Obesity — Causes, BMI Classification, Bariatric Surgery & GLP-1 Treatment Guide — Symptoms, Causes & Treatment | MyMedicPlus

Obesity is a chronic, relapsing, progressive disease characterised by abnormal or excessive adipose tissue accumulation that impairs health. It is defined by body mass index (BMI) — weight in kg divided by height in metres squared — with WHO classifications: BMI 25–29.9 kg/m2: overweight; 30–34.9: class I obesity; 35–39.9: class II obesity; above 40: class III (morbid) obesity. BMI-based definitions have limitations — waist circumference (above 102 cm in men and above 88 cm in women) and waist-to-hip ratio better reflect visceral adiposity and metabolic risk. Obesity affects 650 million adults globally — representing 13% of the world's adult population — with rates continuing to rise. It is causally linked to over 200 comorbidities including type 2 diabetes, hypertension, coronary heart disease, heart failure, obstructive sleep apnoea, non-alcoholic fatty liver disease, 13 types of cancer, osteoarthritis, polycystic ovarian syndrome, and depression. Novel pharmacological advances (GLP-1/GIP receptor agonists achieving 15–22% weight loss) and bariatric surgery have transformed the treatment landscape.

Obesity arises from sustained energy imbalance — caloric intake chronically exceeding energy expenditure — in the context of complex biological, environmental, psychological, and genetic factors. Genetic factors: heritability of BMI is estimated at 40–70%; rare single-gene causes (monogenic obesity) include MC4R (melanocortin-4 receptor) mutations, leptin deficiency, and LEP/LEPR mutations — together accounting for 5–10% of severe early-onset obesity; polygenic risk scores integrate hundreds of common variants. Hormonal and metabolic factors: insulin resistance; hypothyroidism (mildly increases weight); Cushing's syndrome (cortisol excess — truncal obesity, striae, hypertension); hypothalamic injury (tumour, radiation, surgery — causes hyperphagia); polycystic ovarian syndrome (PCOS). Environmental factors: ultra-processed food environment (high-calorie, low-nutrient, hyper-palatable foods — constitute 57% of energy intake in the UK); reduced physical activity and sedentary work; sleep deprivation (increases ghrelin, decreases leptin — raises appetite); socioeconomic deprivation (strong inverse association between deprivation and healthy diet access); obesogenic prenatal environment (gestational diabetes, maternal obesity — increases offspring obesity risk). Medications causing weight gain: olanzapine and other antipsychotics; insulin; sulfonylureas; lithium; valproate; depot progesterone contraception; corticosteroids; beta-blockers; tricyclic antidepressants.

Obesity causes both direct physical effects and consequences of associated comorbidities. Direct effects: exertional dyspnoea (breathlessness on mild exertion or minimal activity); excessive sweating (diaphoresis); musculoskeletal pain — knee, hip, and back pain from mechanical loading; fatigue and reduced exercise tolerance; skin fold infections (intertrigo — fungal and bacterial infections in skin folds); urinary stress incontinence; gastro-oesophageal reflux. Signs of associated comorbidities: hypertension (measured with appropriate large cuff); acanthosis nigricans (dark velvety skin in neck, axillae, groin — marker of insulin resistance); HOMA-IR elevated (hyperinsulinaemia); polycystic ovarian syndrome features (hirsutism, acne, oligomenorrhoea); snoring and apnoeic episodes (obstructive sleep apnoea — Epworth sleepiness scale elevated); cardiovascular signs (raised JVP, peripheral oedema — heart failure). Mental health: depression (bidirectional relationship with obesity — each worsens the other); anxiety; poor self-esteem; binge eating disorder (present in 30% of people with severe obesity seeking bariatric surgery).

BMI calculation (weight kg / height m2): primary screening tool but does not assess fat distribution or metabolic risk. Waist circumference: male above 102 cm (very high risk); female above 88 cm (very high risk) — better predictor of visceral adiposity and cardiovascular risk than BMI alone; particularly important in South Asian individuals (lower thresholds: male above 90 cm, female above 80 cm). Secondary causes investigation: thyroid function tests (TSH, fT4); fasting glucose and HbA1c (type 2 diabetes, pre-diabetes); lipid profile (total cholesterol, LDL, HDL, triglycerides); liver function tests (NAFLD — transaminitis, low albumin in MASH); cortisol (urinary free cortisol or overnight dexamethasone suppression test if Cushing's suspected); sex hormone assessment (PCOS — testosterone, SHBG, LH/FSH ratio); 24-hour ambulatory blood pressure. Sleep study (polysomnography or limited sleep study): for suspected obstructive sleep apnoea — indicated in obesity with excessive daytime sleepiness, apnoeic episodes, or snoring. Assessment for bariatric surgery: comprehensive MDT assessment including psychiatry (eating disorders), dietetics, anaesthetics (airway and cardiovascular risk), and surgical team.

Behavioural weight management: intensive behavioural intervention combining reduced calorie diet (500–600 kcal/day deficit), increased physical activity (150–300 minutes moderate activity per week), and behavioural strategies (cognitive behavioural therapy, motivational interviewing) — typically achieves 5–10% weight loss; effective for reducing type 2 diabetes risk. Very low calorie diets (VLCDs — under 800 kcal/day): initial 12-week VLCD using meal replacement products can achieve 10–15 kg weight loss — as used in the DiRECT trial (achieving T2DM remission in 46% at 12 months). GLP-1 receptor agonists: semaglutide (Wegovy) 2.4 mg weekly subcutaneous injection — achieves 12–17% mean weight loss (STEP trials); liraglutide (Saxenda) 3 mg daily; dual GLP-1/GIP agonist tirzepatide (Mounjaro) 15 mg weekly — achieves 20–22% mean weight loss (SURMOUNT trials); both medications reduce cardiovascular events in people with obesity and CVD (semaglutide: SELECT trial — 20% CV event reduction). Side effects: nausea, vomiting, diarrhoea (usually transient); pancreatitis (rare). Orlistat 120 mg TDS (lipase inhibitor): modest weight loss (3–5 kg vs placebo) — reduces fat absorption; GI side effects (steatorrhoea) limit adherence. Bariatric surgery: most effective long-term intervention for severe obesity — indicated for BMI above 40, or above 35 with significant comorbidities (T2DM, OSA, hypertension) after failure of non-surgical management. Roux-en-Y gastric bypass (RYGB): gold standard — 60–80% excess weight loss; 75% T2DM remission at 2 years; reduces cancer risk, cardiovascular mortality. Sleeve gastrectomy: 50–70% excess weight loss; simpler procedure; lower complication rate; 60% T2DM remission. Adjustable gastric band: now rarely performed (high re-operation rate). Duodenal switch: greatest weight loss (80–85% EWL) and metabolic benefit — used for super-obesity (BMI above 50).

Obesity causes complications across multiple organ systems, substantially reducing life expectancy and quality of life. Type 2 diabetes: BMI above 30 confers a 7-fold increased risk of T2DM; severe obesity (BMI above 40) increases risk 60-fold compared to normal weight — adipocyte-derived inflammatory cytokines (TNF-alpha, IL-6) and ectopic fat deposition in the liver and muscle drive severe insulin resistance and progressive beta-cell failure. Cardiovascular disease: hypertension is 3 times more prevalent in obesity; coronary artery disease risk is doubled; heart failure and atrial fibrillation each occur 2-fold more commonly. Obstructive sleep apnoea (OSA): affects 45% of individuals with BMI above 35 — untreated OSA causes excessive daytime somnolence, cognitive impairment, and substantially increases cardiovascular mortality. Non-alcoholic/metabolic-associated fatty liver disease (MASLD): affects 90% of those with morbid obesity; 20% progress to steatohepatitis (MASH), with risk of cirrhosis and hepatocellular carcinoma. Musculoskeletal: knee osteoarthritis risk is 2–4 times higher (every 5 kg weight gain increases knee OA risk by approximately 35%); chronic low back pain, plantar fasciitis, and joint damage accelerate with increasing BMI. Cancer: obesity is linked to 13 cancer types — endometrial, oesophageal, renal, thyroid, liver, pancreatic, colorectal, postmenopausal breast, and gallbladder among others — accounting for 4–8% of all cancer burden. Reproductive complications: PCOS, infertility, gestational diabetes, pre-eclampsia, and increased caesarean section rates all occur at significantly higher rates. Psychological: depression, anxiety, body image distress, and social stigma are highly prevalent and bidirectionally linked with obesity severity.

Population-level interventions are essential given the pervasive obesogenic environment: taxation on sugar-sweetened beverages (UK Soft Drinks Industry Levy reduced sugar content by 28.8%); food labelling reforms (front-of-pack traffic light labelling); restrictions on marketing of high-fat, sugar, and salt (HFSS) foods to children; calorie labelling in out-of-home eating. Individual prevention: maintain a diet rich in vegetables, fruit, legumes, whole grains, and lean protein (Mediterranean or DASH diet pattern); limit ultra-processed foods, refined carbohydrates, and sugary drinks; achieve 150–300 minutes of moderate aerobic activity per week plus muscle-strengthening twice weekly; limit sedentary behaviour — break prolonged sitting with standing or movement every 30 minutes. Prioritise adequate sleep (7–9 hours per night — sleep deprivation raises ghrelin and promotes weight gain). Early weight management intervention for individuals gaining weight: preventing weight gain is significantly easier than achieving and maintaining weight loss; even 5% weight loss achieves clinically meaningful improvements in blood pressure, glucose, and lipids. Treat mental health conditions concurrently — depression and anxiety both drive emotional eating and reduce physical activity.

Consult your GP if your BMI is above 30 or above 27.5 if you are South Asian — particularly if you also have high blood pressure, type 2 diabetes, joint pain, or breathlessness on minimal exertion. A formal medical review will screen for obesity-related comorbidities and assess eligibility for NHS-funded weight management programmes, GLP-1 medications, or bariatric surgery. Seek urgent review for: new onset breathlessness, chest pain, or leg swelling in a person with obesity (heart failure, pulmonary embolism); signs of obstructive sleep apnoea with excessive daytime sleepiness or witnessed apnoeic episodes; unexplained weight gain with other features of Cushing's syndrome (facial rounding, bruising, striae, hypertension) or hypothyroidism (fatigue, cold intolerance, constipation). People with a BMI above 35 who have T2DM, hypertension, or sleep apnoea should ask their GP for referral to a tier 3 specialist weight management service where available.