Fasting is the practice of avoid certain foods or abstain from eating at certain times. It is often used in religious ceremonies, preparation(s), and/or ritual(s). The custom of fasting has, at one point or another, been a practice of almost every religion.
Theists claim a variety of justifications for fasting. In some religions, fasting is used as a means to cleanse sinful deeds from your soul. Fasting is also observed as a mourning ritual (in some cultures/religions). More commonly, in religious practice, fasting is used as a preparation for the obtainment of an "enhanced spiritual state."
It is argued, by spiritual types, that fasting leads to an enhanced perception of the divine. By depriving the body of food (and sometimes water), it is believed that one is sacrificing earthly needs in an effort to reach higher spiritual connections. After several days of fasting, some religious groups believe that one is primed to see/hear God. Believers often report that they feel "lighter". They ascribe this purity to the absolution of sins from fasting.
- Jewish laws order a fast on Yom Kippur, the day of atonement. Orthodox Jews often order brides and grooms to fast on the day prior to their wedding.
- Many Christians fast during Lent, the period of forty days that Jesus supposedly fasted in the wilderness.
- Muslims fast during Ramadan, the ninth month of their year. Exceptions are optionally allowed if a person is travelling a long distance, pregnant, medically unable  or at war.
- Baha'is fast annually in what is referred to as a 19 day fast.
- Buddhists, especially, fast to attain a higher sphere of "enlightenment" or spiritual alertness.
Both atheists and theists may fast for medical reasons but is usually distinct from religious fasting.
If a deity wants you to reach a higher spiritual plane, why can't he use more straightforward means? Why do some people report visions without fasting? The fact that God's existence is not at all clear is an argument for God's non-existence.
Sensation of fasting is explained by physiology
When you fast for a long period of time, your body goes into a state known as ketosis. Ketosis is the process in which your body changes from burning glycogen to proteins and then fat, which releases ketones.
During the first stages of ketosis, individuals often feel as if they are more alert and/or able to perform physical tasks better. In reality, this stage is temporary and a dangerous signal that the body is trying to release any and all of it's glucose stores.
- "Medical science has long made clear that regular intakes of food and drink are a sine qua non for good health and soundness of mind. "
If one wanted to be more "in touch" with a spiritual realm, or even their own body, one should heed the signals that the body is giving.
Fasting is clearly detrimental to a person's health. A good and just God would not require such a thing be done. In the early stages of fasting, a person often experiences: Fatigue, General discomfort, uneasiness, or ill feeling (malaise), Nervousness, Irritability, or even aggression, Trembling, Headache, Hunger, Cold sweats, Rapid heart rate, Blurry or double vision, Confusion, Convulsions, Coma. Later states of starvation often have the following symptoms: Hallucinations, Excessive sweating, Sleeping difficulty, Paleness, Muscle pain, Memory loss, Palpitations, or feeling your heartbeat (heartbeat sensations), Fainting, Different size pupils, Dizziness, Decreased consciousness.
Regarding Orthodox Jewish weddings: As if the "drop count" wasn't high enough, due to stress and excitement, God is now asking the soon-to-be-wed to not eat, too! Nice God!
Islamic fasting during Ramadan
Studies have found that accidents increase during Ramadan.  People take more risks, such as dangerous driving, when travelling to break their fast. Unsurprisingly, ther is an increase in the number of trafic accidents. 
- "fasting can cause temporary low blood sugar and dehydration, which in turn can result in tiredness, impatience, headaches, faint-headedness and loss of concentration - a lethal cocktail for any driver. "
- "Accidents usually rise in Ramadan in the hours before sunset as motorists exceed the speed limit to reach their homes in time for iftar, when Muslims break their day-long fast. "
Despite being exempt, pregnant women sometimes fast during Ramadan but this has long term adverse effects on their offspring. Medical advice is for women to avoid fasting while pregnant. 
Many Muslims refrain from taking their medication during the daytime while fasting for Ramadan. In some cases, patients changed their drug intake regime, stopped taking it altogether or even took their entire day's mediation at once. This can reduce the effectiveness of medication and puts the person's heath at risk. 
Fasting for Ramadan means that people refrain from drinking liquids, which raises the risk of dehydration, which also causes headaches, dizziness and nausea.
Fasting during Ramadan has significant detrimental impact on worker productivity.  Even in jobs in which a person is responsible for safety, they would rather follow the Ramadan fast rather than being in peak readiness. Fasting increases the risk to surrounding people if the emergency responder cannot perform as effectively.
The fast is often broken after sunset by binge eating, which has further negative health consequences.
- "Peptic ulcer perforation and acute mesenteric ischemia were found to be significantly increased during Ramadan compared with the months before and after Ramadan. On the other hand, violence-induced penetrating injuries like stab wounds and gunshot wounds decreased significantly during Ramadan. "
Research is lacking in the area because of religious sensitivities.
Fasting is ignoring the purpose of hunger
Even if you subscribe to the assumption that God created you, he obviously created you to eat. On that assumption, he also gave you the sensation of hunger to guarantee that you would eat.
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- ↑ 2.0 2.1 
- ↑ 3.0 3.1 Effects of the Ramadan fast on trauma, Health MED - Volume 7 / Number 1 / 2013
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- ↑ Reduction in the Rate of Interpersonal Violence during the Holy Month of Ramadan
- ↑ 11.0 11.1 Effect of Ramadan on surgical emergencies