Overview.
Type 2 diabetes mellitus is the most common metabolic disease, affecting approximately 10% of US adults with an additional 38% in the prediabetes range. It develops from progressive insulin resistance followed by beta-cell failure. Critically, the metabolic dysfunction begins 5-15 years before diagnostic thresholds are crossed — fasting insulin and HOMA-IR detect this early phase when lifestyle intervention is most effective. HbA1c is the cornerstone monitoring marker, reflecting 90-day average glucose control.
T2DM results from the intersection of insulin resistance (tissues fail to respond normally to insulin) and progressive pancreatic beta-cell dysfunction (the pancreas can no longer compensate by producing more insulin). Unlike Type 1 diabetes (autoimmune beta-cell destruction), T2DM develops gradually and is strongly linked to obesity, physical inactivity, and genetic predisposition. The hyperglycaemia damages blood vessels, nerves, kidneys, and eyes over years to decades.
Prevalence: ~10% of US adults (~37 million). ~38% have prediabetes (~96 million) — 80% of whom are unaware. Global prevalence is rising rapidly. Risk doubles with every decade of life after 40. Disproportionately affects African American, Hispanic, Native American, and South Asian populations.
Medical name: Type 2 Diabetes Mellitus (T2DM)
Symptoms.
Early warnings
- HbA1c rising from 5.0% toward 5.7% over successive tests (prediabetes trajectory)
- Fasting insulin elevated (>10 μIU/mL) with normal glucose (hidden insulin resistance)
- HOMA-IR >2.5 (the earliest detectable metabolic abnormality)
- Post-meal fatigue ('food coma') and energy crashes
- Increasing waist circumference despite stable weight
- Acanthosis nigricans developing in neck folds or armpits
Classic symptoms
- Many patients are ASYMPTOMATIC at diagnosis (found on screening)
- Polyuria (frequent urination)
- Polydipsia (excessive thirst)
- Unexplained weight loss (despite increased appetite)
- Fatigue and low energy
- Blurred vision (lens swelling from hyperglycaemia)
- Slow wound healing
- Recurrent infections (UTIs, candidiasis, skin infections)
- Numbness or tingling in hands/feet (early neuropathy)
- Erectile dysfunction
Progression
Insulin resistance → compensatory hyperinsulinaemia → beta-cell exhaustion → impaired fasting glucose → impaired glucose tolerance → overt diabetes. This trajectory spans 5-15 years. At diagnosis, ~50% of beta-cell function is already lost. Fasting insulin and HOMA-IR detect the earliest phase; fasting glucose catches the middle; HbA1c ≥6.5% confirms the endpoint. Complications (nephropathy, retinopathy, neuropathy, CVD) develop after years of poor control.
Risk factors.
- Overweight or obesity (BMI ≥25, or ≥23 in Asian populations)
- Physical inactivity
- Family history (first-degree relative with T2DM)
- Age ≥45 (risk increases with each decade)
- Prediabetes (HbA1c 5.7-6.4%)
- Gestational diabetes history
- PCOS
- African American, Hispanic, Native American, Asian, Pacific Islander descent
- Hypertension (≥140/90 mmHg)
- Dyslipidaemia (HDL <35 mg/dL or triglycerides >250 mg/dL)
- Acanthosis nigricans (dark, velvety skin patches — sign of insulin resistance)
- History of cardiovascular disease
Lab interpretation.
Key biomarkers
- HbA1c — ≥6.5% = diabetes; 5.7-6.4% = prediabetes; <5.7% = normal. Optimal longevity target: <5.3% (primary)
- Fasting Glucose — ≥126 mg/dL = diabetes; 100-125 = prediabetes; <100 = normal. Less sensitive than HbA1c for early detection. (primary)
- Fasting Insulin / HOMA-IR — Elevated insulin with normal glucose = compensated insulin resistance — the EARLIEST detectable stage. HOMA-IR >2.5 = significant resistance. (primary)
- Triglycerides — Often elevated; triglyceride:HDL ratio >3:1 is a strong surrogate for insulin resistance (secondary)
- eGFR / UACR — Annual screening for diabetic nephropathy; declining eGFR and rising UACR indicate kidney damage (secondary)
Diagnostic criteria
- HbA1c ≥6.5% (confirmed on repeat testing unless unequivocal hyperglycaemia with symptoms)
- Fasting plasma glucose ≥126 mg/dL (8-hour fast)
- 2-hour OGTT ≥200 mg/dL (75g glucose load)
- Random glucose ≥200 mg/dL with classic symptoms (polyuria, polydipsia, weight loss)
- Prediabetes: HbA1c 5.7-6.4%, FPG 100-125 mg/dL, or 2-hr OGTT 140-199 mg/dL
- ADA recommends screening all adults ≥35 (or younger with BMI ≥25 + risk factor) every 3 years
Recommended panels
When & next steps.
When to test
- Age ≥35 (ADA 2024 screening recommendation)
- BMI ≥25 with any risk factor (at any age)
- Prediabetes on prior testing (annual retesting)
- Gestational diabetes history (lifelong screening every 3 years)
- PCOS (high T2DM risk regardless of BMI)
- Symptoms: polyuria, polydipsia, unexplained weight loss, recurrent infections
- Acanthosis nigricans on examination
- Family history of T2DM in first-degree relative
If suspected
- HbA1c (preferred single screening test — no fasting needed)
- Fasting glucose if HbA1c is borderline (5.7-6.4%)
- Fasting insulin + glucose for HOMA-IR calculation (earliest detection)
- Lipid panel (dyslipidaemia frequently coexists)
- Confirm abnormal results on repeat testing before diagnosing
If confirmed
- Lifestyle intervention FIRST: 7% weight loss + 150 min/week moderate exercise reduces progression by 58% (DPP trial)
- Metformin: first-line pharmacotherapy (after or alongside lifestyle changes)
- SGLT2 inhibitors or GLP-1 agonists: preferred add-on for cardiovascular or kidney benefit
- HbA1c target: <7.0% for most adults (individualise: <6.5% if safely achievable; <8.0% in elderly/frail)
- Annual complication screening: eGFR + UACR (nephropathy), dilated eye exam (retinopathy), foot exam (neuropathy), lipid panel (CVD risk)
- Blood pressure target: <130/80 mmHg
- Statin therapy: nearly all T2DM patients ≥40 benefit from moderate-intensity statin
FAQs.
What's the difference between prediabetes and diabetes?
Prediabetes (HbA1c 5.7-6.4%) means glucose regulation is impaired but not yet at diabetic levels. Approximately 5-10% of prediabetics progress to diabetes per year without intervention. With lifestyle changes (7% weight loss + exercise), progression is reduced by 58%. Prediabetes is the actionable window — it's not 'a little bit of diabetes', it's the warning that prevention still works.
Why should I test fasting insulin if my glucose is normal?
Because insulin rises YEARS before glucose does. Your pancreas compensates for insulin resistance by producing more insulin, keeping glucose normal. By the time glucose rises, beta-cell function is already significantly impaired (~50% lost). Fasting insulin and HOMA-IR catch the problem at the compensation stage — when lifestyle intervention is most effective.
Can Type 2 diabetes be reversed?
In early stages, yes. Significant weight loss (10-15%+ of body weight), especially through dietary change and exercise, can normalise HbA1c and fasting glucose — termed 'remission'. The DiRECT trial showed 46% remission at 1 year with intensive weight management. The earlier you intervene, the more likely remission is sustainable. After years of poor control, beta-cell damage becomes irreversible.
How often should HbA1c be checked?
Every 3 months if above target or after medication changes. Every 6 months if stable and at target. For prediabetes monitoring: annually. HbA1c reflects 90-day average glucose, so testing more frequently than quarterly doesn't add information.