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Metabolic

Fasting Insulin

Fasting insulin measures the concentration of insulin in your blood after an overnight fast of 8–12 hours. Insulin is a peptide hormone produced by the beta cells of the pancreas in response to rising blood glucose. Its primary role is to shuttle glucose from the bloodstream into cells for energy or storage. When measured in a fasted state — with no incoming glucose from food — insulin reflects your baseline pancreatic output and your tissues' sensitivity to the hormone. A fasting insulin level is one of the earliest markers of metabolic dysfunction, often rising years before fasting glucose or HbA1c moves outside the normal range.

Optimal Ranges

Clinical (NHS) Range

18–173 pmol/L (3–25 mIU/L)

pmol/L

Performance-Optimised Range

20–60 pmol/L (3–9 mIU/L)

pmol/L

The clinical range defines what is considered medically “normal” — broad enough to cover 95% of the population. The performance range reflects where research and clinical experience suggest most people feel and function at their best.

Why It Matters

Why Insulin matters for performance

Fasting insulin is arguably the most underused metabolic biomarker in UK general practice. Most GPs test fasting glucose and HbA1c, but these only become abnormal once the pancreas can no longer compensate for insulin resistance — by which point you may have been metabolically dysfunctional for 5–10 years. Elevated fasting insulin (hyperinsulinaemia) is the body's attempt to force glucose into resistant cells by producing more insulin. It precedes and predicts type 2 diabetes, non-alcoholic fatty liver disease (NAFLD), polycystic ovary syndrome (PCOS), and cardiovascular disease. For active people, insulin sensitivity is directly linked to body composition, recovery, and energy stability — higher sensitivity means more efficient glucose uptake into muscle, steadier energy, and less fat storage. The HOMA-IR index, calculated from fasting insulin and fasting glucose, is the gold-standard surrogate measure of insulin resistance used in clinical research.

Symptoms

Signs your levels may be off

Low / Deficiency

  • Very low fasting insulin may indicate type 1 diabetes or late-stage beta-cell exhaustion
  • Unexplained weight loss despite adequate calorie intake
  • Excessive thirst and frequent urination (polyuria)
  • Fatigue and muscle wasting

High / Excess

  • Often asymptomatic for years — hyperinsulinaemia is a silent condition
  • Central weight gain (visceral fat around the waist)
  • Energy crashes and sugar cravings after meals
  • Difficulty losing weight despite calorie restriction
  • Skin tags and acanthosis nigricans (darkened skin patches on neck or armpits)
  • Brain fog and afternoon fatigue
  • PCOS symptoms in women (irregular cycles, acne, hair thinning)

Dietary Sources

Foods that support Insulin levels

Foods that improve insulin sensitivity (not direct sources of insulin)Non-starchy vegetables — broccoli, spinach, peppers, courgettesOily fish — salmon, mackerel, sardines (omega-3 improves insulin signalling)Nuts and seeds — almonds, walnuts, flaxseed (fibre + healthy fats)Berries — blueberries, strawberries (low glycaemic, high polyphenol)Extra-virgin olive oil (oleic acid improves insulin sensitivity)Apple cider vinegar with meals (modest evidence for reducing postprandial insulin spikes)

Supplementation

Evidence-based supplementation

Insulin sensitivity is primarily improved through lifestyle interventions rather than supplements. Resistance training is the single most effective intervention — it increases GLUT4 transporter expression in muscle, directly improving glucose uptake independent of insulin. Walking after meals (even 10 minutes) significantly blunts postprandial insulin spikes. Sleep quality has a profound impact: a single night of 4-hour sleep reduces insulin sensitivity by ~25% in healthy adults. Among supplements, magnesium (glycinate or threonate, 200–400mg daily) is the most evidence-based — low magnesium is strongly associated with insulin resistance, and supplementation improves HOMA-IR in multiple trials. Berberine (500mg 2–3 times daily with meals) has shown glucose-lowering effects comparable to metformin in some studies, though quality varies. Chromium picolinate (200–1,000mcg daily) has modest evidence for improving insulin sensitivity. Alpha-lipoic acid (600mg daily) may improve glucose uptake. Reducing refined carbohydrate intake and prioritising protein and fibre at each meal reduces the insulin demand on the pancreas. Retest at 90 days after implementing lifestyle changes.

Research

Key study

Homeostasis model assessment: insulin resistance and beta-cell function from fasting plasma glucose and insulin concentrations in man

Matthews DR, Hosker JP, Rudenski AS, et al.

Diabetologia (1985)

DOI: 10.1007/BF00280883

Related Biomarkers

Metabolic

HbA1c (Glycated Haemoglobin)

Optimal: 20-34 mmol/mol (below 5.4%)

Metabolic

Cholesterol (Total, HDL, LDL & ApoB)

Optimal: ApoB: below 1.0 g/L, LDL: below 2.6 mmol/L, HDL: above 1.2 mmol/L

Metabolic

ALT (Alanine Aminotransferase)

Optimal: 10-26 U/L

Inflammation

hs-CRP (High-Sensitivity C-Reactive Protein)

Optimal: Below 1.0 mg/L

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This content is for educational purposes only and does not constitute medical advice. Your data suggests areas for optimisation, but any concerns should be discussed with a qualified healthcare professional. If your results flag values outside safe ranges, we recommend consulting your GP.