As scientists learn more about the way the human body evolved, it makes sense to reflect back on how we evolved in the sense of eating and nutrition. Not only can what we eat affect how we feel, perform, and function, but also when and how often. This is not your grandparents' three square meals a day.
How it works:
The concept of prolonged fasting for health benefits is not a new one. Going back to the days of Greek philosophers Hippocrates, Socrates, and Plato, fasting has been used to improve health. Fasting is defined as abstaining from eating or drinking all or certain kinds of foods and beverages. Throughout history, it has been used in religious observance as a form of purification. But far before the advent of civilization, fasting was simply a time when food was not available. In a hunter/gatherer society, this would happen regularly and is part of the reason why our bodies store fat during times when food is readily available. The big problem is that food is pretty much always readily available in modern times.
Prolonged Fasting and Intermittent fasting are not really a diet so much as an eating protocol. With fasting, you would not eat for a given period of time, giving your body a chance to re-tune and reset. Intermittent fasting (IF) refers to a partial day fast that is repeated day after day, whereas fasting is for a full day or more and only repeated every week, month, or year. Usually, the longer the fast, the more time before you would repeat the fast, although shorter fasts may be interspersed between the longer ones. For example, an IF protocol may give you an eating window of 12 hours a day down to about four, while fasting may mean one day every week, three consecutive days out of every month, or seven consecutive days out of a year. Because of this variation on protocols, it is essential to look at the studies conducted and their purported benefits to understand how to conduct yourself. Some fasts allow only water, others say tea and coffee are ok, others provide a BCAA supplement to their participants.
Metabolic re-tuning. Most of the reported benefits from fasting occur because of this "rest" on our metabolic processes. This gives the body time to get the waste products, excess substrate, and broken down organelles taken care of and "refreshed." Take for example muscle glycogen. Not eating allows the body use blood glucose for energy without continuing to signal for more insulin, and thus insulin resistance. This makes the body more insulin sensitive again (a good thing) so it can better use available blood glucose when it is signaled for. There is also a reset of your immune system that helps to better fight off disease, as well as a general reduction in systemic inflammation. More generally speaking, not eating for a day (or partial day) will obviously help you to reduce your caloric intake. The body has a pretty good system for maintaining itself during these shorter times of no food, so you don't have to worry too much about losing much muscle. Obviously, the reduction in calories will help you dip into your fat stores for energy. All pretty good things, right?
Cons: The negatives of this type of eating are more for specific groups of people. Especially with prolonged fasting, it might be prudent to consult your doctor before attempting. Depending on your lifestyle and job, you may experience fatigue that could be problematic as your body adjusts to the lack of food. People who are already at very low body weight, or at risk for anorexia should not attempt such a diet. Children and adolescents who are still growing need extra nutrition and should definitely not go days without eating. This also includes pregnant women.
Conclusion: There is a lot of positives going on with these type of diets. A lot of the research has been done on animal models, which is useful but must be taken with the understanding that it may not work the same in humans. Many people have had success, but it's all about finding a specific fasting model that works well for you and is sustainable. You can use this eating protocol with any diet, which makes its benefits available to anybody. Start small and build up so that your body can have time to adjust. Start with IF and a 12-hour window and work your way down if you so desire. Try a full day fast every now and then. See how you feel after each change, and find the protocol that helps you feel your best!