Aside from these official days of fasting, Jews may take upon themselves personal or communal fasts, often to seek repentance in the face of tragedy or some impending calamity. For example, a fast is sometimes observed if a sefer torah is dropped. The length of the fast varies, and some Jews will reduce the length of the fast through tzedakah, or charitable acts. Mondays and Thursdays are considered especially auspicious days for fasting. Traditionally, one also fasted upon awakening from an unexpected bad dream although this tradition is rarely kept nowadays.
As far back as the 1930s, scientists have been exploring the benefits of reducing calories by skipping meals. During that time, one American scientist found that significantly reducing calories helped mice live longer, healthier lives. More recently, researches have found the same in fruit flies, roundworms and monkeys. Studies have also shown that decreasing calorie consumption by 30 to 40 percent (regardless of how it’s done) can extend life span by a third or more. Plus, there’s data to suggest that limiting food intake may reduce the risk of many common diseases. Some believe fasting may also increase the body’s responsiveness to insulin, which regulates blood sugar and helps control hunger.
Still, it's important to note that the results of intermittent fasting are no better than a calorie-restricting diet. A 2017 study in JAMA showed that alternate-day fasting doesn't produce superior benefits for weight loss than daily calorie restriction, and similar conclusions were drawn in a 2018 study published in the American Journal of Clinical Nutrition. So if you don't think fasting is right for you, restricting calories works just as well.
In order for a fast for God to be effective, you must also pray. Praying and fasting to God is a powerful weapon against adversity. Joel 1:14 says, "Declare a holy fast; call a sacred assembly. Summon the elders and all who live in the land to the house of the Lord your God, and cry out to the Lord." A fast for God puts your trust in Him as the only one who can help you. During your fast for God, remember that He is faithful and true, willing and able to help you in any way He can.
Fasts may last for varying lengths of time -- one day, three days, seven days, and more. We should prayerfully seek the Lord about the length of our fast. Fasting should have an object in mind; we should have a clear idea of the need and purpose. One of the most pressing reasons for Christian fasting may be revival -- “Will you not revive us again...” (Psalm 85:6).
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When Varady and her colleagues conducted a study that combined alternate day fasting and exercise, they allowed the participants to pick whether they wanted to exercise on a feasting or fasting day, and found there was no strong preference one way or the other. But the researchers were surprised the dieters actually reported feeling more energetic on fasting days.
Determine the length of your fasting experience, whether this is for 1 meal, 1 day, 3 days or a week (Jesus and Moses fasted for a period of 40 days, but that is no sign that anyone else might do that). You may wish to try a shorter fast, and start slowly at first, if you have not previously fasted. You can also pray and ask the Holy Spirit to reveal to you how long you should fast.
The best long-term diets, no matter what their rules entail, are the ones that are least difficult to maintain—and again, in this regard, intermittent fasting isn’t inherently superior to anything else. “Are you making changes in your behavior? Have you learned positive habits so that when you go back to not fasting, you’re going to be a healthier eater?” Sasson asks. “I know people who fast because they think, Okay, I’m going to be really bad and overdrink or overeat, and then two days a week I’m going to have a clean life, and that’s just not how it works.”
Fasting is often used as a tool to make a political statement, to protest, or to bring awareness to a cause. A hunger strike is a method of non-violent resistance in which participants fast as an act of political protest, or to provoke feelings of guilt, or to achieve a goal such as a policy change. A spiritual fast incorporates personal spiritual beliefs with the desire to express personal principles, sometimes in the context of a social injustice.