Fasting in monotheistic religions: Judaism, Christianity and Islam.
Judaism
Fasting for Jews means complete abstinence from food and drink, including water. Observant Jews traditionally fast six days a year. With the exception of Yom Kippur, fasting is never permitted on Saturday.
Christianity
Orthodox, Catholic, Anglican, and Methodist Christian denominations have emphasized the importance of the Friday fast, which includes abstinence from meat, dairy products, and alcohol.
Roman Catholicism
Catholic fasting, is to reduce food intake to one full meal that does not contain meat on Ash Wednesday and Good Friday and Fridays throughout the year unless Friday is celebrated and two small meals should not equal the large meal. Solid food is not allowed between meals. It is necessary to completely refrain from eating meat for this day for those aged 14 and over.
Eastern Orthodox
Orthodox Christians fast on Wednesday (in memory of Jesus' betrayal) and on Friday (in memory of Jesus' crucifixion on Good Friday). Not eating olive oil, meat, fish, milk and dairy products until sunset. In addition, they abstain from sexual relations on Wednesdays and Fridays throughout the year, as do the entirety of Lent, Christmas Lent, and the fifteen days before the feast of the Assumption of Mary.
Islam
Muslims fast the month of Ramadan from dawn to sunset every year, by abstaining from food, drink, and even smoking and sexual intercourse. There is also a broader meaning of fasting that includes abstaining from every falsehood in word and deed, and from fighting. Believers strive to purify the body and soul and increase their piety.
Fasting in earthly religions
Fasting is a feature of ascetic traditions in religions such as Hinduism, Buddhism, and Jainism. Mahayana tradition recommends that laity fast six days each month and three months of fasting each year. Members of the Baha'i Faith fast nineteen days from sunrise to sunset during the month of March each year. Buddhist monks and nuns do not eat every day after the noon meal. It is a disciplined system that aids in meditation and good health. Lay Buddhists fast during times of intense meditation. Where they completely abstain from eating animal products, but they drink milk. Moreover, they also avoid eating processed foods and pungent foods which are: all kinds of garlic and onions.
Hinduism
Certain days of the week are designated for fasting depending on the preferred deity. In southern India, Tuesday is dedicated to the goddess Mariamman. They eat before sunrise and drink liquids only between sunrise and sunset. In the north, Tuesdays and Saturdays are dedicated to Lord Hanuman and milk and fruits are only allowed between sunrise and sunset. In some parts of India, a special fast is practiced for married women in order to prolong the lives of their husbands. The fast is interrupted after the wife sees the moon through a sieve.
Sikhism
Sikhism does not consider fasting a spiritual act and does not encourage intentional starvation. But it encourages moderation in food, that is, not to be hungry and not to overeat.

Yes it is and you have to do it for bloodwork.

From everything I’ve read, yes it is. It’s really difficult for me though!

For me, absolutely not as being diabetic, fasting could result in dangerously low or high blood sugar levels.

In Islam, the Quran states that a sick person who is unable to fast is not required to fast. God said: ‘Fast a certain number of days, but if any one of you is ill or on a journey let him (fast) a similar number of days later on; and for those who are unable (to fast), there is a ransom the feeding of a needy person. Whosoever volunteers good, it is good for him; but to fast is better for you, if you but knew.’
But if the doctors decide that this disease continues and does not go away, then he is like an old person who finds it difficult to fast, so he does not have to fast, but he has to feed one poor person for each day.
The doctor does not recommend fasting for type 1 diabetics. As for the type 2 diabetes patient who takes diabetes tablets orally, doctors say that most patients in this category benefit from fasting, but they have to organize meals. The treating medical team shows them the necessary instructions on how to manage their condition throughout the month of Ramadan.

LUNG1954 has given us a thorough description of fasting as practiced by various religions, and there seems to be some confusion as to what God wants. Perhaps he (God or gods) think/s that appropriate amounts of healthy food are good for health.

Intermittent fasting may put patients into remission for type 2 diabetes, according to a new study published in the Endocrine Society’s Journal of Clinical Endocrinology & Metabolism.

https://www.healthline.com/health-news/type-2-diabetes-intermittent-fasting-may-help-reverse-condition

Patients achieved complete diabetes remission after an intermittent fasting diet intervention, according to a new research study. Complete diabetes remission is defined as an HbA1c (average blood sugar) level of less than 6.5% at least one year after stopping diabetes medication. The details were published on December 14 in the Endocrine Society’s Journal of Clinical Endocrinology & Metabolism.

https://scitechdaily.com/intermittent-fasting-completely-reverses-type-2-diabetes-in-study/

When God imposed fasting, He imposed that a man should not make sex with his wife during the month of Ramadan, neither at night nor during the day, as was the fasting of the Children of Israel in the Torah, and if a man slept at the beginning of the night before he broke his fast, it was forbidden for him to eat after sleeping, and among the Muslims were young men who secretly made sex with their wives at night Because of their lack of patience, so God revealed;
‘It has been made permissible for you to be intimate with your wives during the nights preceding the fast. Your spouses are a garment1 for you as you are for them. God knows that you were deceiving yourselves. So He has accepted your repentance and pardoned you. So now you may be intimate with them and seek what God has prescribed for you. ˹You may˺ eat and drink until you see the light of dawn breaking the darkness of night, then complete the fast until nightfall. Do not make sex with your spouses while you are meditating in the mosques. These are the limits set by God, so do not exceed them. This is how God makes His revelations clear to people, so they may become mindful ˹of Him˺.’ Quran.
1“Garment” is a metaphor for comfort, chastity, and protection.

The Bible says:
Do not deprive each other except by mutual consent for a time, so that you may devote yourselves to prayer. Then come together again so that Satan will not tempt you because of your lack of self-control" (1 Cor 7:3, 5).
Genesis 2:24, "Therefore a man shall leave his father and his mother and hold fast to his wife, and they shall become one flesh."

What does it mean to "become one flesh"?

Is it permissible to practice marital relations during fasting days (especially at night)?
Eastern Church's answer:
Fasting - in the Christian concept - is training to control the body from all lusts and sensual desires (to look, touch, hear and smell). Hence, it prohibits the practice of marital relations during the sacred fasts. And the days of masses, that is, not enjoying sex "in those blessed times."
Refraining from marital relations - during fasting - is known in ancient times (Dan. 6:18). Especially among the Jews, as it was mentioned in the Jewish interpretation of the "Mishnah", which states: "On the day of the penance fast, it is not permissible for the individual to eat, bathe, use perfume, or engage in marital intercourse."
However, this should be done with the agreement of both spouses - before fasting - lest the weak deviate (before the temptation of lust), and cause sin for himself or his life partner, as St. Paul the Apostle saw.
And if the marital relationship is practiced, that day is considered a “breakfast” for both of them.
Anba Sawiris, Bishop of Ashmounin

8th day of fasting and I've stopped feeling hungry. Extremely thirsty though, as the weather is so hot and humid.

Fasting Benefits-

Controls Blood Sugar Reducing Insulin resistance.

Promotes Better Health by Fighting Inflammation.

Enhance Heart Health by improving cholesterol, BP.

Boost functioning of brain.

Helps weight Loss

Improves Hunger

Improves Immune System

Etc.

How to avoid thirst during fasting
Delaying the suhoor time [the meal before sunrise], until after midnight.
It is necessary that the suhoor meal contain fruits, as they reduce the feeling of thirst, most notably grapes and watermelon and more vegetables, especially cucumbers. They also help reduce fluid loss in the body.
Avoid drinking too much water at the suhoor. It is better to distribute drinking water in quantities between iftar [the fast-breaking meal ] and suhoor.
Avoid salty foods because they increase thirst and the body's need for fluids during the day.
Avoid drinks that contain caffeine, such as coffee, Nescafe, and soft drinks, as they are diuretics and make the body lose water quickly.
Do not stay under high temperatures for long periods, and it is preferable to rest in cool places, as well as take frequent showers to cool the body and reduce the feeling of thirst.
Drink herbal drinks such as hibiscus, carob and licorice, as they prevent thirst.
Drink more soup as it is a rich source of water
Reduce sweets because they help to feel thirsty.

Fasting can also be done as a spiritual act! Giving up something for a time period in order to get closer to God. Biblically
It's a personal choice.

Fasting is a powerful spiritual discipline. Through fasting and prayer, the Holy Spirit can transform your life. And the practice of fasting has strong roots in the Bible. Jesus himself spent time in fasting and prayer during his life on earth, and he expected his followers to fast as well.

Today is the last days of Ramadan, the month of fasting, Muslims proclaim the greatness of God at sunset. God Almighty said:
‘’Ramaḍân is the month in which the Quran was revealed as a guide for humanity with clear proofs of guidance and the standard ˹to distinguish between right and wrong˺. So whoever is present this month, let them fast. But whoever is ill or on a journey, then ˹let them fast˺ an equal number of days ˹after Ramaḍân˺. Allah [God] intends ease for you, not hardship, so that you may complete the prescribed period and proclaim the greatness of Allah for guiding you, and perhaps you will be grateful.’’
To proclaim the greatness of God Almighty the Muslims say (God is great) as a metaphor for his oneness in divinity. Then they perform the Eid "feast" prayer.
The Muslims fast is similar to the Christian fast before Easter

you can fast if you want, but clearly fasting is not spritual its phyiscal. (note i wont give the proper word for it here) Yet i rather serve spirtual not physsical.

it does little to help, yet if you fast do it alone so one one can see you fast- it build your humility. It does no good to sit on top of a house and yell i am fasting all that proves is you a brager, and not religious in any way or form.

It is not a requirement for Salvation but it is Biblical. Many people have health issues that maynot allow them to fast without it being harmful. So it is a personal choice.

Sadly Many people don't read or Believe in the Bible or following Jesus so it is not important to them.

I did just shy of a 3 day water only fast and did not see or feel any of the "magical" effects that fasting supposedly brings. Lost a bit of weight of course but nothing else. Many youtube health "gurus" have some amazing claims about fasting but it didn't happen to me unfortunately. Maybe something extraordinary happened on the inside or maybe it just wasn't a long enough fast, who knows?