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. 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.
Such changes in blood chemistry during fasting, when combined with continuing medication, may have dangerous effects, such as increased chance of acetaminophen poisoning. The body continues to use fat for as long as there is fat to consume. The body will generally indicate to the faster when fat levels are running extremely low (less than 7% and 10% of body weight for males and females, respectively) with an increased urge for food. Fasts are usually broken long before this point. If the fast is not broken, starvation begins to occur, as the body begins to use protein for fuel. Medical complications associated with fast-induced starvation include electrolyte imbalances, thinning hair, lanugo, cardiac arrhythmia and renal failure. Death can occur if fasting is pursued to the point of complete starvation.
Excessive fasting for calorie restrictive purposes, accompanied by intense fears of becoming overweight are associated with mental disturbances, including anorexia nervosa.
Research suggests there are major health benefits to caloric restriction. Benefits include reduced risks of cancer, cardiovascular diseases, diabetes, insulin resistance, immune disorders, and more generally, the slowing of the aging process, and the potential to increase maximum life span. According to Dr. Mark P. Mattson, chief of the laboratory of neurosciences at the US National Institute on Aging, fasting every other day (intermittent fasting) shows beneficial effects as strong as those of caloric-restriction diets in mice, and a small study conducted on humans at the University of Illinois indicates the same results  According to the US National Academy of Sciences, other health benefits include stress resistance, increased insulin sensitivity, reduced morbidity, and increased life span. Long-term studies in humans have not been conducted. However, short-term human trials showed benefits in weight loss. The side effect was that the participants felt cranky during the three week trial. According to the study conducted by Dr. Eric Ravussin, "Alternate-day fasting may be an alternative to prolonged diet restriction for increasing the life span".
Adherence to Greek Orthodox fasting periods contributes to an improvement in the blood lipid profile, including a decrease in total and LDL cholesterol, and a decrease in the LDL to HDL cholesterol ratio. A statistically insignificant reduction in HDL cholesterol was also observed. These results suggest a possible positive impact on the obesity levels of individuals who adhere to these fasting periods.
It is strongly advised that those thinking of fasting as a "diet" consult a physician or dietitian before they start due to potential health risks.