Obesity — Causes, Symptoms & Management Guide — Symptoms, Causes & Treatment | MyMedicPlus

Obesity is a chronic, relapsing, progressive, multifactorial disease characterised by excess adipose tissue that impairs health through metabolic, mechanical, and inflammatory mechanisms. The World Health Organization defines obesity as a body mass index (BMI) of 30 kg/m² or above, further classified as Class I (BMI 30-34.9), Class II (BMI 35-39.9), and Class III or 'severe obesity' (BMI 40 and above). Over 650 million adults globally — approximately 13% of the world's adult population — have obesity; 1.9 billion are overweight (BMI 25-29.9). The global prevalence has tripled since 1975. In the United Kingdom, 28% of adults have obesity and a further 38% are overweight. In the United States, 42% of adults are obese. Obesity is now recognised as a major driver of preventable morbidity and premature mortality — significantly increasing risk of over 200 medical conditions including type 2 diabetes, cardiovascular disease, 13 types of cancer, obstructive sleep apnoea, osteoarthritis, and non-alcoholic fatty liver disease. The mechanism is not simply excess energy intake but involves complex neuroendocrine dysregulation, with the brain's weight-regulating centres actively defending a higher body weight set-point. The approval of highly effective GLP-1 receptor agonist medications (semaglutide 2.4mg/week achieving 15-17% weight loss) and dual GIP/GLP-1 agonists (tirzepatide achieving 20-22% weight loss) since 2021 has transformed the therapeutic landscape for obesity.

Obesity results from a sustained positive energy balance — caloric intake chronically exceeding energy expenditure — but its aetiology is profoundly multifactorial, involving genetic predisposition, neuroendocrine dysregulation, environmental exposure, socioeconomic determinants, and psychosocial factors. Genetic factors account for 40-70% of BMI variance — genome-wide association studies (GWAS) have identified over 1,000 genetic loci influencing BMI and fat distribution; rare monogenic obesity disorders (mutations in LEP encoding leptin, LEPR encoding the leptin receptor, MC4R encoding the melanocortin-4 receptor — a key satiety pathway) cause severe childhood-onset obesity. Neuroendocrine drivers: impaired leptin signalling (leptin resistance) prevents the satiety signal from reducing food intake; post-prandial GLP-1 and PYY satiety hormones are attenuated in obesity; ghrelin (the hunger hormone) is elevated after caloric restriction, driving hunger and weight regain — the biological basis for the very high relapse rate after diet alone. Environmental and social determinants: pervasive availability of energy-dense ultra-processed foods (NOVA Group 4) with engineered palatability overriding homeostatic signals; food deserts and insecurity; sedentary occupations, commuting by car, screen-based leisure; shift work and sleep deprivation (short sleep raises ghrelin and lowers leptin); socioeconomic deprivation (poverty correlates strongly with obesity in high-income countries); and stress-driven eating via the hypothalamic-pituitary-adrenal axis. Secondary (endocrine) causes (excluding these prevents misdiagnosis): hypothyroidism; Cushing's syndrome (cortisol excess — buffalo hump, striae, proximal myopathy); polycystic ovary syndrome (PCOS); craniopharyngioma or hypothalamic damage (hyperphagia, obesity in children after brain tumour surgery); and medications (atypical antipsychotics — olanzapine, clozapine; corticosteroids; sodium valproate; insulin therapy; sulfonylureas).

The primary physical sign of obesity is excess body adiposity with elevated BMI, but clinical presentation is shaped by the pattern of fat distribution and developing complications. Central (visceral) adiposity — measured by waist circumference (above 102cm in men, above 88cm in women using IDF/AHA thresholds; lower thresholds for South Asian, East Asian, and African-Caribbean populations) and waist-to-hip ratio — is more metabolically harmful than peripheral (subcutaneous) fat, driving insulin resistance, dyslipidaemia, and inflammation. Common symptoms directly attributable to obesity: exertional breathlessness from reduced thoracic compliance and increased oxygen demand; chronic fatigue; obstructive sleep apnoea — nocturnal snoring, apnoeas witnessed by partners, morning headache, and excessive daytime somnolence from intermittent nocturnal hypoxia; weight-bearing joint pain predominantly affecting the knees, hips, and lumbar spine (osteoarthritis); gastroesophageal reflux disease (GERD — heartburn and regurgitation from increased intra-abdominal pressure); and urinary stress incontinence in women. Cutaneous signs: acanthosis nigricans (velvety brown hyperpigmentation in the axillae, nape of neck, and groin — a marker of insulin resistance and hyperinsulinaemia); intertrigo (skin fold dermatitis); striae (stretch marks from rapid adipose tissue expansion); and hidradenitis suppurativa. Presentations from obesity-related comorbidities: polydipsia and polyuria from undiagnosed type 2 diabetes; palpitations and hypertensive headache; right upper quadrant discomfort from non-alcoholic fatty liver disease; and oedema from venous insufficiency or right heart failure.

Clinical assessment of obesity integrates anthropometric measurement, metabolic screening, complication assessment, and investigation for secondary causes. Anthropometric assessment: BMI (body weight in kg divided by height squared in metres) — the primary classification tool, but imperfect as it does not differentiate fat from muscle mass; waist circumference (measured at the level of the umbilicus — high risk: above 94cm in men, above 80cm in women in European populations; above 90cm and above 80cm respectively in South Asian populations); waist-to-hip ratio; and percentage body fat by DEXA scan or bioelectrical impedance for clinical research or bariatric surgery assessment. Metabolic complication screening (all obese patients at first assessment): fasting plasma glucose or HbA1c (type 2 diabetes — present in 25-30% of obese adults); fasting lipid profile (triglycerides, LDL, HDL — dyslipidaemia in obesity); liver function tests and hepatic ultrasound (non-alcoholic fatty liver disease — NAFLD/NASH in 60-90% of obese patients); uric acid (gout); renal function; urine albumin/creatinine ratio (early nephropathy); and blood pressure (hypertension). Endocrine secondary cause exclusion: TSH (hypothyroidism); 9am cortisol or 24-hour urinary free cortisol (Cushing's syndrome); morning testosterone, SHBG, and LH/FSH in women with irregular periods (PCOS). Sleep study (polysomnography or home nocturnal oximetry) for suspected obstructive sleep apnoea (highly prevalent — affecting 30-40% of obese adults). Echocardiogram and ECG in obesity with cardiac symptoms (obesity cardiomyopathy). Edmonton Obesity Staging System (EOSS) stages 0-4 integrates comorbidities and functional status to guide treatment intensity.

Obesity treatment requires a chronic disease management model — combining lifestyle intervention, pharmacotherapy, and surgery in a stepped approach based on BMI, comorbidities, and patient preference. Lifestyle intervention: structured caloric deficit (500-1000 kcal/day deficit — expected 0.5-1kg/week weight loss); dietary approaches include Mediterranean diet, low-carbohydrate diet (effective short-term), or total diet replacement (800 kcal/day very-low-calorie diet — DiRECT trial: 24% T2DM remission at 2 years); 150-300 minutes of moderate-intensity aerobic exercise per week plus resistance training; behavioural support (cognitive behavioural therapy — CBT for eating behaviour, portion control, emotional eating). Anti-obesity pharmacotherapy: semaglutide 2.4mg subcutaneous injection weekly (Wegovy — GLP-1 receptor agonist) is NICE-approved for BMI above 35 with weight-related comorbidities; STEP-1 trial: 14.9% mean body weight reduction versus 2.4% placebo at 68 weeks; cardiovascular outcome benefit (SELECT trial: 20% MACE reduction in patients with established CVD and overweight/obesity). Tirzepatide 2.5-15mg subcutaneous weekly (dual GIP/GLP-1 agonist): SURMOUNT-1 trial achieved 22.5% weight loss at 72 weeks at highest dose — the most effective anti-obesity pharmacotherapy currently available; NICE approval anticipated. Orlistat (120mg TDS with meals — gastrointestinal lipase inhibitor): reduces fat absorption by 30%; modest efficacy (mean 3-4kg additional loss vs placebo) with significant GI side effects (steatorrhoea, oily spotting); still used where newer agents are contraindicated or unavailable. Bariatric surgery: NICE criteria — BMI above 40 (or above 35 with significant comorbidity: T2DM, sleep apnoea, hypertension, dyslipidaemia) after adequate lifestyle intervention; Roux-en-Y gastric bypass (RYGB): 30-35% total body weight loss at 2 years; induces T2DM remission in 60-80% (partly through foregut hormonal effects, independent of caloric restriction); sleeve gastrectomy: 25-30% total body weight loss; both significantly reduce long-term cardiovascular mortality and cancer risk. Multidisciplinary team input (physician, dietitian, psychologist, bariatric surgeon, specialist nurse) is essential throughout.

Obesity drives an extensive list of serious medical complications through metabolic, mechanical, inflammatory, and hormonal mechanisms. Cardiovascular complications: hypertension (present in 50-60% of obese adults); coronary artery disease (2-3x relative risk); heart failure with preserved ejection fraction (HFpEF — 'obesity cardiomyopathy'); atrial fibrillation (obesity is the most important modifiable risk factor for new-onset AF); stroke; and venous thromboembolism. Metabolic complications: type 2 diabetes mellitus (7x higher risk in obesity; 90% of T2DM patients are overweight or obese); dyslipidaemia (high triglycerides, low HDL, raised small dense LDL); hyperuricaemia and gout; NAFLD (60-90% prevalence in obesity) progressing to NASH, cirrhosis, and hepatocellular carcinoma. Respiratory: obstructive sleep apnoea (in 30-40% of obese adults); obesity hypoventilation syndrome (Pickwickian syndrome — severe hypoventilation requiring NIV). Oncological risk: obesity is causally linked to 13 types of cancer — endometrial (4-6x), oesophageal adenocarcinoma (3-4x), renal cell carcinoma (2-3x), breast (post-menopausal), colorectal, pancreatic, liver, gallbladder, ovarian, thyroid, multiple myeloma, and meningioma. Musculoskeletal: osteoarthritis of knees and hips (dose-dependent — each 5kg/m² BMI increase raises osteoarthritis risk by 35%); chronic low back pain; plantar fasciitis. Reproductive: PCOS, infertility, obstetric complications (gestational diabetes, pre-eclampsia, C-section), and increased neonatal risks. Class III obesity reduces life expectancy by 8-10 years on average, comparable to lifelong smoking. Obesity stigma causes depression (35% higher prevalence), anxiety, and social isolation.

Primary prevention of obesity at population level requires upstream policy interventions: food labelling reform (Nutri-Score, traffic light labelling); sugar-sweetened beverage taxation (UK soft drinks industry levy — associated with reformulation and reduced consumption); restrictions on ultra-processed food advertising to children; mandatory physical activity in school curricula; active transport infrastructure (cycling lanes, pedestrianisation); and planning policies limiting fast food outlet density. For individuals at risk of weight gain: 150-300 minutes weekly of moderate-intensity aerobic exercise plus twice-weekly resistance training; reducing ultra-processed food consumption (associated with positive energy balance, inflammation, and microbiome disruption); consuming a predominantly whole-food Mediterranean, DASH, or plant-based dietary pattern; managing sleep (7-9 hours/night — short sleep raises ghrelin and lowers leptin); stress management techniques (reducing cortisol-driven hyperphagia); and self-monitoring of dietary intake and physical activity using apps or food diaries — shown to support weight maintenance. For patients with established obesity, preventing weight regain after initial loss is the critical challenge: GLP-1 agonists (semaglutide) may need to be maintained long-term as weight is regained when the medication is discontinued; structured multidisciplinary weight management programmes (NHS Tier 3 services in the UK) combine dietetic, exercise, and psychological support. NICE NG246 recommends regular review with monitoring of BMI, waist circumference, blood pressure, HbA1c, lipids, and comorbidity progression at least annually.

See your GP for an initial obesity assessment and management plan if: your BMI is 30 or above (or 27.5 or above if South Asian or African-Caribbean descent, where metabolic risk is higher at lower BMI), especially if you have comorbid conditions including Type 2 diabetes, hypertension, high cholesterol, obstructive sleep apnoea, or fatty liver disease; you have tried lifestyle modification but regained weight — a GP can discuss anti-obesity medication (semaglutide/Wegovy on NHS for BMI 35+ with comorbidities under specialist care, or via tier 3/4 weight management services); unexplained weight gain despite no change in diet or activity — thyroid disease (hypothyroidism), polycystic ovary syndrome, Cushing's syndrome, medication side-effects (olanzapine, valproate, mirtazapine), or insulin resistance should be excluded. Seek urgent medical attention for: chest pain, severe breathlessness, or heart palpitations with significant obesity — increased cardiovascular risk requires urgent cardiac assessment; new or worsening snoring, witnessed apnoeas during sleep, or excessive daytime sleepiness — obstructive sleep apnoea requires sleep study and CPAP; new or worsening joint pain, particularly knees and hips, significantly limiting mobility — orthopaedic review and physiotherapy; severe abdominal pain and jaundice in someone with obesity — possible gallstones, non-alcoholic steatohepatitis (NASH), or pancreatitis.