How Does Intermittent Fasting Work?

With intermittent fasting, your body operates differently when "feasting" compared to when "fasting": When you eat a meal, your body spends a few hours processing that food, burning what it can from what you just consumed. Because it has all of this readily available, easy to burn energy in its blood stream (thanks to the food you ate), your body will choose to use that as energy rather than the fat you have stored. This is especially true if you just consumed carbohydrates/sugar, as your body prefers to burn sugar as energy before any other source.

During the "fasted state," your body doesn't have a recently consumed meal to use as energy, so it is more likely to pull from the fat stored in your body, rather than the glucose in your blood stream or glycogen in your muscles/liver.

Burning fat = win.

The same goes for working out in a "fasted" state. Without a ready supply of glucose and glycogen to pull from (which has been depleted over the course of your fasted state, and hasn't yet been replenished with a pre-workout meal), your body is forced to adapt and pull from the only source of energy available to it: the fat stored in your cells!

Why does this work? Your body reacts to energy consumption (eating food) with insulin production. Essentially, the more sensitive your body is to insulin, the more likely you'll be to use the food you consume efficiently, which can help lead to weight loss and muscle creation. Along with that, your body is most sensitive to insulin following a period of fasting.

Your glycogen (a starch stored in your muscles and liver that your body can burn as fuel when necessary) is depleted during sleep (fasting), and will be depleted even further during training, which can further increase insulin sensitivity. This means that a meal immediately following your workout will be stored most efficiently: mostly as glycogen for muscle stores, burned as energy immediately to help with the recovery process, with minimal amounts stored as fat.

Compare this to a regular day (no intermittent fasting). With insulin sensitivity at normal levels, the carbs and foods consumed will see full glycogen stores, enough glucose in the blood stream, and thus be more likely to get stored as fat. Not only that, but growth hormone is increased during fasted states (both during sleep and after a period of fasting). Combine this increased growth hormone secretion, the decrease in insulin production (and thus increase in insulin sensitivity), and you're essentially priming your body for muscle growth and fat loss with intermittent fasting.