Is Fasting Insulin Testing Worth It?

Fasting insulin measures how much insulin your pancreas produces when you haven't eaten for 8-12 hours. It's essentially measuring how hard your body is working to maintain normal blood sugar.

Here's what makes this marker different: standard glucose tests only tell you if blood sugar is elevated. Fasting insulin tells you how hard your pancreas is working to keep glucose normal—often years before glucose tests show any problem.

In metabolically healthy people, fasting insulin is relatively low. But as cells become resistant to insulin's effects, the pancreas compensates by producing more and more. You can have perfectly normal glucose levels while your pancreas is working overtime—and that compensatory mechanism eventually fails.^[3]

This is why fasting insulin matters: research shows insulin levels typically rise 10-15 years before glucose tests become abnormal.

The landmark Whitehall II study followed over 6,500 people for nearly a decade. The finding: metabolic changes were detectable 3-6 years before diabetes diagnosis—with insulin sensitivity declining well before fasting glucose showed obvious abnormality.^[1]

Other research tracking the natural history of type 2 diabetes found insulin resistance may precede diabetes diagnosis by 10-15 years, with the pancreas compensating by producing progressively more insulin until beta cells eventually fail.^[4]

This is the window that matters. When you catch insulin resistance early—while glucose is still normal—lifestyle intervention is remarkably effective. Wait until glucose is elevated, and you're playing catch-up.

Insulin resistance occurs when cells—particularly muscle, fat, and liver cells—don't respond efficiently to insulin's signal to absorb glucose from the bloodstream.

The major drivers:

• Central adiposity (excess weight around the midsection)—the strongest predictor
• Physical inactivity—muscle is your largest glucose sink
• Poor sleep quality—even a few nights of poor sleep affects insulin sensitivity
• Chronic inflammation—creates insulin signalling dysfunction
• Chronic stress—cortisol directly opposes insulin action
• Genetic factors—some people are more susceptible

Here's what the research shows: insulin resistance is highly responsive to lifestyle intervention, especially when detected early. The Diabetes Prevention Program demonstrated that lifestyle changes reduced diabetes incidence by 58%—significantly more effective than medication.^[5]

But intervention works best before glucose rises. Once you're in the prediabetic range, you've lost years of the most effective intervention window.

HOMA-IR (Homeostatic Model Assessment of Insulin Resistance) combines fasting insulin and fasting glucose into a single score. It's more informative than either test alone because it captures the relationship between insulin production and glucose control.^[6]

A person can have normal glucose but high insulin (the pancreas compensating), which HOMA-IR reveals. This is the pattern that matters for early intervention.

The formula:

HOMA-IR was validated against the gold-standard hyperinsulinaemic-euglycaemic clamp technique and is widely used in metabolic research worldwide. The calculation itself is straightforward—what matters is understanding what the number means and acting on it early.^[6]

The problem: most GPs don't routinely calculate HOMA-IR. You may need to calculate it yourself or specifically request it from a health practitioner with metabolic health experience.

Fasting insulin isn't necessary for everyone. Standard screening is reasonable if you're metabolically healthy with no risk factors.

But if you have risk factors, this test provides genuinely valuable early warning—years before standard screening catches problems.

This is where fasting insulin testing gets complicated—and why understanding reference ranges matters.

Standard laboratory reference ranges for fasting insulin are typically quoted as 2-25 mIU/L (or similar). But these ranges are derived from population statistics—what's 'normal' in a population where metabolic disease is epidemic.

Metabolic health research suggests optimal fasting insulin is considerably lower. Many practitioners cite targets below 8-10 mIU/L as truly optimal for metabolic health.^[11]

Here's what this means: a fasting insulin of 18 mIU/L is technically 'within range' by laboratory standards. But it indicates your pancreas is working hard to maintain glucose levels—this is compensated insulin resistance, the stage before glucose becomes abnormal.

If you request fasting insulin testing from your GP, be prepared for questions—or even pushback. Here's why:

• It's not in standard guidelines: RACGP diabetes screening guidelines recommend fasting glucose or HbA1c, not fasting insulin. The guidelines specifically state there is no role for routine insulin testing in diabetes risk assessment.^[10]
• Medicare coverage is limited: Fasting insulin is typically only rebated for specific conditions (PCOS, investigating hypoglycaemia, suspected insulinoma)—not routine metabolic screening. Expect to pay $50-80 out of pocket.
• Standardisation challenges: Insulin assays vary between laboratories more than glucose tests, making results harder to compare across time or facilities.
• Intervention overlap: Whether insulin is mildly or significantly elevated, initial intervention is similar: diet optimisation, exercise, sleep, stress management.

But here's what the guidelines miss: waiting for glucose abnormalities means losing the most effective intervention window. Research clearly shows insulin rises first—sometimes over a decade before glucose becomes abnormal.^[1][4]

Some GPs with metabolic health interest are familiar with fasting insulin and HOMA-IR. Others may not order it outside specific clinical indications. A health practitioner with functional medicine or metabolic health experience typically offers more detailed interpretation.

For someone without risk factors and normal glucose tests: Routine screening with HbA1c and fasting glucose is probably sufficient. The added value of insulin testing is minimal.

For someone with risk factors (family history, PCOS, central adiposity, previous gestational diabetes, or concerning symptoms despite normal glucose): Yes, absolutely worth it. This test provides 10-15 years of early warning that glucose tests miss entirely.

The cost is typically $50-80 for self-funded testing. Compare that to the cost of developing type 2 diabetes—the medications, complications, reduced quality of life. The cost of knowing is trivial; the cost of not knowing is substantial.

For those optimising metabolic health proactively: Fasting insulin combined with glucose (to calculate HOMA-IR) provides the most complete early-stage metabolic picture available. It's the marker that reveals problems earliest.