Fasting insulin and HOMA-IR: insulin resistance before glucose moves

Fasting insulin is the amount of insulin in the blood after an overnight fast, usually reported in microunits per millilitre (µIU/mL, sometimes written mIU/L) or in pmol/L (1 µIU/mL is about 6 pmol/L). On its own it says how hard the pancreas is working to keep fasting glucose where it is. Combined with fasting glucose, it gives HOMA-IR, an estimate of insulin resistance from a 1985 model by Matthews and colleagues.

HOMA-IR = fasting glucose (mg/dL) x fasting insulin (µIU/mL) / 405. With glucose in mmol/L, divide by 22.5 instead. A fasting glucose of 90 mg/dL with an insulin of 9 µIU/mL gives a HOMA-IR of 2.0.

The ranges, and why there is no guideline

Neither the ADA nor any major society sets a diagnostic threshold for fasting insulin or HOMA-IR. The reasons are practical. Insulin assays from different manufacturers give different numbers for the same sample. A single fasting value moves with the previous day's food, sleep and exercise. And research cutoffs for "insulin resistant" vary by population and assay. The single-digit fasting insulin targets and HOMA-IR "under 1" goals common online are opinion. Your own trend, at the same lab, is the useful thing.

What raises and lowers it

• Raises: excess visceral fat, inactivity, poor or short sleep, some drugs such as steroids, pregnancy, and the early stage of type 2 diabetes, when the pancreas is compensating.
• Lowers: weight loss, regular exercise, and some glucose-lowering drugs. Fasting insulin can also fall late in type 2 diabetes because the pancreas is failing, which is why it is read with glucose.
• Distorts it: insulin injections, and drawing the sample after eating.

What it predicts

A 2012 meta-analysis in PLoS One pooled 65 studies with 516,325 adults without diabetes. Per one standard deviation increase, the relative risk of coronary heart disease was 1.46 for HOMA-IR, 1.21 for fasting glucose, and 1.04 for fasting insulin alone, which was not statistically significant. So the combination carried more information than either part. These are observational associations, and the authors suggested HOMA-IR might be worth adding to risk models, not that it should be treated as a target.

Fasting insulin is also popular because it can rise years before HbA1c or fasting glucose leave their normal ranges. That is plausible physiology, but there is no trial showing that acting on a high fasting insulin, rather than on weight, activity and glucose, improves outcomes.

On GLP-1 drugs and weight loss

Insulin sensitivity improves as weight falls. The clearest published comparison on semaglutide is in adolescents: in a 2026 secondary analysis of STEP TEENS (193 participants without type 2 diabetes), fasting insulin fell 33.6% on semaglutide 2.4 mg against 10.1% on placebo, and HOMA-IR fell 35.0% against 5.3%. In the adult STEP 1 trial, the registry results report fasting insulin at the end of treatment at about 0.73 times baseline on semaglutide. Because these drugs also act on insulin release after meals, a fasting value is only part of the picture.

How to test

• A true fast: 8 to 12 hours, water only. A coffee with milk is enough to move insulin.
• Morning, and the same conditions each time. Avoid a hard training session the day before, a short night, or an unusually large or low-carbohydrate dinner if you want comparable numbers.
• Glucose and insulin from the same draw, or HOMA-IR is meaningless.
• Same lab. Assays differ enough that a change of lab can look like a change in you.
• Expect noise. Insulin varies more from day to day than glucose does, so two results a few weeks apart give a better baseline than one.

Tracking it in Peptide AI

In Peptide AI, fasting insulin sits in the metabolic panel. The app reads it from a lab PDF or manual entry, scores the panel from 0 to 100 alongside an overall grade, flags values that warrant prompt physician consultation, and plots each result against your dose timeline. Enter fasting glucose from the same draw and you can calculate HOMA-IR from the pair. Log the draw date, whether you fasted, and anything unusual in the days before, such as illness, a hard session or a dose step, so the next result is compared like with like. Bloodwork tracking shows the series next to the dose log, weight and side effects, and the panels page lists every marker the app groups. More markers are in the bloodwork guides.