When cells begin to become resistant to insulin's signal, the pancreas compensates by producing more insulin to achieve the same glucose-clearing effect. This compensatory hyperinsulinaemia maintains blood glucose within the normal range for years, sometimes decades, before glucose eventually breaks through. A clinician measuring only fasting glucose during this compensatory phase will see a normal result and conclude there is no metabolic problem. Measuring fasting insulin reveals the compensatory overproduction that is already occurring, providing an earlier diagnostic signal.
HOMA-IR: Combining Glucose and Insulin
The Homeostatic Model Assessment of Insulin Resistance, HOMA-IR, combines fasting glucose and fasting insulin in a simple calculation to estimate insulin resistance. It is not perfect, but it is one of the most practical clinical tools for detecting insulin resistance before glucose reaches diabetic levels. A HOMA-IR above 1.9 suggests insulin resistance is developing. Above 2.9 is consistent with significant insulin resistance. Optimal metabolic health is typically associated with a HOMA-IR below 1.0.
Why Fasting Insulin Is Rarely Measured
Despite its clinical utility, fasting insulin is not included in standard NHS blood panels and is not routinely offered in general practice. This is a consequence of the diagnostic threshold focus of conventional medicine: fasting glucose and HbA1c identify diabetes and prediabetes, which trigger treatment. Insulin resistance without glucose dysregulation does not yet meet a diagnostic threshold, and therefore receives no standard clinical attention. Requesting a fasting insulin test alongside standard metabolic panels requires an informed conversation with a clinician or a private test.
Sources
- Reaven, G.M. (1988). Banting lecture 1988: Role of insulin resistance in human disease. Diabetes, 37(12), 1595-1607.
- Matthews, D.R., et al. (1985). Homeostasis model assessment: insulin resistance and beta-cell function from fasting plasma glucose and insulin concentrations in man. Diabetologia, 28(7), 412-419.
- Stern, S.E., et al. (2005). Identification of individuals with insulin resistance using routine clinical measurements. Diabetes, 54(2), 333-339.