Overview
Obesity is a complex, chronic condition characterised by excess body fat that impairs health. It is defined as a body mass index (BMI) of 30 or above. Obesity substantially increases the risk of type 2 diabetes, cardiovascular disease, several cancers, obstructive sleep apnoea, osteoarthritis, and depression. Modern understanding recognises it as a physiological condition driven by biology, not simply a lack of willpower.
How common is it?
Around 28% of adults in England are obese and a further 36% are overweight. Obesity rates have roughly doubled since 1990. It is now one of the leading preventable causes of disease and death globally.
Causes and risk factors
Obesity results from a chronic positive energy balance driven by a complex interaction of biological, environmental, psychological, and genetic factors.
Common risk factors
- Genetic factors (estimated 40 to 70% heritability for BMI)
- Obesogenic environment: energy-dense food, reduced physical activity, sedentary work and leisure
- Hormonal dysregulation: leptin resistance, altered ghrelin and GLP-1 signalling
- Sleep deprivation (increases ghrelin, reduces leptin)
- Gut microbiome alterations
- Psychological factors: depression, emotional eating, trauma
- Medications: corticosteroids, antipsychotics, insulin, antiepileptics
- Medical conditions: hypothyroidism, Cushing syndrome, polycystic ovary syndrome (rare causes)
- Socioeconomic deprivation (strong inverse relationship)
Symptoms
- Weight above healthy range (BMI 30+)
- Fatigue and reduced exercise tolerance
- Breathlessness on exertion
- Joint pain (especially knees and hips)
- Sleep disturbance or snoring
- Psychological: low self-esteem, depression, social stigma
When to see a doctor
Weight management should be addressed at any BMI above 30 (or above 27.5 for South Asian populations). Medical review is essential for associated conditions (diabetes, hypertension, sleep apnoea) and before starting pharmacotherapy.
Diagnosis
BMI is the primary screening tool. Waist circumference adds cardiovascular risk information (high risk above 94cm in men, 80cm in women). Investigations for metabolic consequences: fasting glucose, HbA1c, lipids, liver function, thyroid function.
Treatments
Structured lifestyle intervention
Intensive dietary and physical activity programmes with behaviour change support. Caloric deficit of 500 to 750 kcal/day produces 5 to 10% weight loss, which has substantial metabolic benefits. NHS low-calorie diet programmes (800 kcal/day, 12 weeks) produce average 10kg weight loss.
GLP-1 receptor agonists (semaglutide, liraglutide)
Semaglutide 2.4mg weekly (Wegovy) produces average 15% body weight loss — the most effective pharmacotherapy to date. Tirzepatide (dual GLP-1/GIP agonist) achieves 20 to 22% in trials. Licensed for BMI above 30 (or 27.5 with comorbidity) alongside lifestyle intervention.
Bariatric surgery
Roux-en-Y gastric bypass and sleeve gastrectomy produce 25 to 35% weight loss with high rates of type 2 diabetes remission. Recommended for BMI above 40 (or above 35 with significant comorbidity) after failure of other approaches.
Psychological support
Cognitive behavioural therapy, motivational interviewing, and treatment of underlying depression or binge eating disorder are integral to sustainable weight management.
Self-care and lifestyle
- Focus on dietary quality over calorie counting: reduce ultra-processed food, increase vegetables, fruit, pulses, and lean protein
- Reduce alcohol (calorically dense and impairs adherence)
- Aim for 150 minutes of moderate exercise plus 2 sessions resistance training weekly
- Prioritise 7 to 9 hours of sleep
Prevention
Population-level interventions addressing the food environment, sugar taxes, food labelling, and reduction of ultra-processed food marketing are the most effective prevention strategies. Individual prevention builds on healthy eating and physical activity habits from childhood.