Obesity is a chronic medical condition affecting millions of people in the UK, characterised by excessive body fat that poses significant health risks. Understanding obesity causes, treatment options and BMI is essential for effective management. Body mass index (BMI) serves as the primary diagnostic tool, with a BMI of 30 kg/m² or above indicating obesity in most adults, though lower thresholds apply for certain ethnic groups. Obesity develops through complex interactions between genetic, environmental, behavioural and socioeconomic factors. Fortunately, evidence-based treatments—including lifestyle modification, medication and bariatric surgery—can achieve meaningful health improvements. This article explores how obesity is diagnosed, what causes it, associated health risks, and the full range of NHS treatment options available.

What Is Obesity and How Is BMI Used to Diagnose It?

Obesity is a chronic medical condition characterised by excessive accumulation of body fat that presents a risk to health. In the United Kingdom, obesity is formally diagnosed using body mass index (BMI), a calculation derived from an individual's weight in kilograms divided by the square of their height in metres (kg/m²). According to NICE guidance, a BMI of 30 kg/m² or above indicates obesity in most adults, whilst a BMI between 25 and 29.9 kg/m² is classified as overweight. Obesity is further categorised into classes: Class I (BMI 30–34.9 kg/m²), Class II (BMI 35–39.9 kg/m²), and Class III (BMI ≥40 kg/m²).

However, BMI has important limitations. It does not distinguish between muscle and fat mass, nor does it account for fat distribution—factors that significantly influence metabolic health. BMI thresholds also do not apply to children and young people; for those under 18 years, obesity is assessed using UK BMI centile charts (overweight is defined as above the 91st centile, and obesity as above the 98th centile). BMI may also be less accurate in pregnancy, older adults, and very muscular individuals.

For individuals of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family origin, NICE recommends using lower BMI thresholds to trigger assessment and intervention: a BMI of 23 kg/m² or above indicates increased risk, and 27.5 kg/m² or above indicates high risk. These lower cut-offs reflect evidence that these populations develop obesity-related complications at lower BMI values, but they do not redefine the diagnostic categories of 'overweight' or 'obesity'—they are action thresholds to prompt earlier intervention.

Waist circumference provides additional diagnostic information, particularly regarding central (abdominal) obesity, which carries greater cardiovascular and metabolic risk. A waist circumference above 94 cm in men or 80 cm in women (or 90 cm and 80 cm respectively in South Asian populations) indicates increased health risk. Healthcare professionals use BMI alongside waist measurement and clinical assessment to provide a comprehensive evaluation of obesity and associated health risks.

It is essential to recognise that obesity is a complex, multifactorial condition influenced by genetic, environmental, psychological and socioeconomic factors—not simply a matter of personal choice or willpower.

References: NICE CG189 (Obesity: identification, assessment and management); NICE PH46 (BMI: preventing ill health and premature death in Black, Asian and other minority ethnic groups); NHS BMI healthy weight calculator.

Common Causes and Risk Factors for Obesity

Obesity develops when energy intake consistently exceeds energy expenditure over time, but this simple equation belies a complex interplay of biological, environmental and social determinants. At the physiological level, the body regulates appetite, metabolism and fat storage through intricate hormonal systems involving leptin, ghrelin, insulin and other mediators. Genetic factors play a substantial role in individual variation in body weight, influencing metabolic rate, appetite regulation and fat distribution.

Environmental and behavioural factors play a substantial role in the rising prevalence of obesity. The modern food environment—characterised by widespread availability of energy-dense, nutrient-poor foods high in sugar, fat and salt—promotes excessive calorie consumption. Portion sizes have increased significantly, whilst marketing practices often target vulnerable populations including children. Sedentary lifestyles, driven by technological advances, car dependency and reduced occupational physical activity, further contribute to positive energy balance.

Socioeconomic determinants are particularly significant in the UK context. Obesity prevalence is higher in areas of greater deprivation, reflecting complex relationships between income, education, food access, housing quality and opportunities for physical activity. Psychological factors, including stress, depression, anxiety and disordered eating patterns, can influence eating behaviours and weight gain.

Certain medical conditions and medications increase obesity risk. Hypothyroidism, Cushing's syndrome, polycystic ovary syndrome (PCOS) and some genetic syndromes (such as Prader-Willi syndrome) are associated with weight gain. Medications including some antipsychotics (e.g. olanzapine, clozapine), antidepressants (e.g. mirtazapine, tricyclic antidepressants), corticosteroids, antiepileptics (e.g. valproate) and some diabetes treatments (e.g. insulin, sulfonylureas, pioglitazone) may promote weight gain as an adverse effect. In contrast, some diabetes medications—such as metformin, SGLT2 inhibitors and GLP-1 receptor agonists—are weight-neutral or associated with weight loss. Sleep deprivation and disrupted circadian rhythms also contribute to metabolic dysregulation and obesity risk.

References: NICE CG189; NHS 'Obesity – Causes'; Office for Health Improvement and Disparities (OHID) 'Health matters: obesity and the food environment'.

Health Risks and Complications Associated with Obesity

Obesity significantly increases the risk of numerous serious health conditions, reducing both quality of life and life expectancy. Type 2 diabetes mellitus is strongly associated with obesity, particularly central adiposity. Excess adipose tissue promotes insulin resistance through inflammatory mediators and altered adipokine secretion. Approximately 90% of adults with type 2 diabetes are overweight or obese, and weight reduction can substantially improve glycaemic control or even achieve remission in some cases.

Cardiovascular disease represents the leading cause of mortality in people with obesity. Obesity increases risk of hypertension, coronary heart disease, heart failure, atrial fibrillation and stroke through multiple mechanisms including increased cardiac workload, dyslipidaemia, endothelial dysfunction and chronic low-grade inflammation. Even modest weight loss (5–10% of body weight) can produce clinically meaningful improvements in blood pressure and lipid profiles.

Respiratory complications include obstructive sleep apnoea (OSA), obesity hypoventilation syndrome and asthma. OSA affects a substantial proportion of individuals with obesity and contributes to daytime fatigue, cognitive impairment and cardiovascular risk. Mechanical effects of excess weight on the chest wall and diaphragm impair respiratory function.

Obesity increases risk of several cancers, including oesophageal, pancreatic, colorectal, postmenopausal breast, endometrial and kidney cancer. The mechanisms involve hormonal changes, chronic inflammation and insulin resistance. Musculoskeletal problems—particularly osteoarthritis of weight-bearing joints—cause significant disability and pain. Non-alcoholic fatty liver disease (NAFLD)—increasingly termed metabolic dysfunction-associated steatotic liver disease (MASLD) in newer guidance—can progress to cirrhosis and liver failure.

Mental health is bidirectionally linked with obesity. Depression, anxiety and low self-esteem are more prevalent, partly reflecting societal stigma and discrimination. Reproductive health may be affected, with obesity associated with reduced fertility, pregnancy complications and polycystic ovary syndrome.

When to seek urgent medical help: Contact 999 or go to A&E immediately if you experience chest pain, acute shortness of breath, or symptoms of stroke (sudden weakness, speech difficulty, facial drooping). Contact NHS 111 or see your GP urgently if you have symptoms of severe hyperglycaemia (extreme thirst, frequent urination, confusion) or signs of obstructive sleep apnoea (loud snoring, witnessed breathing pauses during sleep, excessive daytime sleepiness). Consult your GP promptly if you experience unexplained weight gain, persistent fatigue, or other concerning symptoms.

References: NHS 'Obesity – Health risks'; NHS 'Obstructive sleep apnoea'; Cancer Research UK 'Obesity, weight and cancer'; NHS 'Non-alcoholic fatty liver disease (NAFLD)'.

Treatment Options for Obesity: Lifestyle, Medication and Surgery

Lifestyle modification forms the foundation of obesity management. NICE recommends multicomponent interventions addressing diet, physical activity and behaviour change, ideally delivered over 12 months or longer. Dietary approaches should create a sustainable energy deficit (typically 600 kcal/day) through reduced calorie intake whilst ensuring nutritional adequacy. No single dietary pattern is universally superior; success depends on individual preference and adherence. Options include balanced calorie-controlled diets, Mediterranean-style eating patterns, or low-carbohydrate approaches.

Very low energy diets (VLEDs), providing 800 kcal/day or less, or low-energy diets (typically 800–1,500 kcal/day) may be appropriate for some individuals under medical supervision, usually as part of a time-limited, multicomponent programme. These approaches may be considered for rapid weight loss before surgery or to achieve type 2 diabetes remission, but require careful monitoring and nutritional support.

Physical activity recommendations, according to the UK Chief Medical Officers' guidelines, include at least 150 minutes of moderate-intensity aerobic activity per week (or 75 minutes of vigorous-intensity activity), plus muscle-strengthening activities on two or more days per week. Reducing sedentary time is also important. Activity should be built up gradually, with any increase beneficial. Behavioural strategies—including self-monitoring, goal-setting, stimulus control and relapse prevention—enhance long-term success. Commercial weight management programmes meeting NICE quality criteria may be effective and are sometimes available through NHS referral.

Pharmacological treatment is considered for adults when lifestyle interventions alone are insufficient. UK prescribing must reflect individual medicine licences (Summary of Product Characteristics, SmPC) and NICE commissioning criteria:

• Orlistat (Xenical, 120 mg three times daily) is a pancreatic lipase inhibitor that reduces dietary fat absorption. It may be considered for adults with BMI ≥30 kg/m² (or ≥28 kg/m² with risk factors such as type 2 diabetes or hypertension). Orlistat produces modest additional weight loss (2–3 kg beyond lifestyle changes alone) but commonly causes gastrointestinal adverse effects including oily stools, faecal urgency and flatulence. Treatment should be discontinued if less than 5% of initial body weight is lost after 12 weeks. A lower-dose formulation (60 mg, Alli) is available over the counter from pharmacies.

• GLP-1 receptor agonists were originally developed for type 2 diabetes but demonstrate significant weight loss effects. Semaglutide 2.4 mg (Wegovy) is licensed for weight management and, according to NICE TA875, is recommended via specialist weight management services for adults meeting specific criteria, including BMI thresholds (with lower cut-offs for some ethnic groups) and the presence of at least one weight-related comorbidity. Typical weight loss is approximately 12–15% of body weight. Treatment duration is time-limited (e.g. up to two years). Liraglutide 3 mg (Saxenda) is also licensed for weight management, with typical weight loss of approximately 5–8% of body weight; NHS commissioning criteria apply and vary by locality. These injectable medications work by reducing appetite and slowing gastric emptying. Common adverse effects include nausea, vomiting, diarrhoea and constipation; there is also an increased risk of gallbladder disease. Availability on the NHS is currently limited by cost and eligibility criteria.

• Naltrexone/bupropion (Mysimba) is licensed in the UK for weight management, but NHS commissioning varies by locality; check local policies.

If you experience side effects from any medicine, you can report them via the MHRA Yellow Card Scheme at yellowcard.mhra.gov.uk or through the Yellow Card app.

Bariatric (metabolic) surgery is the most effective treatment for severe obesity, producing substantial, sustained weight loss (typically 20–35% of total body weight) and improvement or remission of comorbidities. NICE recommends considering surgery for adults with:

• BMI ≥40 kg/m² (or ≥37.5 kg/m² for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family origin), or

• BMI ≥35 kg/m² (or ≥32.5 kg/m² for the ethnic groups above) with significant obesity-related comorbidities that could improve with weight loss (e.g. type 2 diabetes, hypertension, obstructive sleep apnoea), and

• All appropriate non-surgical measures have been tried but have not achieved or maintained adequate, clinically beneficial weight loss.

Bariatric surgery may also be considered as a first-line option (with or without lifestyle interventions) for adults with BMI ≥30 kg/m² (or ≥27.5 kg/m² for certain ethnic groups) who have recent-onset type 2 diabetes, as long as they are receiving or will receive assessment in a tier 3 service (or equivalent).

Common procedures include sleeve gastrectomy (removing part of the stomach) and gastric bypass (creating a small stomach pouch connected to the small intestine). These procedures work through multiple mechanisms including restriction, hormonal changes affecting appetite, and altered nutrient absorption. Surgery requires lifelong dietary modifications, nutritional supplementation (including vitamins and minerals) and medical follow-up. Potential complications include nutritional deficiencies, dumping syndrome, gallstones and surgical risks. Assessment by a specialist multidisciplinary tier 3 weight management service is required before surgery.

References: NICE CG189; UK Chief Medical Officers' Physical Activity Guidelines (2019); NICE TA875 (Semaglutide for managing overweight and obesity); MHRA/EMC SmPCs for Xenical, Saxenda, Wegovy; British Obesity and Metabolic Surgery Society (BOMSS) patient information.

NHS Support and Resources for Weight Management

The NHS provides comprehensive support for individuals seeking to manage their weight, organised into a tiered model of services:

Tier 1 (Universal services): The first step is typically consulting your GP, who can assess your overall health, identify any underlying medical causes of weight gain, review medications that might contribute to obesity, and discuss your weight management goals. Your GP can calculate your BMI (using lower thresholds for certain ethnic groups where appropriate), measure waist circumference, and arrange blood tests if needed to check for conditions such as diabetes, thyroid disorders or lipid abnormalities.

Tier 2 (Lifestyle weight management services): Many areas offer NHS-funded weight management programmes, including structured lifestyle interventions delivered by trained practitioners. These typically involve group or individual sessions covering healthy eating, physical activity, behaviour change techniques and psychological support. The NHS Digital Weight Management Programme provides remote support via smartphone app or online platform for adults with obesity and hypertension or diabetes. Your GP can refer you to local services, or you may be able to self-refer depending on your area.

Tier 3 (Specialist weight management services): These are available for individuals with complex needs, severe obesity (typically BMI ≥40 kg/m², or ≥35 kg/m² with comorbidities; lower thresholds apply for some ethnic groups) or multiple comorbidities. Multidisciplinary teams include dietitians, psychologists, physiotherapists and physicians who provide intensive support and can assess suitability for medication or surgery. Referral criteria vary by region.

Tier 4 (Bariatric surgery): Specialist surgical services for eligible individuals, following tier 3 assessment.

The NHS website (nhs.uk) offers extensive free resources including the NHS Weight Loss Plan, BMI calculator, healthy recipes, and physical activity guidance. The Better Health campaign provides tools and support for weight management, including the Couch to 5K running programme and Easy Meals app. Many local authorities offer community-based weight management programmes, physical activity schemes and healthy lifestyle services.

Pharmacists can provide advice on weight management. The only licensed over-the-counter weight management product is orlistat 60 mg (Alli); evidence for most other over-the-counter 'weight loss' products is limited, and some may be harmful. Some pharmacies offer weight management services including regular monitoring and advice.

For individuals struggling with emotional eating or psychological barriers to weight loss, NHS Talking Therapies (formerly IAPT) may provide cognitive behavioural therapy or other psychological interventions. Remember that sustainable weight management is a long-term journey requiring ongoing support—don't hesitate to seek help from healthcare professionals.

References: NHS Digital Weight Management Programme; NHS Weight Loss Plan; NHS BMI healthy weight calculator; NHS Talking Therapies; NICE CG189.

Frequently Asked Questions