A 25-year-old man has been fasting in the desert for a week. Which liver enzyme is most likely upregulated during this time?

• A. Glucose 6-phosphatase
• B. Pyruvate kinase
• C. Phosphofructokinase
• D. Hexokinase

Answer: (A)

During fasting, the body's primary goal is to maintain adequate blood glucose levels to provide energy, especially for the brain and red blood cells, which rely heavily on glucose as their energy source. The liver plays a crucial role in gluconeogenesis (the production of glucose from non-carbohydrate substrates) and glycogenolysis (the breakdown of glycogen to release glucose).

Glucose 6-phosphatase is a key enzyme involved in the final step of gluconeogenesis and glycogenolysis, where it hydrolyzes glucose-6-phosphate to free glucose, allowing glucose to be released into the bloodstream. During fasting, this enzyme's activity increases significantly to promote the conversion of stored glycogen to glucose, thereby ensuring adequate glucose levels in the blood.

The other enzymes listed have different roles in carbohydrate metabolism. Pyruvate kinase, phosphofructokinase, and hexokinase are more active during the fed state, where glucose is plentiful, and their role in glycolysis and glucose utilization does not align with the metabolic demands of a fasting state. Thus, during fasting, the upregulation of glucose 6-phosphatase is crucial for maintaining blood glucose levels, making it the most likely enzyme to be upregulated in