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.
So what’s the first step in getting started? Each method has its own guidelines for how long to fast and what to eat during the “feeding” phase. Below, you’ll find the five most popular methods and the basics of how they work. Keep in mind, intermittent fasting isn’t for everyone. Those with health conditions of any kind should check with their doctor before changing up their usual routine. Note that personal goals and lifestyle are key factors to consider when choosing a fasting method.
To restore balance or to lose weight, we simply need to increase the amount of time we burn food energy. That’s intermittent fasting. In essence, fasting allows the body to use its stored energy. After all, that’s what it is there for. The important thing to understand is that there is nothing wrong with that. That is how our bodies are designed. That’s what dogs, cat, lions and bears do. That’s what humans do.
Those studies above, in working with small sample sizes, and different types of fasting than recommended here, would lead me to believe that fasting affects men and women differently, and that many of the weight loss benefits associated with intermittent fasting (that affect insulin and glucose responses) work positively for men and negatively for women.
As I began my fast, I was not sure I could continue for forty days. But my confidence was in the Lord to help me. Each day His presence encouraged me to continue. The longer I fasted, the more I sensed the presence of the Lord. The Holy Spirit refreshed my soul and spirit, and I experienced the joy of the Lord as seldom before. Biblical truths leaped at me from the pages of God's Word. My faith soared as I humbled myself and cried out to God and rejoiced in His presence.
Eating all your meals in an 8-hour window (say, eating between noon and 8PM, and fasting the other 16 hours a day) causes significant weight loss without counting calories. While this type of intermittent fasting causes weight loss no matter what people eat, research shows that people who do it in a healthy manner lose twice as much weight (7% vs. 3% of their body weight) as those who fast while still eating junk. So it’s still important to follow a high-performance diet like the Bulletproof Diet while you fast.
^ 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.
Fasting for special purposes or before or during special sacred times remains a characteristic of major religions of the world. In Jainism, for example, fasting according to certain prescribed rules and practicing certain types of meditation leads to trances that enable individuals to dissociate themselves from the world and reach a transcendent state. Some Buddhist monks of the Theravada school fast as part of their meditation practices. In India, Hindu sadhus (holy men) are admired for their frequent personal fasts for various reasons.
In addition to these, there is the fast of repentance which a person keeps after committing sin, it being imposed as a penance by the priest for seven days, forty days or one year. There is also a fast which a bishop keeps at the time he is consecrated. Also there are fasts that are widely observed but which have not been included in the canon of the church and which are therefore considered strictly optional such as the "Tsige Tsom" or Spring Fast, also known as "Kweskwam Tsom" which marks the exile of the Holy Family in Egypt.
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. 
Fasting is always practiced prior to surgery or other procedures that require general anesthesia because of the risk of pulmonary aspiration of gastric contents after induction of anesthesia (i.e., vomiting and inhaling the vomit, causing life-threatening aspiration pneumonia). Additionally, certain medical tests, such as cholesterol testing (lipid panel) or certain blood glucose measurements require fasting for several hours so that a baseline can be established. In the case of a lipid panel, failure to fast for a full 12 hours (including vitamins) will guarantee an elevated triglyceride measurement.