Today intentional abstention from food and drink for a given period of time is more likely associated with preparation for a medical test or in conjunction with a weight-loss plan than with spiritual development. But in the Bible, the physical act of fasting was employed in order to enrich an awareness of mortal vulnerability and to sharpen awareness of the Lord’s ability to provide.
Fasts of any duration will cause the body to react in predictable ways. After four to eight hours our bodies naturally seek to replace the glucose needed to maintain brain function by tapping into stored reserves. We first tap into the raw materials produced by the liver; then after about twelve hours we pursue the proteins available in the muscles and then the energy stored in the fat. Our metabolism generally slows during a prolonged fat, leaving us where David found himself” “My knees give away from fasting; my body is thin and gaunt” (Psa 109:24). Thy typical fast reported by the biblical authors lasted for only one day, with food and drink shunned only during daylight hours (Judg 20:26; 1 Sam 14:24; 2 Sam 1:12; 3:35).
That makes fasts of three, seven, and even forty days exceptional and worthy of special notice (Exod 34:28; 1 Sam 31:13; 2 Sam 12:16-21; 1 Chron 10:12; Esther 4:16; Matt 4:2).
In the Bible, fasting is associated with several occasions. People fasted as they reflected on their own sin, the sins of their community, and the need of all to repent. Such penitential fasts were sometimes enacted spontaneously in response to special circumstances (1 Sam 7:6; Ezra 10:6; Dan 9:3; Joel 1:14; 2:12, 15). In addition, God’s people were required to fast on the Day of Atonement when all Israel sought forgiveness. Here the directive to “deny yourselves” included fasting (Lev 16:29-31; 23:27-32).