Summary
Fasting insulin measures the concentration of insulin in the blood after an overnight fast. Elevated fasting insulin is the earliest detectable sign of insulin resistance — the root cause of type 2 diabetes, metabolic syndrome, PCOS, and cardiovascular disease. Crucially, fasting insulin becomes abnormal years before blood glucose or HbA1c rise, making it one of the most powerful early warning tests in preventive medicine.
Insulin resistance occurs when cells stop responding efficiently to insulin’s signals. The pancreas compensates by producing more insulin — so blood glucose may remain normal while fasting insulin quietly rises. This ‘compensated’ insulin resistance phase can last for years or decades before the pancreas eventually fails to keep up and glucose rises.
By testing fasting insulin alongside fasting glucose, the HOMA-IR (Homeostasis Model Assessment of Insulin Resistance) index can be calculated, providing a validated quantitative measure of insulin resistance. This allows identification and monitoring of the insulin-resistant state long before diabetes develops.
High fasting insulin is also a direct metabolic driver of PCOS, hypertension, dyslipidaemia, and chronic inflammation — making it a central target in metabolic health management.
What It Is
Insulin is a 51-amino acid peptide hormone synthesised by beta cells of the islets of Langerhans in the pancreas. It is released in response to rising blood glucose (primarily), amino acids, and incretin hormones, and acts via the insulin receptor to stimulate glucose uptake, glycogen synthesis, lipogenesis, and protein synthesis while suppressing hepatic glucose production and lipolysis.
In the fasted state, insulin is at its lowest physiological level — typically < 60 pmol/L (< 10 mU/L) in insulin-sensitive individuals. Higher fasting insulin indicates that the pancreas is working harder than normal to maintain blood glucose — the signature of insulin resistance. Fasting insulin is measured in pmol/L (preferred) or mU/L (1 mU/L ≈ 6 pmol/L). Reference ranges vary by laboratory but are generally: normal < 60 pmol/L ( 104 pmol/L (> 17.3 mU/L).
Functions
Insulin resistance detection
Elevated fasting insulin is the earliest sign of insulin resistance — identifying the problem years before glucose rises.
HOMA-IR calculation input
Combined with fasting glucose, enables HOMA-IR calculation — the standard validated index of insulin resistance.
Metabolic disease risk stratifier
High fasting insulin predicts the development of type 2 diabetes, PCOS, metabolic syndrome, and cardiovascular disease.
PCOS metabolic assessment
Insulin resistance is central to PCOS — elevated fasting insulin quantifies the severity of the underlying metabolic driver.
Reference Ranges
Fasting Insulin
Measured in pmol/L| Status | Range (pmol/L) | Range (mU/L) | What it means |
|---|---|---|---|
| Optimal | < 60 | < 10 | Good insulin sensitivity — cells responding efficiently to insulin. |
| Borderline | 60–104 | 10–17 | Borderline insulin resistance — lifestyle intervention advised. |
| Elevated | > 104 | > 17 | Significant insulin resistance — metabolic intervention required. |
Reference ranges vary between laboratories. HOMA-IR (calculated from fasting glucose and insulin) provides a more clinically useful index than insulin alone. Strict 10–12 hour fasting is essential for a valid result.
Symptoms of Imbalance
Insulin resistance produces a cluster of metabolic symptoms, though many people are asymptomatic despite significantly elevated fasting insulin.
- Very low fasting insulin in a non-diabetic context is rare but may indicate pancreatic insufficiency
- Type 1 diabetes is characterised by very low or absent insulin
- Often asymptomatic in early insulin resistance
- Central weight gain and difficulty losing weight despite diet
- Fatigue and energy slumps after meals
- Increased hunger and carbohydrate cravings
- Brain fog and poor concentration
- Skin tags and acanthosis nigricans (darkening of skin folds)
- PCOS symptoms in women: irregular periods, acne, excess hair growth
Causes of Imbalance
- Type 1 diabetes (absolute insulin deficiency)
- Pancreatic insufficiency or pancreatectomy
- Very low carbohydrate diet (temporarily reduces insulin requirements)
- Obesity and visceral adiposity — the most common cause
- Type 2 diabetes (early stage — insulin rises before glucose does)
- PCOS — insulin resistance is a core feature
- Metabolic syndrome
- Sedentary lifestyle
- Chronic stress — cortisol drives insulin resistance
- Poor sleep quality and sleep apnoea
- High refined carbohydrate and sugar intake
FAQs
Because insulin resistance — and the elevated insulin that accompanies it — develops years to decades before blood glucose rises. In the early stages of insulin resistance, the pancreas compensates by secreting more insulin, successfully keeping glucose normal. This ‘compensated’ state can persist for 5–10+ years. Testing fasting insulin directly reveals this compensatory hyperinsulinaemia — the earliest detectable metabolic warning sign.
Optimal fasting insulin is < 60 pmol/L (< 10 mU/L). Functional medicine practitioners and sports physicians often target < 42 pmol/L (< 7 mU/L) as an indicator of excellent insulin sensitivity. The key clinical threshold is HOMA-IR 2.5 indicates clinically significant insulin resistance.
Yes — insulin resistance is one of the most modifiable metabolic conditions. The most effective interventions are: sustained weight loss (even 5–10% of body weight dramatically improves insulin sensitivity), regular aerobic and resistance exercise, reducing refined carbohydrates and sugar, improving sleep quality, managing chronic stress, and time-restricted eating or intermittent fasting. Fasting insulin normalises remarkably quickly with these changes — often within weeks.
Yes. Low carbohydrate and ketogenic diets significantly reduce insulin secretion and fasting insulin — often dramatically within days to weeks. This is one of the key mechanisms by which these diets promote weight loss and improve metabolic markers. If you follow a very low carbohydrate diet, your fasting insulin may be very low (< 20 pmol/L), which is metabolically favourable and not a concern.
Chronically elevated fasting insulin is not benign — it independently drives visceral fat accumulation, PCOS, hypertension, dyslipidaemia, and atherosclerosis, in addition to its role as the precursor to type 2 diabetes. Treating hyperinsulinaemia through lifestyle change (and sometimes metformin) reduces risk across all these conditions. The earlier insulin resistance is identified and addressed, the easier it is to reverse.
References
- Muniyappa R, et al. Current approaches for assessing insulin sensitivity and resistance in vivo. Am J Physiol Endocrinol Metab. 2008;294(1):E15–E26. View source
- Reaven GM. Banting Lecture 1988. Role of insulin resistance in human disease. Diabetes. 1988;37(12):1595–1607. View source
- DeFronzo RA, Ferrannini E. Insulin resistance: a multifaceted syndrome responsible for NIDDM, obesity, hypertension, dyslipidemia, and atherosclerotic cardiovascular disease. Diabetes Care. 1991;14(3):173–194. View source