Insulin Sensitivity

Insulin sensitivity describes how readily the body's cells respond to insulin, the hormone that signals cells to take up glucose from the blood. When cells are highly sensitive to insulin, the body needs to release relatively little of it to move glucose out of the bloodstream and into cells for energy or storage. Insulin sensitivity is a normal feature of healthy metabolism and varies from person to person and over time.

Insulin sensitivity is relevant to fasting because insulin is primarily released in response to eating, especially carbohydrate and protein. During a fast, with no food coming in, insulin levels fall and stay low. This low-insulin state is part of what allows the body to access stored fat for fuel, since high insulin tends to favor storage over breakdown. The fed and fasted states are in this sense governed by the rise and fall of insulin.

Insulin sensitivity is also closely linked to metabolic flexibility, the ability to switch cleanly between burning glucose and fat. Cells that respond well to insulin tend to make that switch more easily, which is why the two are often discussed together.

Source: de Cabo & Mattson, NEJM (2019)