Fasting insulin

The amount of insulin circulating after an overnight fast, which usually rises years before fasting glucose does.

Why it matters

In early insulin resistance, the pancreas compensates. It secretes more insulin to keep glucose in range, and it succeeds for years. During that period fasting glucose and HbA1c can look normal while fasting insulin climbs.

That makes fasting insulin one of the earliest routinely available signals that metabolic strain is building, which is precisely the window where lifestyle and pharmacological intervention work best.

HOMA-IR

Fasting insulin is rarely interpreted alone. Combining it with fasting glucose gives HOMA-IR, published by Matthews in 1985, which estimates hepatic insulin resistance from the two values together. It is the standard research proxy for insulin resistance short of a clamp study.

Our calculator does that arithmetic and shows both the conventional and the SI unit forms, because the two divisors in circulation cause endless confusion.

The measurement caveat

Insulin assays are not fully standardized between laboratories, so absolute values are less comparable across labs than glucose or HbA1c. Trends measured at the same laboratory are far more meaningful than a single number compared against something you read online.

There is also no universally agreed cutoff for a high fasting insulin in a nondiabetic adult. This is a gradient marker, not a threshold marker, and it should be read next to waist measurement, triglyceride to HDL ratio and HbA1c.

Fasting insulin is a marker we recommend, and our HOMA-IR calculator does the arithmetic if you choose to have it drawn.

References

  1. Matthews DR, et al. Homeostasis model assessment: insulin resistance and beta-cell function from fasting plasma glucose and insulin concentrations in man. Diabetologia. 1985.
  2. Tabak AG, et al. Trajectories of glycaemia, insulin sensitivity and insulin secretion before diagnosis of type 2 diabetes (Whitehall II). Lancet. 2009.