An informative description of the proclamation of a fast is in Jeremiah 36:9. There the people of Judah convened, apparently for the purpose of national repentance. This at least is what Jeremiah instructed Baruch to encourage them to do (vv. 7-8). Moreover, Jeremiah refers to the anticipated event as a "day of fasting" (v. 6), suggesting a common practice known to him and the people generally. In fact, Isaiah had spoken of such convocations a century earlier ( 58:3-6 ), gatherings on special days for special purposes. Regardless of Isaiah's feelings about the abuse of fasting, it is obvious that he recognized it as a legitimate form of worship and that he found no fault with it being carried out on specially called occasions.
Unger’s Bible Dictionary explains that the word fast in the Bible is from the Hebrew word sum, meaning “to cover” the mouth, or from the Greek word nesteuo , meaning “to abstain.” For spiritual purposes, it means to go without eating and drinking (Esther 4:16 Esther 4:16Go, gather together all the Jews that are present in Shushan, and fast you for me, and neither eat nor drink three days, night or day: I also and my maidens will fast likewise; and so will I go in to the king, which is not according to the law: and if I perish, I perish.
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.
What is ghrelin? It is actually also known as the hunger hormone, because it is responsible for telling your body that it is hungry. Dieting and really restrictive eating can actually increase ghrelin production, which will leave you feeling hungrier. But when you fast, though you might struggle in the first few days, you’re actually normalizing ghrelin levels.