Obesity: Causes, Health Risks, Diagnosis & NHS Treatment

Obesity: Causes, Health Risks, Diagnosis & NHS Treatment

Obesity is a complex, chronic condition characterised by excess body fat that impairs health. It affects around 28% of adults in England — approximately 15 million people — and has significant health, economic, and social consequences. Obesity is a major risk factor for type 2 diabetes, cardiovascular disease, several cancers, fatty liver disease, sleep apnoea, osteoarthritis, and mental health problems. Modern understanding recognises obesity as a disease driven by biological, genetic, and environmental factors — not simply a failure of willpower. Effective treatments are available on the NHS, including structured lifestyle programmes, medication, and surgery.

Key Facts

  • Approximately 28% of adults in England have obesity (BMI ≥30) — one of the highest rates in Europe
  • A further 38% of adults are overweight (BMI 25–29.9)
  • Obesity costs the NHS approximately £6.5 billion per year
  • Obesity is associated with over 60 different health conditions
  • NICE guideline CG189 governs NHS obesity management; new medications (semaglutide — Wegovy) are now available on the NHS
  • Bariatric surgery is the most effective long-term treatment for severe obesity and type 2 diabetes remission

Defining and Measuring Obesity

Body mass index (BMI) is the most widely used clinical tool for classifying obesity — calculated as weight in kilograms divided by height in metres squared. BMI categories used in the UK: underweight (below 18.5); healthy weight (18.5–24.9); overweight (25–29.9); obesity class I (30–34.9); obesity class II (35–39.9); obesity class III or severe obesity (40 and above). For people of South Asian, Chinese, and Black African/Caribbean descent, lower BMI thresholds (overweight: 23; obese: 27.5) are used to reflect higher cardiometabolic risk at lower BMI values.

Waist circumference is an important adjunct to BMI, as central (abdominal) adiposity confers greater metabolic risk than peripheral fat distribution. Increased risk: waist above 80 cm in women, above 94 cm in men. High risk: above 88 cm in women, above 102 cm in men. Waist-to-height ratio (waist/height — ideal below 0.5) is increasingly recommended as a single measure capturing both overall and central adiposity.

Causes of Obesity

Obesity results from a chronic imbalance between energy intake and energy expenditure, but this imbalance is determined by a complex interplay of biological, genetic, psychological, and environmental factors — not simply individual choice. Genetic factors account for approximately 40–70% of the variation in BMI. Over 900 genetic variants associated with BMI have been identified, many affecting hunger and satiety signalling pathways in the brain. Rare single-gene mutations (e.g., in the leptin gene or MC4R) cause severe, early-onset obesity in a small proportion of cases.

The obesogenic environment — characterised by abundant ultra-processed, energy-dense foods, pervasive food marketing, reduced physical activity in daily life, sedentary occupations and transport, and urban design that discourages walking — creates conditions in which biological drives toward weight gain are difficult to overcome through behaviour change alone. Psychosocial factors including adverse childhood experiences, stress, depression, binge eating disorder, and sleep deprivation all promote weight gain through neuroendocrine pathways. Certain medications cause significant weight gain: antipsychotics, antidepressants (particularly mirtazapine and paroxetine), insulin, corticosteroids, beta-blockers, and some anticonvulsants.

Health Consequences of Obesity

Obesity is associated with significantly increased risk of: type 2 diabetes (risk is 7-fold higher in obesity class I, rising with increasing BMI); cardiovascular disease (coronary heart disease, heart failure, atrial fibrillation, stroke); MASLD/fatty liver disease; obstructive sleep apnoea; several cancers (endometrial, postmenopausal breast, colorectal, kidney, oesophageal, and pancreatic cancers are all more common with obesity — obesity accounts for approximately 6% of all UK cancer cases); osteoarthritis (particularly knees and hips); polycystic ovary syndrome; gastro-oesophageal reflux disease (GORD); infertility; depression and anxiety; and pregnancy complications (gestational diabetes, pre-eclampsia, macrosomia). Obesity reduces life expectancy by an average of 3–10 years at severe levels.

NHS Assessment and Treatment (NICE CG189)

NICE guideline CG189 (Obesity: identification, assessment, and management) recommends a tiered care model. All adults with overweight or obesity should receive assessment of BMI, waist circumference, cardiovascular risk, and comorbidities. Treatment should address diet, physical activity, behavioural strategies, and where appropriate, pharmacotherapy and surgery.

Tier 1 — Population and community-based interventions: General public health advice and access to freely available resources (NHS Better Health, Change4Life, NHS weight loss plan app).

Tier 2 — Structured lifestyle interventions: Multicomponent weight management programmes combining dietary advice, physical activity support, and behavioural change techniques. The NHS Weight Management Programme (digital and face-to-face) is available to people with BMI above 30 (or above 27.5 in South Asian groups) with a weight-related comorbidity. Low-calorie diet programmes (e.g., NHS-funded total diet replacement providing 800–900 kcal/day for 12 weeks, based on the DiRECT trial model) are effective for people with type 2 diabetes or prediabetes.

Pharmacotherapy: Two medications are currently available on the NHS for obesity. Orlistat (Xenical or Alli) reduces fat absorption by inhibiting pancreatic lipase — produces modest weight loss of 3–5 kg at 12 months beyond lifestyle alone; side effects include steatorrhoea (oily, loose stools) if a high-fat meal is eaten. Semaglutide 2.4 mg weekly subcutaneous injection (Wegovy — a GLP-1 receptor agonist) is now approved by NICE (TA875, 2023) for adults with BMI ≥35 and at least one weight-related comorbidity, or BMI 30–34.9 with a co-existing condition at high cardiovascular risk. In the STEP-1 trial, semaglutide 2.4 mg produced an average weight loss of approximately 15% at 68 weeks. It is available through specialist NHS weight management services, with access being rolled out through the NHS through a multi-year plan (NHS England pathway). Liraglutide 3 mg (Saxenda) is an alternative GLP-1 RA; tirzepatide (Mounjaro — dual GLP-1/GIP agonist) was approved by NICE in 2024 and produces even greater weight loss (approximately 20%) but access via NHS is currently being established.

Bariatric surgery (Tier 4): The most effective treatment for severe obesity. NICE recommends bariatric surgery for adults with BMI of 40 or above (or 35–39.9 with a significant comorbidity — particularly type 2 diabetes, hypertension, or sleep apnoea) who have not achieved sufficient weight loss through other interventions. For people with type 2 diabetes and BMI 30–34.9 (or lower in Asian populations), surgery should also be considered. Procedures include Roux-en-Y gastric bypass (most effective for T2DM remission), sleeve gastrectomy, and adjustable gastric banding (less commonly used). Average weight loss following bypass is 25–35% of initial body weight; T2DM remission occurs in approximately 50–80% of patients.

Behavioural and Psychological Support

Addressing the psychological dimensions of obesity is essential. Binge eating disorder affects approximately 20–30% of people seeking treatment for obesity and requires specific psychological treatment (CBT-E). Cognitive behavioural therapy (CBT), acceptance and commitment therapy (ACT), and motivational interviewing all have evidence for supporting behaviour change in obesity. Stigma — from healthcare professionals, employers, family, and society — is pervasive and actively harmful, reducing help-seeking and self-efficacy. NICE emphasises that all healthcare professionals should treat people with obesity with dignity and respect, and avoid weight-based blame.

⚠️ Secondary Causes of Obesity

A small proportion of obesity cases have an underlying medical cause that should be identified and treated. Consider screening for hypothyroidism (TSH — common and treatable), Cushing’s syndrome (excess cortisol — rounded face, central adiposity, stretch marks, hypertension, glucose intolerance), polycystic ovary syndrome, and medication-induced weight gain. Hypothalamic obesity can occur after brain injury, tumours, or surgery affecting the hypothalamus. Rare genetic causes of severe early-onset obesity (e.g., MC4R deficiency — treatable with setmelanotide, now NICE-approved) should be considered in children with severe obesity and hyperphagia.

ℹ️ Obesity and Mental Health

The relationship between obesity and mental health is bidirectional and complex. Depression increases the risk of developing obesity by approximately 58%, and obesity increases the risk of depression. Stigma, body image distress, reduced mobility, pain, and social isolation all contribute to poor mental health in obesity. Conversely, depression and anxiety promote emotional eating, reduce motivation for physical activity, and can cause direct neurobiological changes (HPA axis dysregulation) that promote weight gain. Treating mental health conditions and addressing psychosocial factors is an integral — not optional — part of obesity management.

Related Conditions

Frequently Asked Questions

Is obesity a disease?

Yes — major medical organisations including the World Obesity Federation, American Medical Association, and the UK’s Academy of Medical Royal Colleges recognise obesity as a disease. Obesity involves dysregulation of biological systems controlling body weight — including the hypothalamus, gut hormones, adipokines, and the gut microbiome — and is strongly influenced by genetics and environment. Recognising obesity as a disease rather than a personal failing is important for reducing stigma, improving access to treatment, and guiding policy responses.

How effective are GLP-1 medications like semaglutide (Wegovy/Ozempic)?

GLP-1 receptor agonists represent a major advance in obesity pharmacotherapy. Semaglutide 2.4 mg (Wegovy) produces average weight loss of approximately 15% at 68 weeks in the STEP-1 trial — substantially more than any previously available medication. Tirzepatide (Mounjaro), a GLP-1/GIP dual agonist, produces even greater weight loss of approximately 20–22% in clinical trials. These medications work by suppressing appetite, slowing gastric emptying, and reducing food cravings through central and peripheral mechanisms. Weight typically returns when medication is stopped, reflecting the biological nature of the condition — highlighting that obesity requires long-term treatment, as with other chronic diseases.

Can I get bariatric surgery on the NHS?

Yes — bariatric surgery is available on the NHS for eligible patients. NICE criteria require BMI ≥40 (or ≥35 with a significant comorbidity) and demonstrated engagement with weight management services. Access varies by region — some integrated care boards have longer waiting times or stricter local criteria. Referral to a specialist bariatric service is made by your GP. The procedure, hospital care, and follow-up are all funded by the NHS. Some people choose private bariatric surgery to avoid long waiting times, but comprehensive aftercare and lifelong follow-up are essential regardless of funding route.

Why is it so hard to keep weight off?

Sustained weight loss is extremely difficult because weight loss triggers powerful biological adaptations that defend the previous higher body weight — a phenomenon called “adaptive thermogenesis.” After weight loss, resting metabolic rate falls more than predicted, appetite-suppressing hormones (such as GLP-1, PYY, and leptin) decrease, and hunger-stimulating hormones (ghrelin) increase. These changes persist for years after weight loss, creating a strong biological drive to regain weight. This is why obesity treatment requires long-term support rather than one-off interventions, and why pharmacological and surgical options that modify these biological drivers are important tools.

What should I eat to lose weight?

The most effective diet is one that creates a sustained calorie deficit and that the individual can maintain over time. Evidence supports several dietary approaches: low-calorie diets (reduced by 500–600 kcal/day from baseline); very low-calorie total diet replacement (800 kcal/day, medically supervised); Mediterranean diet; low-carbohydrate diets; and intermittent fasting (e.g., 5:2 method). All produce similar weight loss at 12 months if calories are similarly restricted. The NHS recommends working with a dietitian to identify a sustainable approach. Reducing ultra-processed foods, sugar-sweetened beverages, and large portion sizes are evidence-based first steps.