Fasting is the willingness to abstain from food and/or drinking for a duration of time. People choose to do this for a number of reasons, including spiritual and health.
Fasting is generally done for a single day or can be done over the course of several days (or even weeks). An absolute fast (or black fast) is done by abstaining from all food and drinking.
There are a number of complications that can arise from fasting when not done carefully and wisely. Extreme and unplanned fasting can lead to electrolyte imbalances, thinning hair, fatal cardiac arrhythmias, and renal failure.
On the other hand, there are many strong accompanying benefits when fasting is done properly. These include detoxing your body, reduction in calories, loss of fat, giving your digestive track a break, and increased intellectual keenness.
Moreover, according to the US National Academy of Sciences, fasting can help with stress resistance, increased insulin sensitivity, reduced morbidity, and increased life span. Scientists have also found that fasting helps reduce cholesterol in the blood.
Finally, I believe that the strongest benefit of fasting is spiritual sharpening. When fasting, one draws closer to God, the ultimate source of life, energy, and sustenance.