Endocrine physiology — What is the principal metabolic effect of insulin on blood glucose and hepatic glycogen?

Introduction / Context:
Insulin is a peptide hormone from pancreatic β-cells that coordinates fed-state metabolism. Its primary clinical relevance is the reduction of blood glucose concentrations after a carbohydrate-containing meal.

Given Data / Assumptions:

• Insulin promotes glucose uptake in insulin-sensitive tissues (muscle, adipose).
• Insulin stimulates hepatic glycogenesis and inhibits gluconeogenesis.
• Hyperglycemia follows insulin deficiency or resistance.

Concept / Approach:
Insulin lowers plasma glucose by increasing GLUT4-mediated uptake in muscle and adipose tissue, enhancing glycolysis and glycogenesis, and suppressing hepatic glucose output. Therefore, the most direct and overarching description of insulin’s “main function” is to decrease blood glucose. It also increases liver glycogen, but the systemic hallmark is the fall in blood sugar.

Step-by-Step Solution:

Verification / Alternative check:
Clinical practice monitors insulin effect via blood glucose measurements; insulin therapy aims to lower hyperglycemia in diabetes mellitus.

Why Other Options Are Wrong:

• Increase glycogen in liver occurs, but the question asks for the main effect; decreased blood sugar is broader and primary.
• Decrease glycogen in liver is the opposite of insulin’s anabolic action.
• Increase blood sugar describes glucagon, epinephrine, and cortisol effects, not insulin.

Common Pitfalls:
Equating a single tissue effect (hepatic glycogen) with the systemic role; prioritize the whole-body effect on blood glucose.

Final Answer:
Decrease blood sugar