C-peptide and insulin are distinct substances produced by the body, both important for understanding glucose management. While often discussed together, they serve different functions in diagnostic assessments. Understanding their characteristics and relationship helps evaluate metabolic health.
The Relationship Between C-peptide and Insulin
C-peptide and insulin originate from a single precursor molecule, proinsulin, synthesized in the pancreas’s beta cells. Proinsulin is a larger, inactive protein that undergoes enzymatic cleavage. This process splits proinsulin into one molecule of insulin and one molecule of C-peptide.
Following this cleavage, both insulin and C-peptide are released into the bloodstream in equimolar amounts. C-peptide is a byproduct of the body’s insulin production. This co-secretion means C-peptide levels indicate how much insulin the pancreas is producing.
Why C-peptide is Measured
Measuring C-peptide offers an advantage over measuring insulin when assessing the body’s own insulin production. While insulin levels can be influenced by external sources (exogenous insulin), C-peptide levels exclusively reflect insulin produced by the pancreas (endogenous insulin). This distinction helps determine if the body is still making insulin, especially for individuals receiving insulin therapy.
C-peptide also has a longer half-life in the bloodstream than insulin. Insulin is rapidly cleared by the liver, with a half-life of 4 to 10 minutes. C-peptide, primarily cleared by the kidneys, has a half-life of 20 to 35 minutes. This extended presence makes C-peptide a more stable and reliable marker for assessing beta-cell function.
Interpreting C-peptide and Insulin Levels
C-peptide and insulin levels provide insights into pancreatic function and various metabolic conditions. Low C-peptide, often with low insulin, indicates reduced or absent endogenous insulin production. This pattern characterizes Type 1 Diabetes, where the pancreas produces little to no insulin. It also appears in advanced Type 2 Diabetes, where beta cells have lost their ability to produce insulin.
Conversely, high C-peptide and insulin levels suggest the body is producing a large amount of insulin. This is observed in conditions like insulin resistance or early Type 2 Diabetes, where cells do not respond effectively to insulin, prompting the pancreas to overproduce it. Elevated levels can also indicate an insulinoma, a rare pancreatic tumor that secretes excessive insulin.
C-peptide measurements are useful in differentiating between endogenous and exogenous insulin in hypoglycemia. If a person has high insulin levels but low C-peptide, it suggests the high insulin is from an external source. This aids in diagnosing conditions like factitious hypoglycemia, where an individual intentionally administers insulin. In contrast, high C-peptide and insulin levels during hypoglycemia could point to an insulin-producing tumor or diabetes medications that stimulate insulin release.