C-peptide

C-peptide is a piece of protein your pancreas makes together with insulin. The beta cells first build one long protein, proinsulin, and cut a connecting piece out of it just before release. What remains is insulin; the piece cut out is C-peptide, the C for connecting. For every insulin molecule exactly one C-peptide is released. That makes C-peptide a counter of your own insulin production, and on two points a better one than insulin itself. First, it is absent from insulin out of a pen: anyone who injects insulin has an insulin value that comes from outside, but a C-peptide that can only come from their own pancreas. Second, it stays in your blood longer. Insulin is largely captured by your liver before it reaches the rest of your body and is gone within minutes; C-peptide is cleared by your kidneys and stays measurable for half an hour or more, which makes it fluctuate less. Your report shows C-peptide in nmol/L, measured fasting. There is no national threshold and the range differs per lab, so compare your value with your own lab's table, and always read it beside the fasting glucose from the same draw.

The question C-peptide answers is: how much insulin do you still make yourself? For anyone healthy with a normal blood sugar the answer is almost always enough, and then the test adds little to a fasting insulin. To be honest: this is not a value everyone needs to know every year. It becomes important once blood sugar is no longer normal, because then there are two very different explanations that call for opposite approaches. In type 2 diabetes the cells listen poorly and the pancreas makes plenty: a high C-peptide. In type 1 and in LADA, the slow form of type 1 that develops in adulthood, the immune system breaks down the beta cells and production of your own falls away: a low C-peptide. The two look alike from the outside, and the distinction is missed more often than you would think. Of people diagnosed with type 2, an estimated 15 percent actually have LADA and 1 to 5 percent MODY, an inherited form. For anyone with type 2, C-peptide also offers perspective. A pancreas still producing plenty is a pancreas lifestyle can still do a lot with: losing weight and exercising make the cells sensitive again, and your own insulin then does the rest. A low production of your own says the beta cells are exhausted or under attack, and then insulin from outside is the way. C-peptide is the number that keeps those two apart. Where LADA is suspected, the distinction is confirmed with antibodies, not with C-peptide alone. And for anyone who injects insulin, C-peptide is the only way to see what the pancreas itself still does, because an insulin value then says nothing any more.

A high C-peptide means a pancreas making a lot of insulin, and the usual reason is the same as for a high insulin: insulin resistance from belly fat, little exercise, little muscle, plenty of sugar and lack of sleep. Anyone not drawn fasting measures production as a response to that meal, and that sits several times higher. Medication that stimulates release, certain diabetes medicines, lifts the value too. And because your kidneys clear C-peptide, the value rises with reduced kidney function without production changing. A low C-peptide with a normal glucose is, in a healthy person, usually a sensitive metabolism that gets by on little insulin, and so favourable. Low beside a high glucose is the picture of a pancreas that can no longer keep up: exhausted after years of type 2, under attack in type 1 and LADA, or built with a fault in the beta cell as in MODY. A long fast also lowers the value, because without glucose coming in little insulin is needed. With a high C-peptide the levers are those of insulin resistance, and they work: losing weight around the middle, strength training and daily movement, less sugar and fast carbohydrates, more fibre and protein, enough sleep. As your cells become more sensitive, the pancreas has to make less and the number falls with it. A low C-peptide with a high glucose cannot be raised with lifestyle; there production of your own is gone, and insulin from outside is the designated route. In practice C-peptide is a fragile test: the tube has to be processed or frozen within a few hours, so you cannot have it drawn at every location, and a failed measurement means a new draw. Read the value beside the fasting glucose from the same draw, and beside HbA1c for the longer story. Anyone without diabetes who only wants to know how sensitive their metabolism is usually has enough with fasting insulin or the TyG index.