What is Type 2 Diabetes?

Type 2 diabetes mellitus (T2DM) is a chronic metabolic condition in which the body either does not produce enough insulin or does not respond effectively to the insulin it does produce (insulin resistance). As a result, glucose accumulates in the bloodstream instead of being taken up by cells for energy, leading to hyperglycaemia (high blood sugar).

Unlike type 1 diabetes — which is an autoimmune condition causing near-complete loss of insulin-producing beta cells — type 2 is strongly linked to lifestyle factors, metabolic syndrome, and genetics, though the exact interplay is complex. It typically develops gradually in adulthood (though increasingly seen in children and young adults due to rising obesity rates) and may go undiagnosed for years because early hyperglycaemia is often asymptomatic.

Over time, chronic hyperglycaemia causes damage to blood vessels (both large — macrovascular — and small — microvascular) and nerves, leading to serious complications including cardiovascular disease, retinopathy, neuropathy, nephropathy, and foot ulceration.

Symptoms of Type 2 Diabetes

Type 2 diabetes often develops slowly and many people have no symptoms for years. Common presentations include:

• Polydipsia — excessive thirst
• Polyuria — frequent urination, including nocturia (passing urine at night)
• Fatigue — persistent tiredness and lack of energy
• Blurred vision — due to osmotic changes in the lens
• Increased hunger (polyphagia) — despite eating, cells cannot access glucose
• Slow wound healing — infections heal more slowly than normal
• Recurrent infections — particularly thrush (candidiasis), urinary tract infections, and skin infections
• Unintentional weight loss — less common in T2D than T1D; may indicate significant hyperglycaemia
• Tingling or numbness in hands or feet — an early sign of peripheral neuropathy
• Dark skin patches (acanthosis nigricans) — in skin folds (neck, armpits); associated with insulin resistance

Causes and Risk Factors

Pathophysiology

Type 2 diabetes develops through two interrelated processes: insulin resistance (peripheral tissues, especially skeletal muscle, liver, and adipose tissue, respond poorly to insulin signals) and progressive beta-cell failure (the pancreas initially compensates by secreting more insulin but gradually loses capacity over years). The liver also produces excess glucose via gluconeogenesis. Incretin hormone deficiency (GLP-1, GIP) and renal glucose reabsorption contribute to sustained hyperglycaemia.

Non-Modifiable Risk Factors

• Age: risk increases significantly after age 40 (though rates rising in younger people)
• Ethnicity: South Asian, Black African, Black Caribbean, and Chinese people have 2–6× higher risk than White European populations and often develop T2DM at lower BMI
• Family history: first-degree relative with type 2 diabetes increases lifetime risk by 2–3×
• History of gestational diabetes: 50% risk of developing T2DM within 10 years
• Polycystic ovary syndrome (PCOS): insulin resistance is central to PCOS pathophysiology

Modifiable Risk Factors

• Overweight and obesity: BMI ≥25 kg/m² significantly increases risk; visceral (abdominal) fat is particularly harmful
• Physical inactivity: sedentary behaviour impairs insulin sensitivity
• Unhealthy diet: high in refined carbohydrates, sugar-sweetened beverages, processed foods
• Smoking: increases insulin resistance by 30–40%
• Prediabetes: HbA1c 42–47 mmol/mol (6.0–6.4%) or impaired fasting glucose 6.1–6.9 mmol/L
• Hypertension: often co-exists and shares pathological mechanisms
• Dyslipidaemia: raised triglycerides and low HDL cholesterol are key metabolic syndrome features
• Sleep disorders: short sleep duration and sleep apnoea impair glucose metabolism

Diagnosis

Diagnostic Criteria (NICE NG28 / WHO 2023)

Diagnosis is based on blood tests. In asymptomatic individuals, two abnormal tests are required (from the same or different occasions). In symptomatic individuals, one result is sufficient.

Note: HbA1c should not be used for diagnosis in pregnancy, haemolytic anaemia, haemoglobinopathies, recent blood transfusion, or significant renal impairment — use glucose-based tests in these situations.

Investigations at Diagnosis

• HbA1c (baseline for monitoring)
• Renal function: eGFR, creatinine, electrolytes
• Urine ACR (albumin:creatinine ratio) — to detect diabetic nephropathy
• Lipid profile
• Blood pressure measurement
• Liver function tests (fatty liver common)
• QRISK3 cardiovascular risk score
• Thyroid function (if symptomatic or other indication)
• Referral for digital diabetic eye screening (retinal photograph)
• Foot examination including monofilament test, ankle reflexes, foot pulses

All newly diagnosed patients should be offered structured diabetes education — in the UK, the DESMOND programme (Diabetes Education and Self-Management for Ongoing and Newly Diagnosed) provides group-based education that improves HbA1c, weight, and quality of life.

Treatment and Medications

The goal of treatment is to reduce HbA1c to an agreed individualised target (typically 48–53 mmol/mol, or 6.5–7.0%), manage cardiovascular risk factors, prevent complications, and maintain quality of life. NICE NG28 uses a stepped approach.

HbA1c Targets

• With lifestyle or metformin alone: aim for HbA1c ≤48 mmol/mol (6.5%)
• With dual or triple therapy or any drug with hypoglycaemia risk: aim for HbA1c ≤53 mmol/mol (7.0%)
• Frail or elderly patients: individualised; preventing hypoglycaemia is paramount
• With established CVD or high CV risk: SGLT2 inhibitors or GLP-1 receptor agonists are recommended for CV benefit regardless of HbA1c

Step 1: Lifestyle and Metformin

Metformin (500 mg–2 g daily in divided doses with meals; modified-release preferred if GI intolerance) remains first-line for most patients. It reduces HbA1c by 1.0–1.5%, is weight-neutral or causes modest weight loss, has a robust cardiovascular safety record (UKPDS), and is inexpensive. Contraindicated if eGFR <30 mL/min; reduce dose if eGFR 30–45 mL/min; temporarily suspend during contrast procedures.

Step 2: Dual Therapy — Adding a Second Agent

If HbA1c is not controlled on metformin alone after 3 months, add one of the following (choice guided by CV risk, weight, hypoglycaemia risk, eGFR, cost):

• SGLT2 inhibitor (dapagliflozin, empagliflozin, canagliflozin) — first choice if established CVD, symptomatic heart failure (HFrEF), or CKD (eGFR ≥25 mL/min with ACR ≥22.6 mg/mmol). Reduces HbA1c by 0.5–1.0%, promotes weight loss (~2 kg), reduces blood pressure. Key risks: genital thrush, UTI, rare diabetic ketoacidosis (DKA — even with normal glucose), rare Fournier’s gangrene. Sick day rules apply — stop if acutely unwell or nil by mouth.
• GLP-1 receptor agonist (semaglutide [Ozempic/Rybelsus], liraglutide [Victoza], dulaglutide [Trulicity], exenatide) — preferred if BMI ≥35, or significant weight loss required, or established CVD/high CV risk (semaglutide and liraglutide have strongest CV evidence). Reduces HbA1c by 1.0–1.5%, promotes significant weight loss (3–5 kg with liraglutide; up to 6–10 kg with semaglutide). GI side effects (nausea, vomiting) are common, especially on initiation. Injectable except semaglutide oral (Rybelsus).
• DPP-4 inhibitor (sitagliptin, alogliptin, saxagliptin, linagliptin) — well-tolerated, weight-neutral, low hypoglycaemia risk. HbA1c reduction 0.5–0.8%. CV-neutral. Avoid saxagliptin in heart failure. Second-choice agents when SGLT2i/GLP-1 not suitable.
• Sulfonylurea (gliclazide MR preferred over glibenclamide) — low cost, effective (HbA1c ↓1.0–1.5%), but causes hypoglycaemia and modest weight gain (~2 kg). NICE recommends as an option if cost is a priority or other drugs contraindicated. Gliclazide MR 30–120 mg daily.
• Pioglitazone (thiazolidinedione) — improves insulin sensitivity; useful in NASH/NAFLD and PCOS. Causes fluid retention, weight gain, increased fracture risk, and is contraindicated in heart failure. Bladder cancer risk is low but was a historical concern. Rarely used as second-line now.

Step 3: Triple Therapy

If dual therapy is insufficient:

• Metformin + SGLT2i + GLP-1 agonist (if BMI ≥35 or CV risk)
• Metformin + SGLT2i + DPP-4i (if GLP-1 not suitable)
• Metformin + sulfonylurea + SGLT2i, GLP-1, DPP-4i, or pioglitazone

Step 4: Insulin Therapy

Insulin is initiated when HbA1c remains above target on triple therapy, or when type 2 diabetes presents acutely with significant hyperglycaemia. NICE recommends starting with once-daily basal insulin (e.g., insulin glargine [Lantus/Toujeo] or insulin detemir [Levemir]) as the preferred approach, added to existing oral agents (except sulfonylureas, which are usually stopped). Alternatives include twice-daily premix insulin (e.g., NovoMix 30) in those requiring postprandial control. Basal-bolus regimens (multiple daily injections) are used if premix or basal insulin alone is insufficient.

Cardiovascular Risk Management

• Statin therapy: atorvastatin 10 mg for primary prevention in T2DM (if ≥40 years, or <40 with other risk factors); atorvastatin 20–80 mg for secondary prevention
• Antihypertensive therapy: target BP <140/90 mmHg (<130/80 mmHg if CKD with proteinuria); ACEi or ARB first-line
• Antiplatelet therapy: aspirin 75 mg for secondary prevention only (not primary prevention in T2DM)

Type 2 Diabetes Remission

Diabetes remission is defined as HbA1c <48 mmol/mol (6.5%) without glucose-lowering medication for at least 3 months. Evidence shows that substantial weight loss — particularly in the early years after diagnosis — can achieve remission in a meaningful proportion of people.

The landmark DiRECT trial (Diabetes Remission Clinical Trial, UK 2018, Prof Roy Taylor and Mike Lean) showed that an intensive dietary programme (total diet replacement with ~800 kcal/day for 12 weeks, followed by stepped food reintroduction and long-term support) achieved remission in 46% of participants at 1 year and 36% at 2 years. Greater weight loss correlated strongly with higher remission rates.

NICE now recommends offering a low-calorie diet programme (total diet replacement with 800–900 kcal/day for up to 12 weeks, in people with BMI ≥27 kg/m² who have had diabetes for <6 years) as a pathway to remission. The NHS Low Calorie Diet Programme pilots this in England.

Bariatric surgery (gastric bypass, sleeve gastrectomy) achieves remission in 50–80% of patients and is the most effective intervention for long-term remission. NICE recommends considering it in adults with T2DM and BMI ≥35 kg/m², or BMI 30–34.9 in those of Asian origin.

Monitoring Type 2 Diabetes

HbA1c

HbA1c should be checked every 3–6 months until stable, then at least annually. It reflects average blood glucose over the preceding 2–3 months. Each 1% (11 mmol/mol) reduction in HbA1c reduces microvascular complications by ~40%.

Annual Diabetes Review (NICE NG28)

• HbA1c, renal function, lipids
• Urine ACR — nephropathy screening
• Blood pressure
• Weight and BMI
• Foot examination (monofilament, pulses, inspection)
• Digital retinal photography (diabetic eye screening) — annually via NHS programme
• Review of medications and side effects
• Smoking status, alcohol intake, mental health (depression screening with PHQ-2/PHQ-9)
• Immunisations: annual flu vaccine, COVID booster, one-off pneumococcal vaccine

Blood Glucose Self-Monitoring

NICE does not routinely recommend self-monitoring of blood glucose (SMBG) for people with T2DM on diet or metformin alone. It is recommended for:

• Those on insulin or sulfonylureas (hypoglycaemia risk)
• Those who drive and are on hypoglycaemia-risk medications (DVLA requirement)
• During illness (sick day rules)
• During periods of significant treatment change

Flash Glucose Monitoring (FreeStyle Libre)

NICE technology appraisal TA943 (2023) recommends the FreeStyle Libre 2 flash glucose monitor on the NHS for adults with type 2 diabetes on insulin who require frequent blood glucose monitoring (>8 tests/day), or who have problematic hypoglycaemia. This replaces finger-prick testing for eligible patients.

Complications of Type 2 Diabetes

Microvascular Complications

• Diabetic retinopathy: the leading cause of preventable sight loss in working-age adults in the UK. Background retinopathy is common; proliferative retinopathy and diabetic macular oedema (DMO) require treatment (laser, intravitreal anti-VEGF injections). Annual retinal screening via the NHS Diabetic Eye Screening Programme detects early changes.
• Diabetic nephropathy: begins as microalbuminuria (ACR 3–30 mg/mmol) and can progress to macroalbuminuria and CKD. ACEi/ARBs are renoprotective. SGLT2 inhibitors (especially dapagliflozin — DAPA-CKD trial) significantly slow CKD progression.
• Diabetic peripheral neuropathy: distal symmetric polyneuropathy causes burning, tingling, and numbness in the feet and hands (“glove and stocking” distribution). Managed with pregabalin, duloxetine, or amitriptyline for neuropathic pain. Meticulous foot care is essential.
• Autonomic neuropathy: gastroparesis (delayed gastric emptying), postural hypotension, erectile dysfunction, bladder dysfunction, and abnormal sweating.

Macrovascular Complications

• Coronary artery disease: myocardial infarction risk is 2–3× higher than in non-diabetic individuals. “Silent” MI is more common due to autonomic neuropathy masking chest pain.
• Stroke and TIA: 2–4× increased risk of ischaemic stroke
• Peripheral arterial disease: claudication, critical ischaemia; combined with neuropathy creates the diabetic foot

Diabetic Foot Disease

Foot ulceration affects 10% of people with diabetes over their lifetime and is the leading cause of non-traumatic lower limb amputation in the UK. NICE NG19 mandates multidisciplinary diabetic foot teams. High-risk feet (neuropathy, deformity, ischaemia, previous ulcer/amputation) should be referred to specialist podiatry. Any diabetic foot ulcer must be assessed urgently — refer to MDT within 1 working day.

Prevention of Type 2 Diabetes

Type 2 diabetes is largely preventable. For people identified with prediabetes (HbA1c 42–47 mmol/mol or impaired fasting glucose):

• NHS Diabetes Prevention Programme (NDPP): 9-month structured group programme covering diet, exercise, and behaviour change. Reduces T2DM incidence by ~40%. Referred by GP following HbA1c or glucose in the prediabetes range.
• Weight loss: 5% body weight reduction reduces diabetes risk by ~50%. ≥7% weight loss reduces risk by 58% (Diabetes Prevention Program, NEJM 2002).
• Physical activity: ≥150 minutes/week of moderate aerobic exercise; resistance training twice per week.
• Diet: Mediterranean or low-glycaemic index diet; limit ultra-processed foods, sugar-sweetened beverages, and refined carbohydrates; increase fibre intake.
• Metformin: NICE does not routinely recommend metformin for prediabetes prevention in the UK (unlike the US), though it may be considered in selected high-risk individuals.