Some explanation from "The Complete Guide to Fasting" by Jason Fung -- a book I very, very, very highly recommend to anyone unconvinced that fasting is a good idea, or needing guidance on how to do it.

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There appears to be significant research indicating a dramatic drop in inflammation, improvements in insulin signaling, and a near total “reset” of immune function with fasts of 3–5 days. Abnormal and or pre-cancerous cells appear to be pushed towards apoptosis, which essentially selects for healthy cell types. In total this describes a process which should (in theory) reverse many of the signs and symptoms of aging while reducing the processes that appear to be at play in autoimmunity and cancer.

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In a process called apoptosis, also known as programmed cell death, cells that reach a certain age are programmed to commit suicide. While this may sound kind of macabre at first, the process constantly renews cell populations, making it essential for good health. But when just some cellular components need to be replaced, a process called autophagy kicks in.

The word autophagy, coined by Nobel Prize–winning scientist Christian de Duve, derives from the Greek auto (“self”) and phagein (“to eat”). So the word literally means “to eat oneself.” Autophagy is a form of cellular cleansing: it is a regulated, orderly process of breaking down and recycling cellular components when there’s no longer enough energy to sustain them. Once all the diseased or broken-down cellular parts have been cleansed, the body can start the process of renewal. New tissues and cells are built to replace those that were destroyed. In this way, the body renews itself. But it only works if the old parts are discarded first.

Our bodies are in a constant state of renewal. While we often focus on new cell growth, we sometimes forget that the first step in renewal is destroying the old, broken-down cellular machinery. But apoptosis and autophagy are both necessary to keep our bodies running well. When these processes are hijacked, diseases such as cancer occur, and the accumulation of older cellular components may be responsible for many of the effects of aging. These unwanted cellular components build up over time if autophagocytic processes are not routinely activated.

Increased levels of glucose, insulin, and proteins all turn off autophagy. And it doesn’t take much. Even as little as 3 grams of the amino acid leucine can stop autophagy. Here’s how it works: The mammalian target of rapamycin (mTOR) pathway is an important sensor of nutrient availability. When we eat carbohydrates or protein, insulin is secreted, and the increased insulin levels, or even just the amino acids from the breakdown of ingested protein, activate the mTOR pathway. The body senses that food is available and decides that since there’s plenty of energy to go around, there’s no need to eliminate the old subcellular machinery. The end result is the suppression of autophagy. In other words, the constant intake of food, such as snacking throughout the day, suppresses autophagy.

Conversely, when mTOR is dormant— when it’s not being triggered by increased insulin levels or amino acids from ingested food— autophagy is promoted. As the body senses the temporary absence of nutrients, it must prioritize which cellular parts to keep. The oldest and most worn-out cellular parts get discarded, and amino acids from the broken-down cell parts are delivered to the liver, which uses them to create glucose during gluconeogenesis. They may also be incorporated into new proteins. It’s important to note that the dormancy of mTOR is only related to short-term nutrient availability and not the presence of stored energy, such as liver glycogen or body fat. Whether the body has stored energy is irrelevant for mTOR and therefore for autophagy.

This is why the strongest stimulus to autophagy currently known is fasting, and why fasting alone, unique among diets, stimulates autophagy — simple caloric restriction or dieting isn’t enough.

Fung, Jason; Moore, Jimmy. The Complete Guide to Fasting: Heal Your Body Through Intermittent, Alternate-Day, and Extended (p. 151-153). Victory Belt Publishing. Kindle Edition.