What is the primary metabolic fate of pyruvate during fasting?

During fasting, the primary metabolic fate of pyruvate is its shunting to gluconeogenesis. In the absence of food intake, the body relies on stored glucose and reserves to maintain blood glucose levels, particularly for organs that depend on glucose, such as the brain and red blood cells. Pyruvate, produced from glycolysis, can be directed into gluconeogenesis, a metabolic pathway that synthesizes glucose from non-carbohydrate sources.

Gluconeogenesis takes place primarily in the liver and to some extent in the kidney. The process converts pyruvate into glucose, which is then released into the bloodstream to provide energy to tissues that require glucose. Fasting increases levels of glucagon and decreases insulin, favoring gluconeogenesis.

While pyruvate can also be converted to lactate during anaerobic conditions, this is not the primary fate during fasting when there is sufficient oxygen available for aerobic metabolism. The conversion to alanine occurs as part of gluconeogenesis involving transamination, but it is not the main pathway during fasting. The conversion to ethanol is not relevant as it primarily occurs in yeast and certain microorganisms, not in human metabolism during fasting. Therefore, shunting pyruvate to gluconeogenesis best describes the metabolic response during