Physiologically, calorie restriction has been shown in animals to increase lifespan and improve tolerance to various metabolic stresses in the body.  Although the evidence for caloric restriction in animal studies is strong, there is less convincing evidence in human studies. Proponents of the diet believe that the stress of intermittent fasting causes an immune response that repairs cells and produces positive metabolic changes (reduction in triglycerides, LDL cholesterol, blood pressure, weight, fat mass, blood glucose). [3,5] An understandable concern of this diet is that followers will overeat on non-fasting days to compensate for calories lost during fasting. However, studies have not shown this to be true when compared with other weight loss methods. 
This proved to be the most important forty days of my life. As I waited upon the Lord, the Holy Spirit gave me the assurance that America and much of the world will, before the end of the year 2000, experience a great spiritual awakening. This divine visit from heaven will kindle the greatest spiritual harvest in the history of the Church. But before God comes in revival power, the Holy Spirit will call millions of God's people to repent, fast, and pray in the spirit of 2 Chronicles 7:14:
Also, for many people, a full 16 hours of fasting just isn’t realistic, says Cynthia Sass, a New York City– and L.A.-based performance nutritionist. She recommends 12 hours of overnight fasting at most and believes the 16-hour gap is especially tough on those who exercise early in the morning or late at night. “If fasting makes you feel miserable and results in intense cravings and rebound overeating, it's not the right path for you,” she says.
As an expression of lamentation and/or penitence, fasting nearly always is associated with weeping ( Judges 20:26 ; Esther 4:3 ; Psalm 69:10 ; Joel 2:12 ), confession ( 1 Sam 7:6 ; Dan 9:3 ), and the wearing of sackcloth ( 1 Kings 21:27 ; Neh 9:1 ; Esther 4:3 ; Psalm 69:10 ; Dan 9:3 ). In the New Testament Jesus chides the hypocritical Pharisees for disfiguring their faces when they fast ( Matt 6:16-18 ), a reference no doubt to the custom of smearing themselves with ashes. These objects and actions had no intrinsic penitential value but in a culture in which inner feelings were commonly displayed or even dramatized, when done sincerely they effectively communicated contrition. It became easy, however, for the outward exhibition of repentance to take the place of a genuine, inner attitude and thus become an act of hypocrisy.
2) A new desire for God. When we acknowledge through fasting that we need God to live, and to live more abundantly, we can begin to desire God in a new way. When we realize we need God more than we need food, we can start to understand what the Psalmist meant when he wrote, “Like the deer that pants after water, my soul longs for You.” God, the sustainer of all life, wants nothing more than a closer connection with us, and through fasting we can quench that new desire for more of Him in our lives.
I hope this encourages someone. This morning I got into an argument with someone. (that isn't the encouraging part of course) I became so discouraged and did not want to go to my office and try to talk with God about anything at that point. And I wanted a cup of coffee but tried to stay away. Finally I felt the Lord say to me that if a cup of coffee was going to keep me from spending time with Him then have the cup of coffee. I made my cup, went to my office and logged into pastor Jentezen's blog. And I cried and cried. I just couldn't pray but I could listen. I clicked on his sermon, "Right People, Right Place, Right Plan" and God said some of the sweetest and most encouraging things in that sermon. Please stick to and fight through your fast but don't let anything you feel is a slip up keep you from what's really important. If I had let that cup of coffee make me feel unworthy of seeing God this morning then I would have been out of the biggest blessing of hearing His Words to me.
Fasting also appears as a sign of mourning. Following Saul's death, the people of Jabesh- Gilead lamented his passing by fasting ( 1 Sam 31:13 ) as did David and his companions when they heard the news ( 2 Sam 1:12 ). David goes so far as to say that he commiserated with his enemies when they were sick, fasting and dressing himself in sackcloth ( Psalm 35:13 ). Such behavior was a sign of his mourning over them (v. 14). Zechariah describes the commemoration of Israel's tragic days of past defeat and judgment as times of mourning attended by fasting ( 7:5 ). But these days of fasting in the fourth, fifth, seventh, and tenth months will one day be turned to times of joy ( 8:19 ). Jesus speaks of the time of his departure from his disciples as a time of mourning when it will be entirely appropriate to fast ( Matt 9:14-15 ; Mark 2:18-20 ; Luke 5:33-35 ).
In a physiological context, fasting may refer to the metabolic status of a person who has not eaten overnight, or to the metabolic state achieved after complete digestion and absorption of a meal. Several metabolic adjustments occur during fasting. Some diagnostic tests are used to determine a fasting state. For example, a person is assumed to be fasting once 8–12 hours have elapsed since the last meal. Metabolic changes of the fasting state begin after absorption of a meal (typically 3–5 hours after eating).