The Transition to Ketosis During Fasting

During fasting, glycogen stores are depleted, prompting the body to produce ketones from stored fat. This process, known as ketosis, allows the brain and other organs to use ketones as an alternative fuel source when glucose is scarce.

Glycogen Depletion and Ketone Production

Within the first 12 to 24 hours of fasting, glycogen stores are exhausted, leading to lipolysis, where fat is broken down into free fatty acids and glycerol. The liver then converts these fatty acids into ketones through ketogenesis, typically starting around 12 to 24 hours into the fast.

As fasting continues, glucose and insulin levels decrease, prompting the body to use fat for energy. By 12 to 72 hours, approximately 40% of fat is utilized for ketone production. Prolonged fasting promotes adaptation to ketosis, enhancing the body's efficiency in using ketones and fat for energy.