Which of the following is the mechanism by which metformin reduces hepatic glucose output?

Question

Accepted Answer

B. Inhibition of mitochondrial glycerol-3-phosphate dehydrogenase and suppression of gluconeogenesis. ## Metformin's Mechanism on Hepatic Glucose Production

**Key Point:** Metformin's primary antihyperglycemic effect is suppression of hepatic glucose output (HGO) via inhibition of mitochondrial glycerol-3-phosphate dehydrogenase (mGPDH), which reduces the NAD^+^/NADH ratio and impairs gluconeogenesis.

### Detailed Mechanism

1. **Mitochondrial NADH Accumulation:** Metformin inhibits mGPDH, an enzyme in the inner mitochondrial membrane that catalyzes the oxidation of glycerol-3-phosphate to dihydroxyacetone phosphate.
2. **Reduced NAD^+^ Availability:** This inhibition decreases the NAD^+^/NADH ratio, making NAD^+^ unavailable for other metabolic reactions.
3. **Gluconeogenesis Suppression:** The reduced NAD^+^ impairs the conversion of lactate and pyruvate to glucose in the liver.
4. **Decreased HGO:** Net result is a 20–30% reduction in fasting hepatic glucose output.

### Secondary Mechanisms

**High-Yield:** Metformin also:
- Increases AMPK activation (indirectly, via energy stress), which inhibits mTORC1 and reduces protein synthesis
- Improves peripheral insulin sensitivity (modest effect)
- Slows intestinal glucose absorption (minor contribution)
- Reduces lactate production (due to decreased gluconeogenesis)

### Comparison: Metformin vs. Other Agents

| Agent | Primary Mechanism | Effect on HGO | Effect on Insulin Secretion |
|-------|-------------------|---------------|--------------------------|
| **Metformin** | mGPDH inhibition → ↓ NAD^+^ | ↓↓ (major) | No direct effect |
| Sulfonylureas | K-ATP channel blockade | No direct effect | ↑↑ (major) |
| Thiazolidinediones | PPAR-γ agonism | ↓ (modest) | No direct effect |
| DPP-4 inhibitors | Incretin prolongation | ↓ (modest) | ↑ (glucose-dependent) |

**Clinical Pearl:** Metformin is the first-line agent for type 2 diabetes because it reduces HGO without causing hypoglycemia or weight gain, and it has cardiovascular and renal protective effects [cite:Harrison 21e Ch 417].

Answer

A. Blockade of ATP-sensitive potassium channels in hepatocytes

Answer

C. Activation of AMP-activated protein kinase (AMPK) leading to increased insulin secretion

Answer

D. Inhibition of α-glucosidase enzymes in the small intestine

Which of the following is the mechanism by which metformin reduces hepatic glucose output?

Explanation

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