Fasting Insulin

Fasting insulin measures circulating insulin after an 8–12 hour fast. It is a more sensitive early marker of insulin resistance than fasting glucose or HbA1c — elevations appear years or decades before any other marker shifts, making it one of the most valuable screening tests in longevity practice.

Fasting insulin reflects how much insulin the pancreas is producing to maintain baseline glucose control in the fasted state. In a metabolically healthy individual, only small amounts are needed. In someone developing insulin resistance, the pancreas compensates by producing progressively more insulin to achieve the same glucose control — producing hyperinsulinemia while fasting glucose and HbA1c remain "normal."

This is why fasting insulin is the earliest biomarker of metabolic dysfunction. By the time fasting glucose or HbA1c crosses a clinical threshold, the pancreatic compensation has typically been going on for years. Identifying elevated fasting insulin while other markers are still normal allows intervention before dysfunction is entrenched.

Target ranges in longevity practice are tighter than standard labs. Optimal fasting insulin is below 5 µIU/mL; values of 5–8 µIU/mL warrant attention; above 8 µIU/mL indicates meaningful insulin resistance. Standard labs often list "normal" up to 25 µIU/mL, which is functionally too permissive.

HOMA-IR (Homeostatic Model Assessment of Insulin Resistance) combines fasting glucose and fasting insulin into a single index. The formula: (fasting glucose mg/dL × fasting insulin µIU/mL) / 405. Optimal HOMA-IR is below 1.0; above 1.5 suggests insulin resistance.

For women specifically, fasting insulin tends to rise during the perimenopausal decade — a consequence of declining estrogen (which supports muscle insulin sensitivity), rising cortisol patterns, and increasing visceral adiposity. Tracking fasting insulin through the transition provides early warning and allows intervention while improvements are easier. The interventions are the same as for HbA1c: Zone 2 training, strength training, post-meal walks, time-restricted eating, refined carbohydrate reduction.

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