Connecting peptide, called C-peptide, is like a twin to insulin that is released when the early insulin molecule or "pro-insulin" is split into C-peptide and insulin, and then released into the bloodstream. C-peptide is not just a silent twin but has beneficial metabolic effects of its own including supporting red blood cell function and tissue oxygenation, improving blood flow to muscles and skin, supporting kidney function, protecting of cells from excess glucose, and reducing inflammation.
C-peptide reflects the amount of insulin produced. Because of this, it can be used to evaluate whether insulin is too high as it is in early type 2 diabetes, or too low as it is in type 1 diabetes where insulin production eventually stops.
If levels of C-peptide are high and blood glucose also remains high, then you know insulin levels are also high, and that cells are just ignoring the insulin and being "insulin resistant." Elevated C-peptide is also associated with cardiovascular disease risk and all-cause mortality.