1. Fasting is willing abstinence from some or all food, drink, or both, for a period of time.

2. An absolute fast is abstinence from all food and liquid for a defined period.

3. Partially restrictive fast is limited to particular foods or substances.

4. The fast may be intermittent in nature.

5. Fasting medically means the metabolic status of a person who has not eaten overnight, and to the metabolic state achieved after complete digestion and absorption of a meal. A person is assumed to be fasting after 8–12 hours of eating.

6. Metabolic changes after fasting begin after absorption of a meal (typically 3–5 hours after a meal). Medically it is called "post-absorptive state" as against "postprandial" state of ongoing digestion.

7. A diagnostic fast refers to prolonged fasting (from 8–72 hours depending on age)

8. The longest known fast for a human is 132 days (without food).

9. Glucose is the body's primary fuel source and is essential for the brain's functioning. When denied glucose for more than 4–8 hours, the body turns to the liver for glycogen, a storage form of glucose, to be used for fuel. A process called glycogenolysis converts glycogen into a usable form of fuel. At this point, the body also uses small amounts of protein to supplement this fuel. This fuel will last for up to 12 hours before the body needs to turn to glycogen stored in muscles, lasting for a few more days.

10. If glucose is still denied at this point, muscle wasting is prevented by temporarily switching to fat as the fuel source, meaning fat is converted into ketone through catabolism. Ketones, while not sugars, can be used by the brain as a fuel source as long as glucose is denied. The body continues to use fat for as long as there is fat to consume.

11. If the fast is not broken, starvation begins to occur, as the body begins to use protein for fuel.

12. Health complications associated with fast-induced starvation include electrolyte imbalances, thinning hair, cardiac arrhythmias and renal failure. Death can occur if fasting is pursued to the point of complete starvation.

13. Hypovolemia refers to any condition in which the extracellular fluid volume is reduced. It can be produced by dehydration.