Gluconeogenesis is the synthesis of glucose from non-carbohydrate sources like pyruvate, amino acids, and glycerol. During fasting or carbohydrate starvation, amino acids from muscle protein breakdown and glycerol from fat breakdown provide precursors for gluconeogenesis in the liver. The liver is the major site of gluconeogenesis and helps maintain blood glucose levels to meet the metabolic demands of the brain, muscles, and other tissues.