Metabolic States of the Body: Fasting and Starvation

During the initial hours of fasting, the body depletes its glycogen stores for energy.

As glycogen stores diminish, the body begins the breakdown of stored triglycerides.

Breakdown of structural proteins occurs as fasting progresses, which releases amino acids that undergo deamination for gluconeogenesis.

Continued protein catabolism without any dietary influx of amino acids slows down protein synthesis, leading to reduced muscle performance.

By the second day of fasting, the body shifts to lipolysis of triglycerides in adipose tissue. It releases glycerol, which is used for gluconeogenesis, stabilizing the blood glucose level.

Meanwhile, the free fatty acids from lipolysis are used by the muscle fibers and other body cells to produce energy.

During the fasting state, fatty acids are also converted to ketone bodies.

Due to their lipid solubility, ketone bodies can cross membranes and the blood-brain barrier, serving as an alternative fuel for muscles and neurons.

Prolonged food deprivation induces starvation, during which ketone bodies provide up to two-thirds of the brain's energy requirements.