once a day is generally practiced. Timing for fasting is from sunrise to sundown or 24 hours. A good bath in the early morning coupled with prayer is followed by fasting.
02. For many, fasting means not even taking a drop of water for a specified period. One must drink plenty of liquid during long fasting. Keeping the stomach empty for a long time may cause acidity problem and is not advisable.

03. When fasting, do not  indulge in strenuous exercise activities or  work , which will result in over-exertion

04. If diabetic, one should take the advice of a doctor before going on a fast. Keeping the empty stomach for a long time may lower the BP and also blood sugar level.

05. If 24 hour fasting is undertaken, drinking plenty of water  or butter milk  between intervals is a must. Some people don't drink even a drop of water. This is not the right way of fasting.

06. You can end the fasting by drinking some fruit juice or porridge or Kanchi (well cooked rice plus buttermilk).

07. After a break one can take a meal. Avoid heavy meal in the late evening or night. Since you will be retiring to bed soon, it will add calories if you have a sumptuous meal to your heart's content.

08.  When fasting for long hours, if you are on some doctor's medication  don't skip it and take the medicine  as you would normally  after drinking a glass of fruit juice or soup. 

09. After fasting, avoid as much as possible  rich food loaded with calories and also highly carbonated, sugar-rich beverages.

10. When the Hindus do their Pithru Karyams - Thithi or Diwasam (annual remembrance day) to their parents, they go on a fast after their noon meal.   

11. Bagavat Geeta recommends healthy, good food- saatvik diet and simple food - neither too much nor too little - yukta-aahaara.