When are ketone bodies produced?

Ketone bodies are primarily produced during fasting conditions, which occurs when the body has limited access to carbohydrates for energy. During fasting, glycogen stores in the liver become depleted, leading the body to seek alternative energy sources. In this state, fatty acids from adipose tissue are mobilized and transported to the liver, where they undergo β-oxidation to generate acetyl-CoA.

When the levels of acetyl-CoA rise significantly, beyond what can enter the citric acid cycle, the liver converts it into ketone bodies—specifically acetoacetate, β-hydroxybutyrate, and acetone. These ketone bodies are then released into the bloodstream and can be utilized by various tissues, including the brain, as an alternative fuel source, especially in prolonged fasting or during starvation.

This process is significantly different from the body's metabolic state after a meal, during digestion, or even during exercise when readily available glucose from the diet or glycogen stores is typically utilized for energy. Therefore, fasting conditions stimulate the production of ketone bodies, making it the correct context for their synthesis.