What it measures.
Fasting insulin reveals how hard your pancreas must work to maintain normal blood glucose. Elevated insulin is often the first sign of insulin resistance, appearing 10-20 years before glucose levels rise. Testing insulin alongside glucose unmasks early metabolic dysfunction.
Fasting insulin measures the concentration of insulin hormone in blood after an overnight fast. At this point, insulin levels reflect your baseline pancreatic output needed to suppress liver glucose production and maintain glucose homeostasis.
Why it matters.
Fasting insulin is arguably the most important metabolic marker for longevity that's rarely tested. Studies show hyperinsulinemia (elevated insulin) precedes hyperglycemia (elevated glucose) by up to two decades. Detecting high insulin early allows intervention before significant metabolic damage occurs.
Physiology.
Insulin is released by pancreatic beta cells in response to blood glucose. In insulin resistance, cells become less responsive to insulin's signal, requiring the pancreas to produce more insulin to achieve the same glucose-lowering effect. This 'compensatory hyperinsulinemia' can maintain normal glucose for years while driving inflammation, fat storage, and cardiovascular risk.
Testing & preparation.
How to prepare
- Fast for 10-12 hours (critical for accurate insulin measurement)
- Avoid intense exercise for 24 hours before
- Test in the morning, at consistent time
- Note recent carbohydrate intake patterns
When to test
Should be included in metabolic panels, especially with risk factors for diabetes, family history, overweight, or elevated triglycerides.
How often
Annually for screening; every 3-6 months when addressing metabolic syndrome.
Interpretation.
High fasting insulin
Common causes:
- Insulin resistance (most common)
- Early type 2 diabetes (compensatory phase)
- Metabolic syndrome
- Excess carbohydrate intake
- Obesity, especially visceral
- Insulinoma (rare insulin-producing tumor)
- PCOS in women
Implications:
- Pancreas is working overtime—early warning sign
- Increased risk of developing diabetes
- Associated with cardiovascular disease
- Drives fat storage and difficulty losing weight
- May contribute to cognitive decline over time
Low fasting insulin
Common causes:
- Excellent insulin sensitivity
- Type 1 diabetes (insulin deficiency)
- Late-stage type 2 diabetes (beta cell failure)
- Pancreatic disease
- Low-carbohydrate or fasting state
Implications:
- If glucose is normal: indicates excellent metabolic health
- If glucose is high: indicates insulin deficiency, not resistance
- Very low insulin with high glucose requires medical evaluation
Optimization.
Diet
- Reduce refined carbohydrates and added sugars (primary driver)
- Increase protein and fiber to improve satiety and reduce insulin demand
- Consider time-restricted eating (reduces insulin exposure)
- Lower overall carbohydrate intake, especially without exercise
- Avoid frequent snacking (keeps insulin elevated)
Lifestyle
- Build muscle through resistance training (largest glucose sink)
- Regular aerobic exercise improves insulin sensitivity for 24-48 hours
- Prioritize sleep (poor sleep significantly impairs insulin sensitivity)
- Manage chronic stress (cortisol worsens insulin resistance)
- Achieve healthy body composition—especially reduce visceral fat
Supplements
- Berberine: activates AMPK, improves insulin sensitivity
- Magnesium: essential for insulin signaling
- Alpha-lipoic acid: may enhance glucose uptake
- Chromium: supports insulin function
FAQs.
Why isn't fasting insulin tested routinely?
This is a significant gap in standard care. Fasting insulin is inexpensive but often not included in standard panels. Many physicians focus on glucose alone, missing the early warning of elevated insulin. You can request it specifically, or use direct-to-consumer labs.
What's the difference between high insulin and diabetes?
High insulin typically comes first (insulin resistance). Your pancreas produces extra insulin to keep glucose normal. This can work for years or decades. Diabetes develops when the pancreas can no longer keep up—insulin production falls or becomes insufficient—and glucose rises. Catching high insulin early prevents this progression.
Can thin people have high fasting insulin?
Absolutely. 'Metabolically obese normal weight' (MONW) individuals can have significant insulin resistance despite normal BMI. These individuals often have excess visceral fat relative to their frame. Fasting insulin can reveal this hidden metabolic dysfunction.
How does exercise lower insulin?
Exercise improves insulin sensitivity through multiple mechanisms: depletes muscle glycogen (creating demand for glucose uptake), increases GLUT4 transporters, reduces inflammation, and builds metabolically active muscle tissue. Both resistance and aerobic training help, with effects lasting 24-48 hours per session.
What's a good target for fasting insulin?
Most longevity-focused practitioners target <8 µIU/mL, with optimal being <5 µIU/mL. The wide lab 'normal' range (up to 25) allows significant insulin resistance to go undetected. Focus on the lower end of the range for metabolic health.