Fasting for religious and spiritual reasons has been a part of human custom since pre-history. It is mentioned in the Bible, in both the Old Testament (the Tanach) and New Testament, the Qur'an, the Mahabharata, and the Upanishads. Fasting is also practiced in many other religious traditions and spiritual practices.
Fasting is also used in a medical context to refer to the state achieved after digestion of a meal. A number of metabolic adjustments occur during fasting and many medical diagnostic tests are standardized for fasting conditions. For most medical purposes a person is assumed to be fasting after 8-12 hours. A diagnostic fast refers to prolonged fasting (from 8-72 hours depending on age) conducted under medical observation for investigation of a problem, usually hypoglycemia. Fasting has occasionally been recommended as a therapeutic intervention by physicians of many cultures, though it is uncommonly resorted to for this purpose by modern doctors.
Along with obligatory prayer, it is one of the greatest obligations of a Bahá'í. The Guardian of the Bahá'í Faith, Shoghi Effendi, explains: "It is essentially a period of meditation and prayer, of spiritual recuperation, during which the believer must strive to make the necessary readjustments in his inner life, and to refresh and reinvigorate the spiritual forces latent in his soul. Its significance and purpose are, therefore, fundamentally spiritual in character. Fasting is symbolic, and a reminder of abstinence from selfish and carnal desires."
Buddhist monks and nuns following the Vinaya rules commonly do not eat each day after the noon meal. This is not considered a fast, but rather a disciplined regimen aiding in meditation. Fasting is not practiced by lay Buddhists because it is seen as a deviation from the Middle Path. This is because prior to attaining Buddhahood, prince Siddhartha practiced a regime of four years of strict austerity during which he consumed very little food. Later on this practice was abandoned since it achieved nothing. Henceforth, prince Siddhartha practiced moderation in eating which he later advocated for his disciples.
The Vajrayana practice of Nyung Ne is based on the tantric practice of Chenrezig. It is said that Chenrezig appeared to Gelongma Palmo, an Indian nun who had contracted leprosy and was on the verge of death. Chenrezig taught her the method of Nyung Ne in which one keeps the eight precepts on the first day, then refrains from both food and water on the second. Although seemingly against the Middle Way, this practice is to experience the negative karma of both oneself and all other sentient beings and, as such is seen to be of benefit. Other self-inflicted harm is discouraged.
Perhaps due to sectarian differences, some lineages of Buddhism consider taking the eight precepts, even for a limited period of time, to be a fast. In fact, they are occasionally referred to as "fasting precepts." The eight precepts closely resemble the ten vinaya precepts for novice monks and nuns. The novice precepts are the same with the prohibition against handling money. (For further information, see The Way to Buddhahood: Instructions from a Modern Chinese Master by Venerable Yin-shun.)
The "acceptable fast" is discussed in the biblical Book of Isaiah, chapter 58:3-7, and is discussed metaphorically. In essence, it means afflict the soul through abstaining from fulfilling the needs or wants of the flesh. The blessings gained from this are claimed to be substantial. Christian denominations that practice this acceptable fast often attest to the spiritual principles surrounding fasting and seek to become a testament to those principles. The opening chapter of the Book of Daniel, vv. 8-16, describes a partial fast and its effects on the health of its observers. Fasting is a practice in several Christian denominations or other churches. Other Christian denominations do not practice it, considering it an external observance, but many individual believers choose to observe fasts at various times at their own behest, and the Lenten fast observed in the Catholic Church and the Eastern Orthodox Church is a forty-day partial fast to commemorate the fast observed by Christ during his temptation in the desert. This is similar to the partial fasting within the Ethiopian Orthodox church (abstaining from meat and milk) which takes place during certain times of the year, and lasts for weeks.
And I brought him to Your disciples, and they were not able to cure him.
And Jesus answered, O you unbelieving and perverse generation! How long am I to remain with you? How long am I to bear with you? Bring him here to Me.
And Jesus rebuked the demon, and it came out of him, and the boy was cured instantly.
Then the disciples came to Jesus and asked privately, Why could we not drive it out?
He said to them, Because of the littleness of your faith. For truly I say to you, if you have faith like a grain of mustard seed, you can say to this mountain, Move from here to yonder place, and it will move; and nothing will be impossible to you.
But this kind does not go out except by prayer and fasting." (15-21)
For Roman Catholics, fasting is the reduction of one's intake of food to one full meal (which may not contain meat during Fridays in Lent) and two small meals (known liturgically as collations, taken in the morning and the evening). Eating solid food between meals is not permitted. Fasting is required of the faithful on specified days. Complete abstinence is the avoidance of meat for the entire day. Partial abstinence prescribes that meat be taken only once during the course of the day. To some Roman Catholics, fasting still means consuming nothing but water.
Pope Pius XII had initially relaxed some of the regulations concerning fasting in 1956. In 1966, Pope Paul VI in his apostolic constitution Paenitemini, changed the strictly regulated Roman Catholic fasting requirements. He recommended that fasting be appropriate to the local economic situation, and that all Catholics voluntarily fast and abstain. In the United States, there are only two obligatory days of fast - Ash Wednesday and Good Friday. The Fridays of Lent are days of abstinence: those observing the practice may not eat meat. Pastoral teachings since 1966 have urged voluntary fasting during Lent and voluntary abstinence on the other Fridays of the year. The regulations concerning such activities do not apply when the ability to work or the health of a person would be negatively affected.
Prior to the changes made by Pius XII and Paul VI, fasting and abstinence were more strictly regulated. The church had prescribed that Roman Catholics observed fasting and/or abstinence on a number of days throughout the year.
In addition to the fasts mentioned above, Roman Catholics must also observe the Eucharistic Fast, which involves taking nothing but water and medicines into the body for one hour before receiving the Eucharist during the Mass. The ancient practice was to fast from midnight until Mass that day, but as Masses after noon and in the evening became common, this was soon modified to fasting for three hours. Current law requires merely one hour of eucharistic fast, although some Roman Catholics still abide by the older rules.
The Book of Common Prayer prescribes certain days as days for fasting and abstinence, but since the separation of the Church of England from the Roman Catholic Church, there have been no regulations prescribing the mode of observance of these days, nor is any distinction made between fasting and abstinence. Observance of fast days declined until the 19th century, when under the influence of the Oxford Movement many Anglicans began once again taking the prescribed fast days more seriously.
The Book of Common Prayer sets out the prescribed days as follows:
A Table of the Vigils, Fasts, and Days of Abstinence, to be Observed in the Year.
- The Evens or Vigils before:
- The Nativity of our Lord.
- The Annunciation of the Blessed Virgin.
- Easter Day.
- Ascension Day.
- Note, that if any of these Feast-Days fall upon a Monday, then the Vigil or Fast-Day shall be kept upon the Saturday, and not upon the Sunday next before it.
- Days of Fasting, or Abstinence.
- I. The Forty Days of Lent.
- II. The Ember-Days at the Four Seasons, being the Wednesday, Friday, and Saturday after the First Sunday in Lent, the Feast of Pentecost, September 14, and December 13.
- III. The Three Rogation Days, being the Monday, Tuesday, and Wednesday, before Holy Thursday, or the Ascension of our Lord.
- IV. All the Fridays in the Year, except Christmas Day.
In the process of revising the Book of Common Prayer in various parts of the Anglican Communion the specification of abstinence or fast for certain days has been retained, though because each province is free to set its own calendar, there is no universal Anglican rule for which days are fast days. Generally Lent and Fridays are set aside, though Fridays during the Easter season are sometimes avoided. Often the Ember Days or Rogation Days are also specified, and the eves of certain feasts.
Individual Anglicans are free to determine for themselves what particular measures of abstinence they will follow in the observance of these days, though certain parishes and dioceses are more encouraging of fasting than others. One diocese, that of Sydney in Australia, discourages its people from fasting during the season of Lent.
For Eastern Orthodox and Greek-Catholic Christians, fasting is an important spiritual discipline, found in both the Old Testament and the New, and is tied to the principle in Orthodox theology of the synergy between the body (Greek: soma) and the soul (pnevma). That is to say, Orthodox Christians do not see a dichotomy between the body and the soul, but rather consider them as a united whole, and believe that what happens to one affects the other (this is known as the psychosomatic union between the body and the soul). Saint Gregory Palamas argued that man's body is not an enemy, but a partner and collaborator with the soul. Christ, by taking a human body at the Incarnation, has made the flesh an inexhaustible source of sanctification. This same concept is also found in the much earlier homilies of Saint Macarius the Great.
Fasting can take up a significant portion of the calendar year. The purpose of fasting is not to suffer, but according to Sacred Tradition to guard against gluttony and impure thoughts, deeds and words. Fasting must always be accompanied by increased prayer and almsgiving (donating to a local charity, or directly to the poor, depending on circumstances). To engage in fasting without them is considered useless or even spiritually harmful. To repent of one's sins and to reach out in love to others is part and parcel of true fasting.
Wednesdays and Fridays are also fast days throughout the year (with the exception of fast-free periods—see below). In some Orthodox monasteries, Mondays are also observed as fast days (Mondays are dedicated to the Angels, and monasticism is called the "angelic life").
Other days occur which are always observed as fast days:
When a feast day occurs on a fast day, the fast is often mitigated (lessened) to some degree (though meat and dairy are never consumed on any fast day). There are two degrees of mitigation: allowance of wine and oil; and allowance of fish, wine and oil. The very young and very old, nursing mothers, as well as those for whom fasting would endanger their health, are exempt from the strictest fasting rules.
On weekdays of the first week of Great Lent, fasting is particularly severe, and many observe it by abstaining from all food for some period of time. According to strict observance, on the first five days (Monday through Friday) there are only two meals eaten, one on Wednesday and the other on Friday, both after the Presanctified Liturgy. Those who are unable to follow the strict observance may eat on Tuesday and Thursday (but not, if possible, on Monday) in the evening after Vespers, when they may take bread and water, or perhaps tea or fruit juice, but not a cooked meal. The same strict abstension is observed during Holy Week, except that a meal (with wine and oil) is allowed on Great Thursday.
On Wednesday and Friday of the first week of Great Lent the meals which are taken consist of xerophagy (literally, "dry eating"). That is to say, vegetables cooked with water and salt. In a number of monasteries, and in the homes of more devout laypeople, xerophagy is observed on every weekday (Monday through Friday) of Great Lent, except when wine and oil are allowed.
With exception of the Fifty days following Easter in the Coptic Orthodox Church fish is not allowed during Lent, Wednesdays, Fridays, and Baramon days. Other than that Fish and Shellfish are allowed during Fasting days.
The discipline of fasting entails that apart from Saturdays, Sundays, and Holy feasts should keep a total fast from all food and drink from midnight the night before to a certain time in the day usually three O'clock in the afternoon (the hour Jesus died on the Cross). Also, it is preferred to practice the reduction of one's daily intake of food (typically, by eating only one full meal a day).
In Protestantism, the continental 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.
Likewise, Lutheran churches encourage fasting during Lent. They also encourage it before partaking in the Eucharist, as Luther writes in his Small Catechism: Who, then, receives such Sacrament worthily? Fasting and bodily preparation is, indeed, a fine outward training; but he is truly worthy and well prepared who has faith in these words: Given, and shed for you, for the remission of sins.
Other Protestants consider fasting, usually accompanied by prayer, to be an important part of their personal spiritual experience, apart from any liturgical tradition. The United Methodist fast in the old Wesleyan way of sundown to sundown on Mondays to Tuesdays and Thursdays to Fridays to promote discipline among Christ's followers.
Individuals in mainline Pentecostal denominations undertake both short and extended fasts as the Spirit leads them. In the Normal Fast pure water alone is consumed. During the "Black Fast" nothing, not even water is consumed. In addition to the Normal Fast and the Black Fast Pentecostals sometimes undertake what they call the Daniel Fast (or Partial Fast) in which only one type of food (ie, fruit or fruit and non starchy vegetables) is consumed.
For Charismatic Christians fasting is undertaken at the leading of God. Fasting is done in order to seek a closer intimacy with God, as well as an act of petition. Some take up a regular fast of one or two days a week as a spiritual observance. Holiness movements, such as those started by John Wesley, Charles Wesley, and George Whitefield in the early days of Methodism, often practice such regular fasts as part of their regimen.
For members of the Church of Jesus Christ of Latter-day Saints, fasting is total abstinence from food and drink. Members are encouraged to fast on the first Sunday of each month, designated as Fast Sunday. During Fast Sunday, members fast for two consecutive meals. The money saved by not having to purchase and prepare meals is donated to the church as a fast offering, which is to be used to help people in need. The late LDS President Gordon B. Hinckley asked: “What would happen if the principles of fast day and the fast offering were observed throughout the world[?] The hungry would be fed, the naked clothed, the homeless sheltered. … A new measure of concern and unselfishness would grow in the hearts of people everywhere.” (“The State of the Church,” Ensign, May 1991, 52–53.)
Fasting is also encouraged for members any time they desire to grow closer to their Father in heaven and to show self-mastery of spirit over body. Members may also implement personal, family or group fasts any time they desire to solicit special blessings from God, including health or comfort for themselves and/or others.
Individuals can also use fasting as a part of their repentance process or to show gratitude towards God.
Fasting is a very integral part of the Hindu religion. Individuals observe different kinds of fasts based on personal beliefs and local customs. Some are listed below.
Methods of fasting also vary widely and cover a broad spectrum. If followed strictly, the person fasting does not partake any food or water from the previous day's sunset until 48 minutes after the following day's sunrise. Fasting can also mean limiting oneself to one meal during the day and/or abstaining from eating certain food types and/or eating only certain food types. In any case, even if the fasting Hindu is non-vegetarian, he/she is not supposed to eat or even touch any animal products (i.e. meat, eggs) on a day of fasting. (Milk is an exception for animal products).
In Sri Vidya, one is forbidden to fast because the Devi is within them, and starving would in return starve the god. The only exception in Srividya for fasting is on the anniversary of the day one's parents died.
In Islam, fasting for a month is an obligatory practice during the holy month of Ramadan, from fajr (dawn), until the maghrib (sunset). Muslims are prohibited from eating, drinking, smoking, and engaging in sexual intercourse while fasting. Fasting in the month of Ramadan is one of the Pillars of Islam, and thus one of the most important acts of Islamic worship. By fasting, whether during Ramadan or other times, a Muslim draws closer to their Lord by abandoning the things they enjoy, such as food and drink. This makes the sincerity of their faith and their devotion to God (Arabic: Allah) all the more evident.
The Qur'an states that fasting was prescribed for those before them (i.e., the Jews and Christians) and that by fasting a Muslim gains taqwa, which can be described as the care taken by a person to do everything God has commanded and to keep away from everything that He has forbidden. Fasting helps prevent many sins and is a shield with which the Muslim protects him/herself from jahannam (hell).
O who believe, fasting is decreed for you as it was decreed for those before you; perchance you will guard yourselves. ...
The month of Ramadan is the month in which the Koran was sent down, a guidance for the people, and clear verses of guidance and criterion. [Quran: Chapter 2, 183]
Muslims believe that fasting is more than abstaining from food and drink. It also includes abstaining from any falsehood in speech and action, from any ignorant and indecent speech, and from arguing and fighting, and lustful thoughts. Therefore, fasting strengthens control of impulses and helps develop good behavior.
Fasting also inculcates a sense of fraternity and solidarity, as Muslims feel and experience what their needy and hungry brothers and sisters feel. However, even the poor, needy, and hungry participate in the fast. Moreover, Ramadan is a month of giving charity and sharing meals to break the fast together.
The Siyam is intended to teach muslims patience and self-control, and to remind them of the less fortunate in the world. The fast is also seen as a debt owed by the muslim to God. Faithful observance of the Siyam is believed to atone for personal faults and misdeeds, at least in part, and to help earn a place in paradise. It is also believed to be beneficial for personal conduct, that is, to help control impulses, passions and temper. The fast is also meant to provide time for meditation and to strengthen one's faith.
While fasting in the month of Ramadan is considered Fard (obligatory), Islam also prescribed certain days for non-obligatory, voluntary fasting, such as:
Fasting is forbidden on these days:
Although fasting is fard (obligatory), exceptions are made for persons in particular circumstances:
Penalty of purposefully breaking fast at Ramadan:
Self-starvation by fasting is known as Sallekhana and is supposed to help shed karma according to Jain philosophy. Another form of fasting is Santhara, the Jain religious ritual of voluntary death by fasting. Supporters of the practice believe that Santhara cannot be considered suicide, but rather something one does with full knowledge and intent, while suicide is viewed as emotional and hasty. Due to the prolonged nature of Santhara, the individual is given ample time to reflect on his or her life. The vow of Santhara is taken when one feels that one's life has served its purpose. The goal of Santhara is to purify the body and, with this, the individual strives to abandon desire.
Yom Kippur is considered to be the most important day of the Jewish year and fasting as a means of repentance is expected of every Jewish man and boy above the age of bar mitzvah and every Jewish woman and girl above the age of bat mitzvah. It is so important to fast on this day, that only those who would be put in danger by fasting are exempt, such as the ill, elderly, or pregnant or nursing women, as endangering one's life is against a core principle of Judaism. Those that do eat on this day are encouraged to eat as little as possible at a time and to avoid a full meal. For some, fasting on Yom Kippur is considered more important than the prayers of this holy day. If one fasts, even if one is at home in bed, one is considered as having participated in the full religious service. In addition to fasting and prayer, Yom Kippur -- as the "Sabbath of Sabbaths" -- has the same restrictions regarding work as the Sabbath, such as striking a fire, carrying objects outside the home, using tools, and so on. Traditionally, leather shoes are not worn on this day. Men may wear a white gown (kittel) over their clothes, symbolic of a burial shroud on this Day of Judgement. Women may either wear all white, or they may simply wear a large white scarf over their heads, and many do not put on make-up or jewelry. The aura of the day is serious, humble, sacred and repentant, yet happy in the knowledge that sincere repentance brings redemption.
The second major day of fasting is Tisha B'Av, the day nearly 2000 years ago on which the Romans destroyed the Holy Temple in Jerusalem and the Jews were banished from their homeland. Tisha B'Av ends a three-week mourning period beginning with the fast of the 17th of Tammuz. Unlike the fast of Yom Kippur, there are no restrictions on activities, although one should try to avoid doing regular work the first part of the day, sit in a low chair or on the floor, and wear no leather shoes. This is also the day when observant Jews remember the many tragedies which have befallen the Jewish people, including the Holocaust. The atmosphere of this holiday is serious and deeply sad.
Both of these holy days are considered major fasts and are observed from sunset to sunset the following day by both men and women. The remaining four fasts are considered minor and fasting is only observed from sunrise to sunset. Men are expected to observe them, and women should observe them, but a rabbi may often give dispensions if the fast represents too much of a hardship to a sick or weak person.
On the two major fast days sexual relations are also forbidden.
Other fast days are:
Minor fast days, not universally observed, include:
It is traditional for a bride and groom to fast on their wedding day before the ceremony as the day represents a personal Yom Kippur. In some congregations, repentance prayers that are taken from the Yom Kippur service are included by the bride and groom in the service before the ceremony.
Aside from these official days of fasting, Jews may take upon themselves personal or communal fasts, often to seek repentance in the face of tragedy or some impending calamity. For example, a fast is sometimes observed if the scrolls of the Torah are dropped. The length of the fast varies, and some Jews will reduce the length of the fast through tzedakah, or charitable acts. Mondays and Thursdays are considered especially auspicious days for fasting.
Nevertheless, fasting is conducive to atonement, for it tends to precipitate contrition in the one who fasts (see Joel, 2:12-18). This is why the Bible requires fasting (lit. self affliction) on Yom Kippur (see Leviticus, 23:27,29,32; Numbers, 29:7; Tractate Yoma, 8:1; ibid. (Babylonian Talmud), 81a). Because, according to the Hebrew Bible, hardship and calamitous circumstances can occur as a result of wrongdoing (see, for example, Leviticus, 26:14-41), fasting is often undertaken by the community or by individuals to achieve atonement and avert catastrophe (see, for example, Esther, 4:3,16; Jonah, 3:7). Most of the Talmud's Tractate Ta'anit ("Fast[s]") is dedicated to the protocol involved in declaring and observing fast days.
The second purpose in fasting is commemorative mourning. Indeed, most communal fast days that are set permanently in the Jewish calendar fulfil this purpose. These fasts include: Tisha B'Av, Seventeenth of Tammuz, Tenth of Tevet (all of the three dedicated to mourning the loss of the destroyed Temple in Jerusalem), and Fast of Gedaliah. The purpose of a fast of mourning is the demonstration that those fasting are impacted by and distraught over earlier loss. This serves to heighten appreciation of that which was lost. This is in line with Isaiah (66:10), who indicates that mourning over a loss leads to increased happiness upon return of the loss:
The third purpose in fasting is commemorative gratitude. Since food and drink are corporeal needs, abstinence from them serves to provide a unique opportunity for focus on the spiritual. Indeed, the Midrash explains that fasting can potentially elevate one to the exalted level of the Mal'achay HaSharait (ministering angels) (Pirkei d'Rabbi Eliezer, 46). This dedication is considered appropriate gratitude to God for providing salvation. Additionally, by refraining from such basic physical indulgence, one can more greatly appreciate the dependence of humanity on God, leading to appreciation of God's benificience in sustaining His creations. Indeed, Jewish philosophy considers this appreciation one of the fundamental reasons for which God endowed mankind with such basic physical needs as food and drink. This is seen from the text of the blessing customarily recited after consuming snacks or drinks:
People can also fast for medical reasons, which has been an accepted practice for many years. One reason is to prepare for surgery or other procedures that require anesthetic. Because the presence of food in a person's system can cause complications during anesthesia, medical personnel strongly suggest that their patients fast for several hours (or overnight) before the procedure.
Another reason for medical fasting is for certain medical tests, such as cholesterol testing (lipid panel). People are often asked to fast so that a baseline can be established. In the case of cholesterol, the failure to fast for a full 12 hours (including vitamins) will guarantee an elevated Triglyceride measurement.
It has been shown in many empirical, scientific studies that fasting can improve health and help to eliminate a variety of diseases. Although some fasting methods use juice or various amounts of food, the health of such methods is questionable, according to Dr. Joel Fuhrman. A true fast, he contends, consists of an intake solely of water, and can last (healthily) for extended periods of time when undertaken with the correct knowledge. According to Fuhrman, it is critically important to consume no calories (to put the body in Ketosis), or more than 800 calories per day, to avoid Hypoglycemia resulting in brain damage. Thus, diets of 100-800 calories/day are very dangerous. Any fasts of such nature should be preceded and followed by a healthy diet, and should also be supervised by a knowledgeable physician to make sure that deficiencies of any nutrients do not take place and detract from the healthful benefits of such a fast
Some doctors believe that pure water fasting can not only detoxify cells and rejuvenate organs, but can actually cure such diseases and conditions as cardiovascular disease, rheumatoid arthritis, asthma, high blood pressure, type 2 diabetes, colitis, psoriasis, lupus and some other autoimmune disorders when combined with a healthy diet. They believe that "Fasting is Nature's Restorer." There is empirical evidence to corroborate the beliefs of these scientists.
Recent studies on mice show that fasting every other day while eating double the normal amount of food on non-fasting days led to improved insulin and blood sugar control, neuronal resistance to injury, and health indicators superior to mice on 40% calorie restricted diets. Alternate-day calorie restriction may prolong lifespan and attenuate diseases associated with inflammation, oxidative stress and aging.
People near the end of their lives sometimes consciously refuse food and/or water. The term in the medical literature is "patient refusal of nutrition and hydration".
In naturopathic medicine, fasting is seen as a way of cleansing the body of toxins and dead or diseased tissues, and giving the gastro-intestinal system a rest. Such fasts are either water-only, or consist of fruit and vegetable juices. A juice fast can also be used as a stepping-stone to a water fast, reducing the amount of discomfort experienced.
Common terms used in research are: reduced diet therapy (RDT), Fasting Therapy (FT) and caloric restriction (CR). Research tends to originate from Russia, Japan and Germany.
Activists have also used fasting to bring attention to a cause and to pressure authority or government to act. For example, Canadian medical doctor and politician David Swann has launched a seven-day fast in December 2007 to bring attention to the world's inaction on the humanitarian crisis in Darfur.
In Northern Ireland in 1981 a prisoner, Bobby Sands, was part of the 1981 Irish hunger strike, protesting for better rights in prison. Sands had just been elected to the British Parliament and died after 66 days of not eating. His funeral was attended by 100,000 people and the strike ended soon afterwards.
Glucose is the body's primary fuel source, and is essential for the brain's functioning. When denied glucose for more than 4-8 hours, the body will turn to the liver for glycogen, a storage form of glucose, to be used for fuel. A process called glycogenolysis converts glycogen into a usable form of fuel. At this point, the body will also use small amounts of protein to supplement this fuel. This fuel will last for up to 12 hours before the body needs to turn to muscle stores of glycogen, lasting for a few more days. If glucose is still denied at this point, muscle wasting is prevented by temporarily switching to fat as the fuel source meaning fat is converted into ketone through catabolism. Ketones, while not sugars, are able to be used by the brain as a fuel source as long as glucose is denied.
As a protective biomechanism, many toxins are stored within fat. During catabolism, these toxins are liberated and then released into the blood stream. This increases the likelihood of acetaminophen poisoning, possibly because of depletion of hepatic glutathione reserves.. The body will continue to use fat for as long as there is fat to consume. The body will generally indicate to the faster when fat levels are running extremely low (less than 2%) with an increased urge for food. Fasts are usually broken long before this point. If the fast is not broken, starvation will begin to occur, as the body begins to use protein for fuel. It will begin with the least important proteins, then muscles, and eventually organs. Death may occur before the body turns to organs as a fuel source however.
Research conducted by University of California, Berkeley suggests there are major health benefits to fasting. Benefits include a reduced risk of cancer, the slowing of the aging process and the potential to increase maximum life span. Currently, the reduction of caloric intake is the only proven method of increasing the lifespan of an organism According to Dr. Mark P. Mattson, chief of the laboratory of neurosciences at the National Institute on Aging, fasting every other day (intermittent fasting) shows as strong if not stronger beneficial effects as caloric-restriction diets According to The National Academy of Sciences other health benefits include stress resistance, increased insulin sensitivity, reduced morbidity, and again increased life span Long term studies in humans have not been conducted. However, short term human trials showed benefits in weight loss. The side effect was the participants felt cranky during the three week trial. According to the study conducted by Dr. Eric Ravussin "Alternate-day fasting may be an alternative to prolonged diet restriction for increasing the life span."
Adherence to Greek Orthodox fasting periods contributes to a reduction in the blood lipid profile including a non-significant reduction in HDL cholesterol and possible impact on obesity .
Prolonged fasting can result in serious health problems. It is highly advised that anyone thinking of fasting as a "diet" consult a physician or dietitian before they start. Fasting can result in harmful and irreversible health problems if maintained too long, or not done properly with the advice of a health professional.