By the 21st century, although fasting clearly was applicable in some instances of disease, such as in certain acute diseases (particularly when accompanied by a loss of appetite), whether fasting in other instances was beneficial to human health remained unclear. For example, whereas research in humans had suggested that intermittent fasting carried out over 15 days improved insulin-mediated glucose uptake into tissues, studies in rodents had indicated that such fasting, carried out over the long-term, promoted glucose intolerance and the release of damaging oxidants from tissues.

Second, fasting seems foreign to many of us simply because nobody talks about it that much. The reason for this is that nobody stands to make much money by telling you to not eat their products, not take their supplements, or not buy their goods. In other words, fasting isn't a very marketable topic and so you're not exposed to advertising and marketing on it very often. The result is that it seems somewhat extreme or strange, even though its really not.
We tend to think of fasting as going without food. But we can fast from anything. If we love music and decide to miss a concert in order to spend time with God, that is fasting. It is helpful to think of the parallel of human friendship. When friends need to be together, they will cancel all other activities in order to make that possible. There’s nothing magical about fasting. It’s just one way of telling God that your priority at that moment is to be alone with him, sorting out whatever is necessary, and you have cancelled the meal, party, concert, or whatever else you had planned to do in order to fulfil that priority. [James I (J. I.) Packer].

6. Are you hungry all the time? A scale of 1 to 10 is helpful in moments like this. One is ravenous, while 10 is extremely full. What is your number before and after meals? What is your number at moments during the fast? While hunger may be part of a fast given the caloric restriction, it shouldn’t be constant, ongoing and terrible. Also, notice portions when eating. If you haven’t created the right balance, you might overeat when breaking a fast, which is hard on your digestion and metabolism. Ideally, once acclimated, you’d stay within the 3 to 7 range at all times, 3 when fasting and 7 after meals.
Now, intermittent fasting isn’t magical—your calories still count. It’s just an alternate way to reach that same calorie restriction, says Grant Tinsley, Ph.D., C.S.C.S., an intermittent fasting researcher and assistant professor of exercise physiology at Texas Tech University. “With traditional calorie restriction, you’re following a normal, healthy eating pattern but eating less at each meal. With intermittent fasting, you’re eating roughly the same number of reduced calories, just in a confined time frame. You’re just giving yourself different parameters to live within.”
Weight-loss resistance can often be due to an underlying hormone imbalance. Leptin resistance occurs when your brain stops recognizing leptin's signals to use your body’s fat stores for energy. This causes your body to continually store fat instead of using it. Intermittent fasting has been shown to improve chronic inflammation that can dull the brain’s leptin receptor sites.
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You’re consuming less food and thus spending less money. Rather than overeating to put on 1 pound of muscle and 4 pounds of fat in a week or two, you’re aiming to eat exactly enough to put on 1 pound of muscle without adding much fat on top of it.  Yeah, it’s a delicate balance, but there’s far less swing involved. You are just slowly, steadily, and consistently building muscle and strength over many months.

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).