The hidden biochemistry of insulin resistance: why a standard blood-sugar test misleads

Most people assume that if their fasting glucose is normal (below 5.6 mmol/L), their carbohydrate metabolism is perfectly healthy. This is a dangerous myth. Type-2 diabetes and metabolic syndrome develop silently for 10–15 years before blood sugar ever starts rising.

Compensatory hyperinsulinaemia: the silent phase

How does it happen? As cellular resistance develops, the pancreas is forced to produce ever-larger amounts of insulin to push glucose into cells. This state — compensatory hyperinsulinaemia — can persist for decades without any abnormality in glucose readings (DeFronzo, Diabetes Care 2009; Petersen & Shulman, Physiol Rev 2018).

Blood sugar stays perfect, but chronically high insulin blocks lipolysis, causes fat accumulation around the waist, damages the vascular endothelium, promotes dry skin, acanthosis nigricans, and constant afternoon fatigue after carb-rich meals.

Why HbA1c and fasting glucose miss 60% of undiagnosed cases

Kraft (1975, later replicated by Crofts & Schofield, BMJ Open Diabetes 2016) showed that a 5-hour oral glucose tolerance test with insulin measurement detects insulin resistance in about 75% of participants whose glucose is 'normal'. Fasting glucose and HbA1c only rise once beta-cell reserve has been exhausted.

• Fasting glucose < 5.6 mmol/L ≠ healthy metabolism
• HbA1c < 5.7% does not rule out hyperinsulinaemia
• Insulin typically rises 5–10 years before glucose
• Visceral fat can accumulate in normal-weight people (TOFI — thin outside, fat inside)

Diagnosis: HOMA-IR and fasting insulin

Glucose alone is not enough. Fasting insulin must be measured and the HOMA-IR index calculated: (glucose mmol/L × insulin mU/L) / 22.5. Other valuable markers include the triglyceride/HDL ratio (> 1.5 suggests insulin resistance), fasting C-peptide and waist circumference.

• HOMA-IR optimal functional level: < 1.0
• HOMA-IR 1.0–1.9: subclinical resistance, lifestyle intervention needed
• HOMA-IR > 1.9: active insulin resistance
• Fasting insulin > 10 mU/L: warning signal

"Insulin is the hormone that stores energy. Chronically high insulin means the body is locked in storage mode — even when you are not eating."

The 5 main root causes of insulin resistance

• Visceral fat — releases inflammatory adipokines that block insulin signalling
• Chronic eating window > 14 h/day — beta cells get no rest
• Non-alcoholic fatty liver disease (NAFLD) — about 25% of adults per Younossi et al., Hepatology 2019
• Chronic sleep debt — a single bad night reduces insulin sensitivity ~25% (Donga et al., JCEM 2010)
• Inactivity and low muscle mass — fewer GLUT4 transporters in skeletal muscle

Practical reversal protocol

• Eating window 10–12 h, last meal at least 3 h before sleep
• Carbs mainly during active periods, not in the evening
• 10–15 min walk after each main meal — Reynolds, Diabetologia 2016
• Strength training 2–3× per week, even short sessions work
• Magnesium (glycinate 200–400 mg) and vitamin D if serum < 75 nmol/L
• Polyphenol-rich Mediterranean pattern: olive oil, berries, fish, legumes

What to avoid during the reversal phase

• Liquid carbs (juice, sweet drinks, smoothies on an empty stomach)
• Frequent snacking — every 'little snack' is an insulin stimulus
• Late dinner after 21:00 (Morris et al., JCEM 2015 — evening glycemic response ~17% stronger)
• Chronic alcohol — burdens the liver and disrupts gluconeogenesis

Clinical bottom line

Insulin resistance is fully reversible in most cases if caught early and the right markers are used. A normal fasting glucose is not a certificate of health — it is a starting point. Look deeper.

When to consider medication — metformin, GLP-1s and the bariatric route

Lifestyle comes first, but not everyone responds fast enough. If HOMA-IR stays above 2.5 after 6 months of structured intervention, or HbA1c > 6.0%, it is reasonable to discuss pharmacological support. Metformin lowers hepatic gluconeogenesis and improves insulin sensitivity ~15–25% (UKPDS 34, Lancet 1998). GLP-1 agonists (semaglutide, tirzepatide) add another 10–20% weight loss and reduce cardiovascular events (SUSTAIN-6, NEJM 2016; SURMOUNT-1, NEJM 2022).

Important: medication is not a substitute for diet and movement — it opens a window in which lifestyle change can consolidate. Without new habits the effect fades once the drug is stopped.

Common mistakes in managing insulin resistance

• Cutting carbs to zero without compensating with protein and fiber — quick win, muscle loss
• Trusting 'diabetic' products — sugar alcohols (maltitol, sorbitol) still stimulate insulin
• Tracking only fasting glucose without insulin — misses 5–10 years of the compensatory phase
• Ignoring chronic stress — cortisol keeps gluconeogenesis running even during a fast

CGM (continuous glucose monitor) — when it makes sense without diabetes

A 14-day CGM (Freestyle Libre, Dexcom) gives a personalized picture of how your body responds to specific meals, sleep, training and stress. Zeevi et al. (Cell 2015) showed that the same meal can produce up to a 4× different glucose response between individuals. It does not diagnose diabetes, but it provides behavioral feedback no lab panel can replace.