In Hinduism, a religious fast is observed on ekadasi (the eleventh day of each lunarfortnight) and, if observed strictly, involves taking no food or water from the previous day's sunset until 48 minutes after the following day's sunrise.
For Roman Catholics, fasting refers to those days set aside by the church when the faithful must reduce their intake of food to one full meal (which may contain meat) and two small meals (known liturgically as collations, taken in the morning and the evening) as distinct from abstinence which was the complete avoidance of meat on Fridays, especially during Lent.
Ash Wednesday and Good Friday are still days of fasting and abstinence, as specified in the Code of Canon Law (cc. 1250 to 1253). On these two solemn days Roman Catholics are enjoined to both fast (reduce the size of their daily meals) and to abstain (to completely avoid the consumption of meat in those meals).
The current regulations concerning Lenten fasting and abstience for Catholics in the United States generally are as follows,
Abstinence from meats is to be observed by all Catholics 14 years old and older on Ash Wednesday and on all Fridays of Lent.
Fasting is to be observed on Ash Wednesday and Good Friday by all Catholics who are 18 years of age but not yet 59. Those who are bound by this may take only one full meal. Two smaller meals are permitted if necessary to maintain strength according to one’s needs, but eating solid foods between meals is not permitted.
For Catholics whose health or ability to work would be negatively affected by fasting and/or abstinence, the regulations above don't apply.
Until the Second Vatican Council Ash Wednesday and all the subsequent Fridays and Saturdays of Lent were days of "Fasting and Abstinence" whereas all the other weekdays of Lent were days of "Fasting without Abstinence". An exception to this rule was granted to the Bishops of Ireland (see Irish calendar) by the Vatican in 1918, when the obligation of fasting and abstaining on the Lenten Saturdays was transferred to the Wednesdays of Lent instead.
In recent years, Saint Patrick's Day has at times fallen on a Friday of Lent. Some Priests have granted dispensations for their parishoners from the abstinence obligations so that Catholics could enjoy traditional Irish dishes.
For Orthodox Christians, fasting at various times refers to abstention from animal products, olive oil (or all oils, according to some Orthodox traditions), wine and spirits -- see Eastern Orthodoxy (Fasting).
In Protestantism, the Reformers criticized fasting as a purely external observance that can never gain a person salvation. The Swiss Reformation of the "Third Reformer" Huldrych Zwingli began with an ostentatious public sausage-eating during Lent.
Another reason that people fast for medical reasons is for certain medical tests. People are asked to fast often so that a baseline can be established.
Longer term fasting for health reasons typically lasts a week or longer and includes some food intake, such as fruit or vegetable juices.
