Prolonged fasting and gluconeogenesis…

A 14-year-old high school girl who is extremely conscious about her appearance has gone a full day fasting to fit in to a dress she intentionally brought a size smaller than her actual size for a dance party. Which of the following organs/tissues contribute to the glucose that is being synthesized through gluconeogenesis during periods of prolonged fasting?

A. Spleen

B. Red blood cells

C. Skeletal muscle

D. Liver

E. Brain

D-Liver is the correct answer.

Gluconeogenesis is the process of converting noncarbohydrate precursors to glucose or glycogen. The major substrates are the glucogenic amino acids, lactate, glycerol, and propionate. Liver and kidney are the major gluconeogenic tissues.

Gluconeogenesis meets the needs of the body for glucose when sufficient carbohydrate is not available from the diet or glycogen reserves. A supply of glucose is necessary especially for the nervous system and erythrocytes. Failure of gluconeogenesis is usually fatal.

Muscle gluconeogenesis does not contribute to blood glucose levels due to absence of Glucose-6-Phosphatase enzyme. Glucose-6-P produced is channeled towards pathway of glycolysis for muscle’s own energy needs. Muscle indirectly however contributes to maintenance of blood glucose levels through Cori’s cycle (figure-1) and Glucose Alanine cycle (figure 2).

As regards other option glucose is not at all synthesized in spleen, red blood cell and brain.

Cori cycle

Figure-1-Cori cycle- Lactate produced from glycolysis in muscle is transported to liver where it is used as a substrate for gluconeogenesis. Glucose thus produced is poured in to blood to be transported back to muscle to be channeled towards the pathway of glycolysis. The formation and utilization of lactate continues in a cyclic manner involving muscle, blood and liver. Thus indirectly muscle contributes glucose to blood. The pathway of gluconeogenesis apart from producing glucose also helps in removal of waste products from tissues like muscle (lactate) and glycerol from red blood cells and adipose tissue.

glucose-alanine cycle

Figure-2- Glucose alanine cycle contributes to maintenance of blood glucose levels besides transporting amino group of amino acids in the form of alanine to liver  for their disposal.


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