Metabolic Switch

The metabolic switch is the point during a fast when the body shifts its main fuel source from glucose to fat. After a meal, cells run primarily on glucose drawn from carbohydrate. Once that supply and the body's stored carbohydrate, known as glycogen, run low, the body turns to breaking down fat for energy, releasing fatty acids and producing ketones. This changeover is often called the glucose-to-ketone switch.

The switch is the central event that makes fasting metabolically distinct from simply eating less. It usually begins many hours into a fast, once glycogen is sufficiently depleted, though the exact timing depends on the person, their last meal, and their activity. It is driven by the same nutrient-sensing signals that govern the rest of the fasted state, with the growth pathway mTOR quieting down and the energy sensor AMPK becoming more active.

Repeatedly crossing this switch is thought to be part of why fasting trains the body to move between fuel sources more readily over time, a capacity known as metabolic flexibility.

Source: Anton et al., Obesity (2018)