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This is a clinician-written, evidence-based summary aligned to the 2026 MLA Content Map. It is intended for medical students and junior doctors preparing for the UKMLA. Always cross-reference with NICE guidance, local protocols, and clinical judgement.
The Bottom Line
- BMI classification: overweight 25–29.9, obese I 30–34.9, obese II 35–39.9, obese III ≥40. Lower thresholds for South Asian/Black populations (≥27.5)
- Waist circumference adds metabolic risk assessment: >80 cm (women) or >94 cm (men) = increased risk
- Management: tiered approach — lifestyle (diet + activity + behavioural), pharmacological (GLP-1 RAs, orlistat), and surgical (bariatric)
- GLP-1 receptor agonists (semaglutide 2.4 mg weekly) and dual GIP/GLP-1 agonists (tirzepatide): 15–20%+ weight loss
- Bariatric surgery: consider if BMI ≥40, or ≥35 with comorbidity, or ≥30 with recent T2DM (NICE). Roux-en-Y or sleeve gastrectomy
- Person-centred approach: avoid weight stigma, treat as chronic disease, address psychological and social factors (NICE NG246)
Overview
Obesity is now recognised as a chronic, relapsing disease characterised by excess adiposity that impairs health. It is defined by a BMI ≥30 kg/m² in most populations, with lower thresholds for South Asian, Chinese, and Black populations (≥27.5 kg/m²). Waist circumference is an important adjunct — central (visceral) adiposity confers greater metabolic risk than peripheral fat distribution. The pathophysiology involves complex interactions between genetic susceptibility, epigenetics, neuroendocrine regulation of appetite and energy expenditure, environmental factors (obesogenic food environment, sedentary lifestyle), psychological factors, and socioeconomic deprivation. Obesity is associated with over 200 comorbidities including T2DM, cardiovascular disease, several cancers, osteoarthritis, NAFLD, obstructive sleep apnoea, and mental health disorders. NICE NG246 (2024) provides the current UK framework.
Epidemiology
Approximately 26% of UK adults are obese (BMI ≥30) and a further 38% are overweight. Childhood obesity affects approximately 10% of reception-age and 23% of Year 6 children. Prevalence is higher in socially deprived areas, and in Black and South Asian populations. Obesity reduces life expectancy by 2–4 years (BMI 30–35) or 8–10 years (BMI 40–45). It is the second largest preventable cause of cancer after smoking. The economic burden on the NHS is estimated at >£6 billion annually.
Clinical Features
Symptoms
Weight gain — often gradual and insidious
Reduced exercise tolerance, breathlessness
Joint pain (weight-bearing joints — knees, hips, lower back)
Snoring, daytime somnolence (OSA)
Psychological: low mood, reduced self-esteem, body dissatisfaction, eating disorders
Symptoms of complications: polyuria/polydipsia (T2DM), GORD, urinary incontinence
Signs
BMI ≥30 kg/m² (≥27.5 in South Asian/Black populations)
Increased waist circumference: >80 cm (women), >94 cm (men). Very high: >88 cm (women), >102 cm (men)
Acanthosis nigricans (insulin resistance marker)
Intertrigo (skin fold infection/inflammation)
Signs of complications: hypertension, hepatomegaly (NAFLD), varicose veins
Investigations
First-line
BMIHeight and weight. Overweight 25–29.9, obese I 30–34.9, obese II 35–39.9, obese III ≥40. Use ethnic-specific thresholds
Waist circumferenceMeasured at midpoint between lower rib margin and iliac crest. Assesses central adiposity and metabolic risk
Second-line
HbA1c or fasting glucoseScreen for T2DM/pre-diabetes
Lipid profileDyslipidaemia (raised TG, low HDL, raised LDL)
LFTsScreen for NAFLD (raised ALT/GGT)
TFTsExclude hypothyroidism as contributory factor (rare as sole cause)
BP measurementHypertension screening
Specialist
Cortisol (overnight DST)If Cushing's clinically suspected (but obesity alone is NOT an indication for Cushing's screening)
Sleep studyIf symptoms of OSA (snoring, witnessed apnoeas, daytime somnolence, Epworth score ≥10)
1
Tier 1–2: Lifestyle interventions (all patients)
- Individualised healthy eating plan (no single diet is superior). Calorie deficit for weight loss
- Physical activity: ≥150 min/week moderate-intensity (build up gradually)
- Behavioural support: self-monitoring, stimulus control, problem-solving, goal-setting
- Address psychological factors: screen for depression, disordered eating, emotional eating
- Person-centred approach: minimise weight stigma, shared decision-making. Treat obesity as a chronic disease
2
Tier 3: Pharmacotherapy (if lifestyle insufficient)
- GLP-1 receptor agonists: semaglutide 2.4 mg weekly SC (Wegovy) — ~15% weight loss. Tirzepatide (dual GIP/GLP-1) — up to 20%+
- Orlistat 120 mg TDS (pancreatic lipase inhibitor): ~5% weight loss. GI side effects (steatorrhoea). Continue only if ≥5% weight loss at 3 months
- Naltrexone/bupropion combination: 5–6% weight loss
- Anti-obesity medications should be used alongside lifestyle measures, not as standalone
3
Tier 4: Bariatric surgery
- Consider if: BMI ≥40, or ≥35 with significant comorbidity (T2DM, OSA, hypertension), or ≥30 with recent-onset T2DM
- Expedited assessment if BMI ≥35 + recent T2DM (within 10 years)
- Roux-en-Y gastric bypass (RYGB): gold standard, ~30% sustained weight loss
- Sleeve gastrectomy: ~25% weight loss, fewer nutritional deficiencies than RYGB
- Adjustable gastric band: less effective, declining use
- Post-operative: lifelong nutritional monitoring (B12, iron, calcium, vitamin D, folate), dietary support
- Bariatric surgery achieves T2DM remission in 50–80%
Complications
- Type 2 diabetes: Obesity is the strongest modifiable risk factor — 80–85% of T2DM risk is attributable to obesity
- Cardiovascular disease: Hypertension, coronary artery disease, heart failure, stroke, AF
- Cancer: Increased risk of ≥13 cancers including colorectal, breast (post-menopausal), endometrial, oesophageal, pancreatic, renal
- NAFLD/NASH: Up to 80% of obese individuals have some degree of fatty liver
- Obstructive sleep apnoea: ~70% of OSA patients are obese
- Osteoarthritis: Weight-bearing joints — knee OA is 6× more common in obese
- Mental health: Depression, anxiety, reduced quality of life, weight stigma, eating disorders
- Reduced life expectancy: BMI 30–35 reduces by 2–4 years; BMI 40–45 by 8–10 years
UKMLA Exam Tips
- 1BMI ≥30 = obese. But use ≥27.5 for South Asian/Black populations (lower threshold for equivalent metabolic risk)
- 2Waist circumference is MORE predictive of metabolic risk than BMI alone — central adiposity matters most
- 3Bariatric surgery criteria: BMI ≥40, or ≥35 + comorbidity, or ≥30 + recent T2DM. Know the thresholds
- 4GLP-1 RAs (semaglutide): significant weight loss + cardiovascular benefit — increasingly important
- 5Orlistat: continue only if ≥5% weight loss at 3 months. GI side effects common (steatorrhoea)
- 6Obesity is a DISEASE, not a lifestyle choice. Genetic, neuroendocrine, and environmental factors all contribute
- 7Post-bariatric surgery: lifelong monitoring for nutritional deficiencies (B12, iron, Ca, vitamin D)
practicetest your knowledge on obesityApply what you've learnt with UKMLA-style questions from the iatroX Q-Bank — endocrine and beyond.
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