Al-Bukhari, may Allah have mercy upon him, has reported that the Messenger of Allah, sallallaahu ‘alaihi wa sallam, said: “Paradise has eight gates and one of them is Ar-Rayyaan through which none will enter but those who observe fating.” As-Suyooti, may Allah have mercy upon him, has reported that Al-Haarith Ibn Kalda, may Allah have mercy upon him, stated that: “Imposing abstinence from food” was the essence of medicine. Fasting, in medical terms, may mean a variety of things. 
Some healthcare practitioners might recommend juice fasting. This type of fast eliminates all solid foods and the person consumes only fruit and vegetable juices. Others might recommend elimination of all foods except fruits and water for a specified time. Some might even recommend eliminating all foods and drinks for specific periods. The reported purpose for these forms of fasting is to cleanse toxins and poisons form the body and the digestive tract. Islamically speaking, fasting both voluntary and obligatory is an act of worship. Although there may be health benefits, its primary purpose must always be to please Allah.
Alternative medicine views fasting as a detoxification method. This simply means that it is used to purify the body by eliminating “toxins.” This process enhances the healing processes of the body. Fasting generally is perceived to spare any harm to vital organs but uses fatty tissue, etc. For energy production. Naturopathic health care practitioners generally proscribe short fasts. Because body temperature may drop during a fast, as does blood pressure, pulse, and respiration, it is considered important to stay warm. Fasting is seen as possible treatment for a variety of disorders and diseases including: obesity, chemical poisoning, arthritis, allergies, psoriasis, eczema, leg ulcers, irritable bowel syndrome, bronchial asthma and even depression.
Fasting is a protection for the body and the mind. At-Tirmithi, may Allah have mercy upon him, has reported that the Prophet, sallallaahu ‘alaihi wa sallam, said: “Fasting and the Qur’an intercede for a man. Fasting says, ‘O my Lord I have kept him away from his food and his passions by day, so accept my intercession for him.’ The Qur’an says, ‘I have kept him away from sleep by night so accept my intercession for him.’ Then their intercession is accepted.” Research has shown that fasting can improve the control of diabetes. Another study has shown that fasting improved the health of persons with irritable bowel syndrome and constipation. Another study suggests that fasting may improve cholesterol levels.
Al-Bukhari, may Allah have mercy upon him, has reported that the Messenger of Allah, sallallaahu ‘alaihi wa sallam, said that Allah said: “All the deeds of the children of Aadam are for them, except fasting, which is for Me and I will give the reward for it.” Al-Bukhari, may Allah have mercy upon him, has also reported that the Prophet, sallallaahu ‘alaihi wa sallam, narrated that Allah said: “The fast is for Me and I will give the reward for it, as he (who observes the fast) leaves his (sexual) desire, food and drink for My Sake. Fasting is a shield (from Hell) and there are two pleasures for a fasting person, one at the time of breaking his fast and the other at the time when he will meet his Lord. The smell of the mouth of a fasting person is better in Allah’s Sight than the smell of musk.”
There are specific requirements for the Muslim to consider when fasting. First is the intention to worship Allah and obey His commandments. Second, is that the fasting must take place between dawn to dusk. Muslims are prohibited from fasting for more than two continuous days without breaking the fast. This is based on the narration reported in Al-Muwatta, which states that the Prophet, sallallaahu ‘alaihi wa sallam, forbade fasting for two days or more without breaking the fast in between. Likewise, fasting is prohibited on cer­tain days: ‘Eid Al-Fitr, ‘Eid Al-Adha, and on Jumu’ah (Fridays unless the person has fasted the day before or will fast the day after). Married women are admonished not to fast without the permission of their hus­bands, as the husbands have a right over their wives.
Breaking the fast with dates or sips of water is better. Again, this is based on the Prophet’s Sunnah, sallallaahu ‘alaihi wa sallam, and the narration reported by Abu Daawood, may Allah have mercy upon him: “When one of you is fasting, he should break his fast with dates, but if he cannot get any, then (he should break his fast) with water, for water is purifying.” Muslim, may Allah have mercy upon him, has reported that the Prophet, sallallaahu ‘alaihi wa sallam, said: “The Ajwah date is an excellent remedy.” The best days for fasting are Mondays and Thursdays. At-Tirmithi, may Allah have mercy upon him, also reported that the Prophet, sallallaahu ‘alaihi wa sallam, said: “The deeds of men are presented to Allah on Mondays and Thursdays, and I like mine to be presented when I am fasting.”  The thirteenth, fourteenth, and fif­teenth of each lunar month are also recommended for fasting. 
While both the obligatory fasting during Ramadan and the voluntary fast­ing during the remainder of the year are acts of worship, they may provide healing to the body. Whatever healing may occur because of fasting is a blessing from Allah, however this should not be the intention or the goal of fasting. Fasting gives us a greater ability to control our desires and emotions. It should not be con­sidered a “cure-all” or a diet programme. Those who fast will receive their reward from Allah as long as their intentions are pure and their fasting is not harmed or nullified by lying, backbiting, or evil deeds.
Article source: http://www.islamweb.net/emainpage/