Insulin Test · Blood

Your blood sugar might look perfectly normal and still be hiding a problem. When your cells start resisting insulin's signal, your pancreas compensates by pumping out more of it. For years, even decades, this extra insulin keeps your glucose numbers in the normal range. But that compensation comes at a cost: elevated insulin itself drives fat storage, inflammation, and arterial damage long before a diabetes diagnosis ever appears on your chart.

Measuring fasting insulin gives you a window into this hidden phase. Over half of people with normal glucose tolerance may already have elevated insulin levels, according to an analysis of over 4,000 individuals with normal glucose clearance. A standard metabolic panel that checks only glucose and HbA1c will miss this entirely.

What Insulin Does in Your Body

Insulin is a peptide hormone, a small protein made up of 51 amino acids arranged in two chains. Your pancreas produces it in specialized clusters of cells called the islets of Langerhans. Between meals, your pancreas releases a low, steady trickle of insulin. After you eat, rising blood sugar triggers a much larger burst.

Insulin's primary job is moving sugar out of your bloodstream and into cells that need it for energy. But it does far more than that. It tells your liver to stop releasing stored sugar, signals your body to build protein and store fat, and influences cell growth throughout the body. When insulin levels stay chronically high, these secondary effects become problematic: excess fat storage, increased inflammation, and faster-than-normal cell growth.

Heart Disease Risk

The connection between high insulin and cardiovascular disease is one of the strongest in metabolic medicine, and it holds up even after accounting for the usual suspects like obesity, high blood pressure, and cholesterol.

The Helsinki Policemen Study followed 970 men for 22 years and found that those with the highest insulin levels were about 2.7 times as likely to die from cardiovascular causes during the first decade of follow-up (age-adjusted HR 2.67), with the association weakening but remaining significant over the full study period (age-adjusted HR 1.73). After further adjustment for BMI, blood pressure, cholesterol, smoking, and other risk factors, the 10-year cardiovascular mortality HR was 2.30, though the 22-year association was attenuated and no longer statistically significant (HR 1.39).

A meta-analysis pooling 22 prospective studies with over 22,000 participants found that people in the highest fasting insulin category were about 50% more likely to develop coronary heart disease (pooled RR 1.50). Each 50 pmol/L increase in fasting insulin was linked to a 16% increase in coronary heart disease risk. A separate analysis of the 30-year Da Qing Study showed that people with high insulin but still-normal glucose had roughly double the risk of cardiovascular death (HR 2.03) compared to those with low insulin and low glucose.

Mendelian randomization studies, which use genetic variants to test cause and effect, support a direct causal link between elevated insulin and cardiovascular risk, not just a correlation.

All-Cause Mortality

Elevated fasting insulin predicts dying earlier from any cause, not just heart disease. A meta-analysis of 7 studies covering nearly 27,000 non-diabetic adults found that those with the highest fasting insulin levels had a 13% increase in all-cause mortality risk (RR 1.13). When researchers used HOMA-IR (a calculated score combining insulin and glucose that estimates how resistant your cells are to insulin's signal), the association was stronger: a 34% increase in all-cause mortality (RR 1.34).

The NHANES III cohort of nearly 14,000 people followed for a median of 25 years confirmed a similar pattern, with higher fasting insulin showing a 7% increase in all-cause mortality risk (HR 1.07) after adjusting for multiple factors. These associations held in both diabetic and non-diabetic populations.

Cancer Risk

Insulin promotes cell growth, and chronically elevated levels appear to increase cancer risk across several organ systems.

Sources: Tsujimoto et al. (NHANES 1999-2010); Ma et al. (Physicians' Health Study); Kabat et al. (Women's Health Initiative).

What this means for you: the cancer associations are independent of obesity. In the NHANES analysis, non-obese individuals with elevated insulin still had about 1.9 times the risk of cancer death. This suggests insulin itself, not just body weight, plays a role in cancer risk.

Hypertension and Metabolic Syndrome

High fasting insulin is also linked to developing high blood pressure. The same meta-analysis of 22 prospective studies found that people in the highest insulin category were about 63% more likely to develop hypertension (pooled RR 1.63). Each 50 pmol/L increase in fasting insulin carried a 25% increase in hypertension risk. Elevated insulin clusters with abdominal obesity, high triglycerides, low HDL cholesterol, and high blood pressure in a pattern called metabolic syndrome, which compounds cardiovascular and diabetes risk.

Why HOMA-IR Often Outperforms Insulin Alone

Fasting insulin reflects several overlapping processes: how much insulin your pancreas produces, how quickly your liver and kidneys clear it from the blood, how resistant your cells are to its signal, and your current blood sugar level. The strongest single driver of fasting insulin is actually insulin clearance (how fast your body removes it), not insulin resistance.

This is why HOMA-IR, which combines fasting insulin and fasting glucose into a single score, consistently predicts disease outcomes better than insulin alone. In the meta-analysis by Gast et al. covering over 516,000 participants, HOMA-IR showed a 46% increase in coronary heart disease risk per standard deviation increase, while fasting insulin alone showed only a 4% increase. For cardiovascular mortality, the gap was even wider. When you order an insulin test, getting glucose measured at the same time (which allows HOMA-IR calculation) significantly increases the clinical value.

Reference Ranges

There is no globally standardized insulin assay, and values from different labs and platforms cannot be directly compared. The American Diabetes Association's 2023 guidelines explicitly acknowledge that attempts to harmonize insulin assays produce "greatly discordant results." Among commercial assays, variation between platforms can range from 12% to 66%. This means your specific number matters less than your trend at the same lab over time.

With that caveat, here are reference ranges drawn from large population studies of healthy adults.

These tiers are drawn from published research across multiple populations. Your lab may use different assays and cutpoints. Compare your results within the same lab over time for the most meaningful trend. Proposed metabolic syndrome screening cutoffs range from 7.35 µIU/mL in a Taiwanese community study to 8 mU/L for men and 10 mU/L for women in a Brazilian cohort.

The longevity literature suggests that lower insulin levels and higher insulin sensitivity are associated with healthy aging. A review in the Journal of the American College of Cardiology noted that normal glucose metabolism, lower insulin levels, and higher insulin sensitivity may constitute markers of healthy aging and longevity.

Age and Sex Differences

Fasting insulin tends to decrease with age in healthy adults, according to the Tehran Lipid and Glucose Study. In children, insulin rises before puberty, then typically decreases in girls while stabilizing or continuing to increase in boys. Most adult studies show modest sex differences, with men generally having slightly higher median values than women, though not all populations confirm this pattern.

Obesity has a more pronounced effect on fasting insulin in men than in women, with insulin secretion increasing about 2.4-fold across the BMI spectrum. Genetic studies using Mendelian randomization have found that the link between elevated insulin and chronic kidney disease risk is far stronger in men (OR 7.23) than in women (OR 1.05), suggesting sex-specific metabolic consequences.

Tracking Your Trend

A single fasting insulin reading is a rough snapshot, not a reliable portrait. The day-to-day biological variation for fasting insulin is about 26%, meaning your number can swing substantially from one draw to the next based purely on normal physiology. For a reading to represent a true biological change (rather than random noise), it needs to shift by at least 68% in someone with normal glucose tolerance, and even more in someone with impaired glucose regulation.

This high variability makes serial measurement essential. Get a baseline reading, retest in 3 to 6 months if you are making diet or exercise changes, and then at least annually. Always test at the same lab, fasting, and ideally at a consistent time of morning. The trend across three or more readings tells you far more than any single number. A rising trajectory, even within the "normal" range, is a warning sign worth acting on. A declining trajectory after a lifestyle change confirms the intervention is working.

When Results Can Be Misleading

Because fasting insulin has a day-to-day variability of about 26% (meaning your result can swing that much from one draw to the next based on normal physiology alone), it is one of the more volatile routine lab values. Beyond normal variability, several factors can push your reading in a direction that does not reflect your actual metabolic health.

• Recent illness or infection: Acute infections can reduce insulin sensitivity by roughly 50%, equivalent to the metabolic effect of severe obesity. This insulin resistance can persist for 1 to 3 months after recovery. Avoid testing during or shortly after any significant illness.
• Surgery or physical trauma: Surgical stress suppresses insulin secretion by 40 to 50% while simultaneously causing marked insulin resistance, effects that can last 24 to 48 hours. Delay testing for at least a week after any procedure.
• Recent intense exercise: A single hard workout can lower fasting insulin by roughly 10 to 20% the following day, with enhanced insulin sensitivity lasting up to 48 hours. This is a transient effect, not a permanent metabolic improvement. Test on a day when you have not exercised intensely in the prior 24 to 48 hours for the most representative result.
• Circadian timing: Insulin secretion increases by about 60% during sleep, and glucose tolerance is naturally lower in the evening. Morning fasting draws give the most consistent, interpretable results. The "dawn phenomenon," a normal early-morning rise in blood sugar driven by cortisol and growth hormone, can also shift results depending on exact draw time.

Several common medications can also shift your insulin reading without necessarily meaning your metabolic health has changed.

• Statins: Atorvastatin, simvastatin, and rosuvastatin increase insulin resistance by about 24% and compensatory insulin secretion by 9 to 12% as a side effect unrelated to their cholesterol-lowering action. Pravastatin and pitavastatin appear neutral. If you are on a statin, your fasting insulin may read higher than it would otherwise, though this does appear to reflect a real metabolic shift rather than a pure measurement artifact.
• Corticosteroids: Even low-dose prednisolone (6 mg daily) increases insulin resistance and causes the liver to release more sugar into the blood. The hyperglycemic pattern typically peaks in the afternoon and evening. If you are taking steroids for another condition, your insulin will likely be elevated.
• Proton pump inhibitors (PPIs): Some evidence links PPIs to increased fasting insulin and insulin resistance, though the data are mixed. If you take a PPI regularly, this is worth noting when interpreting your results.