Which of the following is the most common site of gluconeogenesis in the human body?

Explanation

## Anatomical Distribution of Gluconeogenesis **Key Point:** The liver is responsible for approximately 90% of total gluconeogenesis under normal fed and fasted states, making it the predominant gluconeogenic organ. ### Quantitative Contribution by Organ | Organ | Gluconeogenic Capacity | % of Total | Notes | |-------|------------------------|-----------|-------| | Liver | Very high | ~90% | Primary site; continuous synthesis | | Kidney cortex | Moderate | ~10% (increases to 40% in prolonged fasting) | Becomes significant during starvation | | Small intestine | Low | <5% | Minor contributor; mainly postprandial | | Skeletal muscle | None | 0% | Lacks glucose-6-phosphatase; cannot release free glucose | ### Why the Liver Dominates **High-Yield:** The liver contains all four key gluconeogenic enzymes: 1. **Pyruvate carboxylase** — converts pyruvate → oxaloacetate 2. **PEPCK** (phosphoenolpyruvate carboxykinase) — oxaloacetate → phosphoenolpyruvate 3. **Fructose-1,6-bisphosphatase** — fructose-1,6-bisphosphate → fructose-6-phosphate 4. **Glucose-6-phosphatase** — glucose-6-phosphate → free glucose (final step) The presence of **glucose-6-phosphatase** is critical — this enzyme is found only in liver and kidney, allowing these organs to release free glucose into the bloodstream. ### Role of Kidney in Extended Fasting **Clinical Pearl:** During prolonged fasting (>48 hours) or metabolic acidosis, the kidney cortex increases its gluconeogenic contribution from ~10% to ~40% of total glucose production. This becomes clinically important in: - Starvation states - Diabetic ketoacidosis - Severe sepsis ### Why Other Sites Cannot Perform Complete Gluconeogenesis - **Skeletal muscle:** Lacks glucose-6-phosphatase; produces glucose-6-phosphate but cannot release free glucose. Instead, it releases alanine (glucose-alanine cycle). - **Small intestine:** Limited enzymatic capacity; contributes minimally except in the postprandial period. - **Brain, RBC, adipose tissue:** Cannot perform gluconeogenesis at all. **Mnemonic:** **GLUK** — **G**lucose-6-phosphatase **L**iver/**K**idney **U**nique — only these two organs can complete gluconeogenesis and release free glucose.