How It Works: Fast for 14 (women) to 16 (men) hours each day, and then “feed” for the remaining eight to 10 hours. During the fasting period, you consume no calories. However, black coffee, calorie-free sweeteners, diet soda and sugar-free gum are permitted. (A splash of milk in your coffee won’t hurt, either.) Most practitioners will find it easiest to fast through the night and into the morning. They usually break the fast roughly six hours after waking up. This schedule is adaptable to any person’s lifestyle, but maintaining a consistent feeding window time is important. Otherwise, hormones in the body can get thrown out of whack and make sticking to the program harder, Berkhan says.

One potential disadvantage of this schedule is that because you typically cut out a meal or two out of your day, it becomes more difficult to get the same number of calories in during the week. Put simply, it's tough to teach yourself to eat bigger meals on a consistent basis. The result is that many people who try this style of intermittent fasting end up losing weight. That can be a good thing or a bad thing, depending on your goals.
But it wasn’t until the 1940s that the first experiments began as an outgrowth of CR studies. Researchers started withholding food from lab animals on alternate days, says Michelle Harvie, a research dietitian in Manchester, England. And in 1946, The Journal of Nutrition published the first study on fasting, showing that rats deprived of food every third day lived longer and were less likely to develop tumors than control animals. Later work showed that fasting spurs metabolic changes similar to those of CR.
Distinguish between fasting and abstinence. Fasting means not eating food (and sometimes not drinking) for a period of time (usually not more than 12 hours) in order to respond to a moment so sacred that indulging in food or drink would profane it. While some people say that they’re fasting when giving up something besides all food – like watching TV, having sex, or eating a specific kind of food, such as chocolate – for a while, that’s really abstinence, not fasting. Fasting is a more complete expression of devotion to God that can free you to focus your attention better on God.
I sometimes hear people say, "I'm giving up chocolate" and they regard this as a type of fasting. I think this is a rather frivolous approach. The first and foremost purpose of a biblical or spiritual fast is to get a breakthrough on a particular matter that one lifts up to the Lord in prayer. A spiritual fast involves our hearts and the way in which we relate to and trust God. It relates to discerning and receiving strength to follow through on what God might reveal to us about circumstances in our lives or a direction we are to take.

Chris Pratt! Hugh Jackman! Halle Berry! Kourtney Kardashian! What these celebrities have in common, other than a gratuitous exclamation point after their names, is a professed fondness for intermittent fasting, the diet craze turning the fitness world on its sweaty, well-toned head. For help determining whether you, too, should incorporate this into your 2019 resolution-related plans, we asked a few experts to explain what it is, why people love it, and whether it’s really worth the pain of forgoing on-demand snacks for the rest of the winter.
In 2017, Longo was a co-author on the first human trial of whether fasting might reduce the risk factors for diseases like diabetes, cancer, and cardiovascular disease. The researchers randomized 100 people into one of two groups for three months: The first group ate anything they wanted, and the second fasted for five consecutive days each month. (By “fasted” here, I mean, they followed the fasting-mimicking diet.) After three months, the first group was crossed over into the fasting group, so the researchers could gather even more data on fasting.
No cravings. Fat doesn’t spike your blood sugar levels. In fact, a keto diet is so effective at stabilizing blood sugar that it got type II diabetics off their medication entirely, according to a recent study.[15] If you pair a keto diet with fasting, your blood sugar will stay stable and low (but not too low) all day. Say goodbye to the cravings, fatigue, and mood swings that make high-carb fasting so difficult.
5) A new satisfaction. When you finish your fast, renewed, full of energy, detoxed, with a new desire, a new praise and a sensitivity to God’s voice, you’ll find that the absence of food was small in comparison to what you gained. When Christ’s disciples noticed that He hadn’t eaten all day and tried to get Him to slow down to eat, Christ said, “I have food to eat that you know nothing about….My food is to do the will of Him who sent me and to finish His work.”
Wesley Duewel, a twentieth-century writer, said, “You and I have no more right to omit fasting because we feel no special emotional prompting than we have a right to omit prayer, Bible reading, or assembling with God’s children for lack of some special emotional prompting. Fasting is just as biblical and normal a part of a spiritual walk of obedience with God as are these others.”
On its web site, the Academy of Nutrition and Dietetics says that although there is evidence that intermittent fasting diets may help prevent chronic disease, more research is needed. It doesn't recommend the diet because "it is not a realistic long-term solution.” The academy also notes that "any variation of fasting may make a person irritable, cause daytime sleepiness/sleeplessness at night, and can even lead to dehydration."
The problem is that too many people jump on the fasting bandwagon without understanding how to assess its impact. Once a plan has been integrated into your everyday life, you should take time every week or month to analyze how things are going. Even if weight loss is your primary goal, it’s important to consider how fasting is affecting other aspects of your health, such as your energy levels, ability to exercise and digestion. Here are some questions you should ask yourself.
If we are to learn the lost art of fasting and enjoy its fruit, it will not come with our ear to the ground of society, but with Bibles open. Then, the concern will not be whether we fast, but when. Jesus assumes his followers will fast, and even promises it will happen. He doesn’t say “if,” but “when you fast” (Matthew 6:16). And he doesn’t say his followers might fast, but “they will” (Matthew 9:15).
Although the findings with mice are well established there’s less evidence that time restricted eating works for humans. One human study found significant weight loss when participants fasted for 11 hours overnight compared with control conditions. Another study found a 4.1 percent weight loss resulting from consuming a single meal in the afternoon although self-reported hunger was greater the next morning. Other studies haven’t supported night-time fasting for humans so it’s too early to draw definitive conclusions but you might want to experiment to see if it would work for you. 
Biblical fasting brings us into a closer union with God. While our bodies are being deprived for the purpose of drawing near to God, He has promised in return to draw near to us. This is a spiritual certainty. As we decrease, the Spirit increases. As individuals we are strengthened and renewed. “...Though outwardly we are wasting away, yet inwardly we are being renewed day by day” (2 Corinthians 4:16).
Biblical fasting is a spiritual discipline which was encouraged by Jesus, Himself, while He was on earth. When questioned as to why the Pharisees and the disciples of John the Baptist fasted while Jesus disciples did not, Jesus answered, "How can the guests of the bridegroom mourn while he is with them? The time will come when the bridegroom will be taken from them; then they will fast” (Matthew 9:15). 

^ Jump up to: a b c Harris, L; Hamilton, S; Azevedo, LB; Olajide, J; De Brún, C; Waller, G; Whittaker, V; Sharp, T; Lean, M; Hankey, C; Ells, L (February 2018). "Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis". JBI Database of Systematic Reviews and Implementation Reports. 16 (2): 507–547. doi:10.11124/JBISRIR-2016-003248. PMID 29419624.
Zero-calorie beverages are okay.  I drink green tea in the morning for my caffeine kick while writing. If you want to drink water, black coffee, or tea during your fasted period, that’s okay.  Remember, don’t overthink it – keep things simple! Dr. Rhonda Patrick over at FoundMyFitness believes that a fast should stop at the first consumption of anything other than water, so experiment yourself and see how your body responds.
"There is a good evolutionary explanation for the difference in the calorie restriction response when comparing short-lived and long-lived species: famines are seasonal, and a season is a large fraction of a mouse lifespan but a small fraction of a human life span. Thus only the mouse evolves a relatively large plasticity of life span in response to food scarcity."

The Bible has only one command regarding fasting: God’s people are commanded to fast on the Day of Atonement from sundown to sundown (Leviticus 23:27-32 Leviticus 23:27-32 27 Also on the tenth day of this seventh month there shall be a day of atonement: it shall be an holy convocation to you; and you shall afflict your souls, and offer an offering made by fire to the LORD. 28 And you shall do no work in that same day: for it is a day of atonement, to make an atonement for you before the LORD your God. 29 For whatever soul it be that shall not be afflicted in that same day, he shall be cut off from among his people. 30 And whatever soul it be that does any work in that same day, the same soul will I destroy from among his people. 31 You shall do no manner of work: it shall be a statute for ever throughout your generations in all your dwellings. 32 It shall be to you a sabbath of rest, and you shall afflict your souls: in the ninth day of the month at even, from even to even, shall you celebrate your sabbath.
Once when the Buddha was touring in the region of Kasi together with a large sangha of monks he addressed them saying: I, monks, do not eat a meal in the evening. Not eating a meal in the evening I, monks, am aware of good health and of being without illness and of buoyancy and strength and living in comfort. Come, do you too, monks, not eat a meal in the evening. Not eating a meal in the evening you too, monks, will be aware of good health and..... living in comfort.[25]
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