The role of insulin in regulating carbohydrate, lipid, and protein metabolism is a fundamental physiological fact well-established in standard biological literature, rendering a specific citation unnecessary.
The claim states that insulin regulates the breakdown and storage of carbohydrates, fats, and proteins. This is a foundational, textbook biological fact about endocrine function and metabolism. Per instructions, trivial or universally established biological facts of this nature are classified as common knowledge, so no citations are attached and the balance verdict is set to COMMON_KNOWLEDGE.