What health benefits and risks can you associate with fasting?

Recently, I have been curious about the practice of fasting. Not for religious or cultural reasons, just curiosity. Some of my friends who have done this have described various sensations of euphoria or “inner calm” associated with fasting.

What health benefits and risks can you associate with fasting? I imagine that dehydration is the biggest concern for example. What other risks or special considerations one should take when fasting?

Fasting has no health benefit and the risks are dependent upon how long the fast lasts, whether the person drinks water while fasting, age, health status, environmental temperature and humidity as well as physical exercise. If you are sweating, you need to replace water lost thru sweat. The good way to determine water needs is to weigh yourself before going out in hot and humid temperatures or exercising, drink water while you are out, and then weigh yourself when you return inside in the same clothing. For every 1 pound weight loss, you should drink an additional 2 cups of cool water.  Dehydration in temperate climates will occur in a few days and thirst is not a good indicator. By the time you experience thirst, you are already dehydrated. The very minimum amount of water needed per day in temperate climates is 1 1/2 quarts (women) and 2-quarts (men) which includes the 20% water that normally you would consume in food. An individual’s need may be higher especially for pregnant and breastfeeding women who should never fast. I would suggest leaving fasting a curiosity.

The euphoria or inner calm, I cannot explain other than perhaps not having enough energy (fuel) to function. After 1 day when the body has depleted glycogen (stored glucose in the liver), the body starts breaking down protein structures to provide glucose the brain and nervous system need for fuel. At the same time, the body also breaks down body fat, but not completely to produce ketones which build up in the blood and are excreted by the kidneys. Other body systems can run on ketones, but generally, a person feels lethargic and mentally dull due to the lack of glucose in the blood. Meanwhile, the body is deprived of the basic building blocks (protein, carbohydrates, fat, vitamins, and minerals) for maintaining essential body functions.

There is no cleansing of the body that occurs, only depletion and toxins or heavy metals may increase in the blood as body fat is broken down for fuel.