Fasting Insulin: The Early Warning Number

A standard panel checks fasting glucose and often an A1c. Both can read normal for a decade while the problem underneath them grows. The number that moves first is fasting insulin, and most people have never had it drawn.

Why insulin rises before glucose

Insulin's job is to move glucose out of the blood and into muscle, liver, and fat. When those tissues start responding less, the pancreas does the obvious thing and makes more insulin. The higher dose gets the job done, so glucose stays in range. From the outside everything looks fine. From the inside, the pancreas is working two or three times as hard to produce the same result.

That compensation can hold for years. Glucose only starts climbing when the pancreas can no longer keep up, which is why prediabetes often shows up on labs long after insulin resistance began. Fasting insulin shows the effort. Glucose shows the failure.

What we read as high

Many labs call fasting insulin normal up to about 25 uIU/mL. Our target for the people we treat is under 5. A fasting insulin of 15 beside a glucose of 92 passes a standard panel, and we read it as a pancreas under strain.

HOMA-IR puts the two numbers together. Multiply fasting glucose in mg/dL by fasting insulin, then divide by 405. Under 1 is where we want it. Above roughly 2 to 2.5 suggests meaningful insulin resistance. The person above, at 92 and 15, scores 3.4. A person at 92 and 4 scores 0.9. Same glucose, very different picture.

High insulin matters on its own. It tells fat cells to store and blocks them from releasing fat, which is one reason weight loss stalls. It raises triglycerides, lowers HDL, nudges blood pressure up, and drives fat into the liver. Each of those appears on later panels as a separate problem, and they usually share this one root.

What brings it down

• Muscle. It is the body's largest glucose sink. Resistance training pulls glucose into muscle through a pathway that does not need insulin, and a muscle that just trained keeps absorbing glucose for a day or more.
• Sleep. A single week of short nights measurably worsens insulin sensitivity in healthy adults. Sleep debt reads like a diet problem on labs.
• Carbs matched to training. We set carbohydrate at 1 to 2 grams per pound of lean mass depending on activity, with most of it placed around training sessions, when muscle is ready to take it.
• Losing fat around the organs, and a 10 minute walk after meals.

Fasting insulin is an inexpensive add on to a routine draw, and it responds within a few months of real change, so we retest at about 3 months.

That's why we check it.

This article is educational and is not medical advice.

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