Fasting is a very ancient practice. Most religions encourage some form of fasting. It can range from the absolute fasts, to fasts that just exclude some foods.
Fasting actually goes beyond just the benefits from Hormesis. Any stress forces the body to mount a defense against it. This reactionary force helps rejuvenate the body, provided there is adequate rest between periods of stress. Occasional Intermittent Fasting also helps by rejuvenating the body. Different types of fasting provides different benefits to the body.
Types of Fasts
The most common method of fasting is what is called a Water Fast. In this type of Fasting, only water is allowed. It is actually better to add lemon and salt, to provide electrolytes and acids. It is possible for some people to continue water fasts for more than a month. Water fast is a super set of other types of fasts.
Absolute fasts which avoid water as well can be very hard on the body, they can be used very rarely, but avoid doing it for more than 16-24 hours. They are not as helpful as water fasts. This type of fast happens over the Ramadan.
The next most common type of fast is the protein fast. Here we eat only fruits. Fruits are very low in proteins. It would be possible to add fats like ghee and coconut oil. This is useful for creating longer term fasts compared to water fasts. These fasts are not really low on anything except protein. Indians do Navratri fasts, which avoid grains and meat, which results in a low protein diet. Catholics avoid meat, which is their main source of protein during the season of Lent which occurs twice an year.
Another type of fast is where people avoid all grains. Navratri fasts fall into this category as well. Chinese observed Bigu as it was supposed to cure several illnesses.
Intermittent Fasting is the method of fasting at regular or irregular intervals. Most religious prescribed fasting is intermittent in nature.
Fast: What it means for the body
When we don't eat any food, we don't get any energy. The body must find alternative sources of energy. After eating digestion continues for a few hours, depending on the content of food. During digestion we get energy from the food. Once that energy has tapered off normally around 4-6 hours after eating, the liver starts to provide glucose for energy from its storage. The energy from the liver is mostly for the use of the brain and neurons. Most of the rest of the body shifts to using stored fat and glucose. The ratio of fat to glucose increases as muscles lose excess stored glucose. This continues till another 6-10 hours. After this glucose supply starts to drop off, and the body starts to shift to using fat stores predominately. After about 24 hours of fasting glucose supply drops sufficiently that almost all glucose is used by brain only and that too not more than 25% of its peak requirement. The rest of the energy comes from ketones and fat. Brain can only consume ketones and glucose. Interesting thing to note is that the Metabolic rate does not reduce till about 72 hours of fasting.
The body uses muscles as its store of amino-acids. This would logically result in reduced muscles during fasting. But the body does not use muscles indiscriminately to create amino-acids. It avoids using well functioning proteins, but breaks down proteins that are unutilized or not functioning properly or inefficient/weak muscle fibers. Breaking down of these proteins is called Autophagy.
Protein fasts can also be used for strengthening the muscles, but remember that including carbs during the protein fast can be counter productive if insulin sensitivity is low. Overweight people are by definition insulin resistant and for these people protein fasts cannot contain carbs. So the only things that these people can consume during fasts are pure fats.
Benefits of Fasting
It has been observed that it is a lot easier to build muscles that we have been lost compared to building new muscles. When we fast, we lose muscle fibers that are weak. After the fast the muscles that were broken down during the fast are the first targets of rebuilding. The second time around the body is more likely to build stronger muscle fibers. In this way a fast actually results in increased strength. John Berardi a very popular fitness coach talks about his experiments with water fasting, he used to be a 6 times a day dieter before.
Another very important benefit of fasting is losing fat. Since fasting reduces energy input it turns metabolic functions to fat stores. It has been seen that fasting does not reduce metabolic rate, but eating after a fast increases metabolic rate. This results in increased average metabolic rate with regular fasting. Fasting in effect provides a lot of benefit to overweight people.
Fasting is also known to increase Human Growth Hormone. HGH is directly related to the age of a person. It could be that Fasting can make you younger.
Traditionally body builders go through cutting and bulking phases. During bulking phase body builders eat a lot, and build muscles and fat. During the cutting phase they eat a lot of high protein low calorie foods in an attempt to burn of the extra fat. Martin Berkhan is a fitness expert who pioneered the method of body building via everyday fasting. His method boasts of muscle increase while reducing fat.
Fasters claim that fasts make their brains clearer and sharper. This probably is because the brain works better when it has a good ratio of glucose and ketones as fuel. People with insulin resistance are not able to provide enough ketones to their brains as their glucose levels remain high. Fasting Autophagy may also provide benefit to the neurons. Fasting is also helpful in several brain related disorders, Alzheimer's, Huttington's, and Trauma.
Fasting has been shown to be a very effective in cancer treatment. It is also shown that fasting damages cancer cells but not normal cells.
To recap. Fasting has been considered very beneficial since time immemorial. The fact is that all ancient religions require fasting in some form. Science also agrees and provides ample evidence of its benefits. Intermittent Fasting is slowly gaining acceptance for body building and weight loss. Ease of Fasting shows the health of the body. When a person is not very healthy, fasting is very difficult. A 24 hour fast is a must for a healthy body, but be aware that if you are not able to do it, fix the problem first rather than plowing through it.