Obesity and bariatric surgery

• Obesity is a chronic disease of excessive adiposity, not a personal failing; it raises the risk of type 2 diabetes, cardiovascular, liver, renal and many cancers.
• Body mass index (BMI) is used to screen: WHO defines overweight as a BMI of 25 or more and obesity as 30 or more, with classes 1, 2 and 3.
• Non-surgical care is the foundation for everyone: dietary change, increased activity, behavioural support; medicines, including GLP-1 receptor (GLP-1R) agonists, are added when these are started and reviewed.
• Metabolic and bariatric surgery is strongly recommended for a BMI above 35 kg/m2 whether or not comorbidities are present, and should be considered for BMI 30 to 34.9 kg/m2 when non-surgical care has not worked.
• Surgery is a lifelong commitment: it needs a multidisciplinary team, a follow-up package of at least 2 years, then at least annual monitoring of nutritional status.

By the end of this page you should be able to:

1. Define obesity and classify it by BMI, by waist-to-height ratio, by age group and by ethnicity.
2. List the main health consequences of excess adiposity.
3. Outline a stepwise management plan: lifestyle, medicines, and bariatric surgery.
4. Compare sleeve gastrectomy with Roux-en-Y gastric bypass on mechanism, expected weight loss, and complications.
5. State the indications, exclusions and follow-up required for bariatric surgery.
6. Recognise and manage common postoperative complications, including dumping and nutritional deficiency.
7. Adjust thresholds and advice for children, adolescents, older adults, pregnancy and ethnic minority groups.

NICE NG246 and WHO agree on the main cut-offs (WHO additionally describes obesity at all levels); the NICE classes are used below. ^{[1] [3]}

A BMI below 35 kg/m2 should be read alongside waist-to-height ratio, because fat around the midline drives cardiometabolic risk. ^[3]

• Healthy central adiposity: waist-to-height ratio 0.40 to 0.49 (no increased health risk).
• Increased central adiposity: 0.50 to 0.59 (increased health risks).
• High central adiposity: 0.60 or more (further increased health risks).

The practical rule to give patients is to keep the waist under half the height (a ratio under 0.50). ^[3]

Waist circumference is measured midway between the bottom of the ribs and the top of the hips, after breathing out naturally. ^[3]

BMI is interpreted with age- and sex-specific charts, not adult cut-offs. ^[3]

• Overweight: BMI at or above the 91st centile (1.34 standard deviations above the median). ^[3]
• Clinical obesity: 98th centile (2.05 SD above the median). ^[3]
• Severe obesity: 99.6th centile (2.68 SD above the median). ^[3]

Central adiposity is classified in children aged 5 years or more by the same waist-to-height ratio thresholds as adults: 0.40 to 0.49 healthy, 0.50 to 0.59 increased, 0.60 or more high. ^[3]

WHO uses BMI-for-age for children 5 to 19 years: overweight is above 1 SD and obesity above 2 SD of the WHO Growth Reference median; for children under 5, overweight is weight-for-height above 2 SD and obesity above 3 SD of the WHO Child Growth Standards median. ^[1]

After 65 years of age BMI should be read with care, accounting for comorbidities, functional capacity, and the possible protective effect of a slightly higher BMI in older people. ^[3]

Obesity is common and becoming more common. WHO describes it as affecting 1.1 billion people in 2024, with 2.6 billion adults (45% of adults) overweight, of whom 940 million (16% of all adults) were living with obesity. ^[1]

• Adult obesity worldwide more than doubled between 1990 and 2024. ^[1]
• Adolescent obesity quadrupled over the same period (about 8% of children and adolescents aged 5 to 19 years, 170 million, had obesity in 2024). ^[1]
• Among pre-school children, 35 million under the age of 5 were overweight in 2024. ^[1]
• In 2024, overweight and obesity were estimated to contribute to substantial disease burden: higher-than-optimal BMI was responsible for an estimated 3.7 million deaths through cardiovascular disease, diabetes, cancers and other conditions. ^[1]

Prevalence varies by region. ^[1] It is higher in the Americas (about 68% of adults overweight in 2024) and lower in the WHO South-East Asian and African regions (about 32%). ^[1] The fastest rise is now in low- and middle-income countries and in rural populations. ^[1]

In 2023, higher-than-optimal BMI was associated with an estimated 3.7 million deaths. ^[1]

Low-resource settings therefore carry a growing share of obesity care: many low- and middle-income countries face a double burden of malnutrition, with undernutrition and obesity at the same time. ^[1]

Obesity arises when energy intake chronically exceeds expenditure, but biology, environment and psychology all push this in one direction. ^[1]

• Genetic susceptibility: obesity is highly heritable at population level, with many common variants of small effect; individual susceptibility varies. ^[1]
• Environment: cheap, energy-dense, nutrient-poor food; limited safe opportunities for activity; built environments that favour sedentary behaviour. ^[1]
• Sleep and stress: poor sleep, shift work, chronic stress and low mood all bias towards weight gain. ^[1]
• Medications that promote gain include systemic glucocorticoids, several antipsychotics, some antidepressants and anti-epileptics, and insulin secretagogues when used without dietary review.
• Endocrine causes (hypothyroidism, Cushing syndrome, polycystic ovary syndrome, hypothalamic disease) often mimic or complicate obesity.
• Pregnancy and menopause add risk, especially where gestational weight gain was excessive.

Obesity is recognised by WHO and other bodies as a disease in its own right, driven by dysregulation of appetite, energy balance and adipose tissue function, not by willpower alone. ^[1]

The harm comes from the fat, especially visceral and ectopic fat in liver, muscle and the epicardium.

• Adipose tissue expansion leads to hypoxia, macrophage recruitment and a chronic low-grade inflammatory state with raised cytokines (interleukin-6, tumour necrosis factor-alpha) and acute-phase reactants. ^[4]
• The inflammatory state disrupts insulin signalling and promotes insulin resistance, dyslipidaemia and hypertension. ^[4]
• Leptin resistance blunts satiety signalling; weight loss lowers metabolic rate and increases hunger, which is part of why treatment is long-term.
• GLP-1 receptor (GLP-1R) agonists and the dual agonist tirzepatide reduce appetite and body fat through hypothalamic and brainstem pathways and also improve glucose control. ^{[6] [7]}

Each complication of obesity can be traced back to this combination of metabolic dysfunction and the mechanical load of excess weight.

Patients rarely present saying "I am obese"; they present with a complication. Ask, in their own words, about:

• weight history: when it changed, jobs and shifts, pregnancies, diets tried and whether weight was maintained;
• sleep (duration, snoring and daytime sleepiness, restless legs);
• snacking patterns, alcohol, sugary drinks;
• appetite and control of eating; disordered eating and binge episodes;
• mood, mobility and pain;
• medications and endocrine symptoms (heat/cold intolerance, menstrual change, striae, easy bruising);
• fertility plans;
• family history of obesity and diabetes.

• Height, weight and BMI; waist circumference if BMI under 35.
• Blood pressure (both arms where indicated); pulse and rhythm.
• Waist-to-height ratio and waist circumference.
• Neck circumference where sleep apnoea is suspected.
• Skin: acanthosis nigricans (insulin resistance), striae, hirsutism, skin tags, intertrigo.
• Joints and gait; varicose ulcers and venous disease.
• Cardiovascular and respiratory signs (cardiac failure, sleep apnoea); hepatomegaly; fundoscopy where diabetes is present.

Cardiovascular (hypertension, dyslipidaemia, coronary and cerebrovascular disease, atrial fibrillation, heart failure), respiratory (obstructive sleep apnoea, asthma), endocrine (type 2 diabetes, polycystic ovary syndrome, infertility), liver (metabolic dysfunction-associated steatotic liver disease), renal, musculoskeletal (osteoarthritis, back pain), reproductive, mood disorders and cancers (breast, colorectal, endometrial and others). ^[4]

Weight gain with obesity is usually primary. Suspect a secondary cause, or an eating disorder, when age of onset, body fat distribution or the history do not fit.

The aim of investigation is threefold: confirm the degree of adiposity, screen for comorbidities, and make sure nothing else needs treating.

• Anthropometry: height, weight and BMI; waist circumference and waist-to-height ratio if BMI is under 35 kg/m2. ^[3]
• Baseline bloods: full blood count, renal profile, liver function (including platelets and fibrosis markers where indicated), fasting glucose or HbA1c, lipid profile, thyroid function if symptoms suggest.
• Comorbidity work-up as directed: blood pressure, ECG; sleep study if snoring and daytime sleepiness; liver ultrasound/elastography if liver enzymes are raised; screening for breast, bowel and other age-appropriate cancers as per national programmes.
• Bioimpedance is not recommended as a substitute for BMI in adults or children. ^[3]
• Nutritional status if bariatric surgery is being considered: iron, folate, vitamin B12, vitamin D, calcium, magnesium, zinc.

There is no single "gold standard" for obesity beyond adiposity measures; the diagnosis rests on BMI, adjusted for ethnicity and age. ^{[1] [3]}

Staging the disease means deciding how much it is harming this patient:

• Degree of obesity (WHO/NICE classes, ethnic thresholds, paediatric centiles).
• Degree of central adiposity (waist-to-height ratio).
• Presence and severity of obesity-related conditions (cardiovascular, metabolic, sleep, liver, joint, psychological).
• Functional impact: mobility, quality of life, ability to work and self-care.

This staging drives which step of management is offered next.

Non-surgical care comes first for everyone; it continues before, during and after any surgery. ^[3]

• Calorie-reduction tailored to the patient; high-quality dietary advice, not fad diets.
• Physical activity built up to regular moderate activity each week, plus resistance work where possible.
• Behavioural support (self-monitoring, stimulus control, goal setting, problem solving) delivered individually or in groups.
• For adults with BMI below 35 kg/m2, measure waist-to-height ratio and, where it indicates increased risk, offer cardiometabolic risk assessment. ^[3]
• NICE suggests considering a supplement providing the reference nutrient intake of all vitamins and minerals where micronutrient intake may be inadequate, especially in older and young people. ^[3]
• Support to maintain weight loss should be offered when medicines are stopped. ^[3]

Dietary, exercise and behavioural approaches are started and evaluated before medicines are added, and medicines are used only alongside a reduced-calorie diet and increased physical activity. ^[3]

Medicines are an adjunct, always alongside diet, activity and behavioural care, and only after these have been started and evaluated. ^[3]

Non-diabetic hyperglycaemia is defined as HbA1c 42 to 47 mmol/mol (6.0% to 6.4%) or fasting plasma glucose 5.5 to 6.9 mmol/L. ^[3]

Practical points common to all:

• Medicines are reviewed regularly; monitor adherence and reinforce behavioural advice. ^[3]
• Stop or review: tirzepatide and semaglutide need at least 5% weight loss by 6 months on the highest tolerated or standard dose; liraglutide and orlistat need at least 5% by 12 weeks. ^[3]
• GLP-1R agonists delay gastric emptying and can reduce absorption of oral drugs; with tirzepatide, use a non-oral or barrier method of contraception for 4 weeks on initiation and after each dose escalation. ^[6]
• Do not use GLP-1R agonists in pregnancy; stop tirzepatide at least 1 month and semaglutide at least 2 months before a planned pregnancy. ^{[6] [7]}
• In children, weight-management medicines are not generally recommended under 12 years; orlistat may be considered from age 12 in a specialist paediatric setting with comorbidities. ^[3]

Note on GLP-1 medicines for type 2 diabetes (for example, semaglutide 1 mg licensed for diabetes or tirzepatide for glycaemic control) are a separate prescribing question from the obesity labels above; doses and monitoring differ. ^{[6] [7]}

NICE NG246 offers adults referral for a comprehensive multidisciplinary assessment for bariatric surgery when they: ^[3]

• have a BMI of 40 kg/m2 or more, or 35 to 39.9 kg/m2 with a significant health condition that could be improved by weight loss (box 2 conditions: cardiovascular disease, hypertension, idiopathic intracranial hypertension, metabolic dysfunction-associated steatotic liver disease, obstructive sleep apnoea, type 2 diabetes), and ^[3]
• agree to the necessary long-term follow-up after surgery, such as lifelong annual reviews. ^[3]

The ASMBS/IFSO 2022 consensus broadened this: surgery is recommended for BMI above 35 kg/m2 regardless of the presence, absence or severity of comorbidities; for BMI 30 to 34.9 kg/m2 with metabolic disease that has not responded to nonsurgical treatment; and it does not apply separate age limits without considering frailty. ^[4]

Offer an expedited assessment for adults with: ^[3]

• BMI 35 kg/m2 or more with recent-onset type 2 diabetes diagnosed within the previous 10 years; ^[3]
• BMI 30 to 34.9 kg/m2 with recent-onset type 2 diabetes can be considered; and ^[3]
• BMI thresholds reduced by 2.5 kg/m2 for the ethnic backgrounds above.

A multidisciplinary team (surgeon, physician or obesity specialist, dietitian, psychologist, anaesthetist) assesses medical, nutritional, psychological and surgical suitability. ^{[3] [4]}

• nutritional status and correction of micronutrient deficiency;
• mental health, disordered eating and substance use;
• previous attempts at weight management and responses;
• factors affecting adherence (language, disability, deprivation);
• fitness for anaesthesia and surgery.

NICE specifically advises performing a full medical evaluation, including genetic screening or assessment where relevant, to exclude rare treatable causes of obesity (for example monogenic or hypothalamic causes) before operating in young people. ^[3]

The 1991 NIH criteria (BMI above 40, or 35 with comorbidities) are the historical standard that many funders and insurers still use; both NICE and ASMBS/IFSO have now moved well beyond them. ^{[2] [4]}

A useful bridge indication also occurs when obesity blocks another operation, such as joint arthroplasty or hernia repair: weight loss first can improve surgical outcomes. ^[4]

Sleeve gastrectomy (SG) and Roux-en-Y gastric bypass (RYGB) are the two operations performed most often worldwide (together roughly 90% of all operations). ^{[4] [5]}

Mechanism and anatomy:

• SG removes 75% to 85% of the stomach along the greater curvature, leaving a sleeve (tubular stomach), acting mainly by restriction plus neurohormonal changes. ^[5]
• RYGB creates a small gastric pouch joined to a loop of small bowel (the Roux limb), bypassing most of the stomach and duodenum; it combines restriction with malabsorption and a stronger incretin effect. ^[5]

The choice is made jointly with the patient, taking into account comorbidities (especially reflux and type 2 diabetes), nutritional history and local surgical expertise. ^{[3] [4]}

• No insurance-mandated preoperative weight loss is evidence-based; multidisciplinary assessment with optimisation of risk factors is preferred. ^[4]
• Registered dietitian assessment and correction of micronutrient deficiencies before surgery. ^[4]
• Only surgeons with extensive experience should do revisional surgery in specialist centres, because complications and mortality are higher than primary surgery. ^[3]

NICE sets a follow-up care package of at least 2 years within the bariatric service, then at least annual monitoring shared with primary care. ^[3]

The package includes: ^[3]

• monitoring of nutritional intake, including protein and vitamins, and mineral deficiencies;
• monitoring of comorbidities;
• medications review (for example adjusting or stopping diabetes and hypertension drugs as weight falls);
• individualised dietary and nutritional assessment, advice and support;
• physical activity advice and support;
• psychological support;
• information about support groups.

Give patients information about expected dietary change, macronutrient and micronutrient monitoring, individualised supplementation, and support groups from the outset. ^[3]

Nutritional supplements after surgery are individualised, but minimum patterns commonly needed after RYGB (and often after SG) include a daily multivitamin with iron and calcium, vitamin B12, and vitamin D; protein targets and extra iron, folic acid, thiamine, copper and zinc are checked against postoperative levels rather than guessed. ^{[3] [5]}

Left untreated, obesity steadily damages almost every system: cardiovascular disease and stroke, type 2 diabetes, metabolic dysfunction-associated steatotic (fatty) liver disease and cirrhosis, obstructive sleep apnoea, osteoarthritis, renal impairment, gallstones, subfertility, and several cancers including breast, colorectal and endometrial. ^[4]

Leaks are among the most feared complications. Anastomotic leak after RYGB occurs in roughly 1.6% of operations, and staple-line leak after SG in about 0.8% to 4%. ^[5] RYGB leaks present on average 9.5 plus or minus 7.4 days after surgery, so warning signs (tachycardia, raised C-reactive protein, abdominal pain, shoulder-tip pain, hiccups) must be taken seriously even after discharge. ^[5]

• Reflux disease: reflux is improved more often by RYGB than by SG; about 19% of SG patients experience worsening of pre-existing reflux and 23% develop new reflux, with esophagitis and Barrett change possible beyond 2 years. ^[5]
• Marginal ulcer (RYGB): occurs at the gastrojejunal anastomosis in 0.6% to 25% of patients; treat with smoking and NSAID cessation, high-dose proton pump inhibitor (liquid or open-capsule form), test for Helicobacter pylori, and re-scope at 2 to 3 months. ^[5]
• Stenosis: sleeve stenosis (often functional, usually at the incisura), or anastomotic stricture when the lumen is under 6 mm; endoscopic balloon dilation is first line with through-the-scope balloons (target above 8 mm). ^[5]
• Internal hernia (RYGB): 4% to 17%, depending on whether mesenteric defects were closed; CT may miss it, so maintain a low threshold for surgical re-exploration. ^[5]
• Dumping syndrome after RYGB: early (usually within 1 hour, often within 15 minutes, osmotic) or late (1 to 3 hours, hypoglycaemic); modify diet first; acarbose or a somatostatin analogue (octreotide) for refractory cases. ^[5]
• Gallstones: may develop in up to 40% after bariatric surgery; ursodeoxycholic acid 600 mg daily for 6 months postoperatively reduces both formation and symptomatic disease; prophylactic cholecystectomy is not routinely supported. ^[5]
• Weight regain: at least 1 in 6 patients regain 10% or more by about 5 years; GLP-1R agonists, endoscopic outlet reduction or revisional surgery are options. ^[5]
• Chronic abdominal pain occurs in 11% to 34% after RYGB and needs multidisciplinary assessment, including internal hernia, marginal ulcer, gallstones, small intestinal bacterial overgrowth and abdominal wall pain. ^[5]

Nutritional deficiencies occur often after RYGB because of changes in anatomy, motility, pH and enzymes, and are seen after SG too; they require prompt assessment against published guidelines. ^[5]

Common problems are iron, folate, vitamin B12, thiamine, copper, zinc, calcium and vitamin D deficiency, with metabolic bone disease and anaemia if missed. ^{[3] [5]}

Bariatric surgery is the most effective treatment for severe obesity: sustained weight loss of 15% or more is common, and comorbid disease (type 2 diabetes, hypertension, dyslipidaemia, obstructive sleep apnoea, non-alcoholic fatty liver disease) improves or remits in many patients. ^[4]

The Sweden-based Swedish Obese Subjects study (cited in ASMBS/IFSO) reported a 30.7% lower adjusted overall mortality at an average of 10 years after surgery compared with non-surgical controls; a meta-analysis found a 6.1-year median gain in life expectancy. ^[4]

Prognosis after surgery depends on adherence to follow-up, supplementation, and lifestyle change; without these, weight can be regained and deficiencies develop.

WHO calls for prevention at every stage of life, from before conception to older age: healthy weight before pregnancy, exclusive breastfeeding for six months, healthy diets and activity throughout childhood, limitation of sugar-sweetened beverages and energy-dense foods, adequate sleep, and no tobacco or harmful alcohol. ^[1]

At health system level, identify overweight and obesity early, assess comorbidities, provide evidence-based counselling, and refer to comprehensive multimodal care (behavioural, nutritional, psychological, pharmacological and surgical) where appropriate. ^[1]

Screening should also include weight-status assessment built into routine care (for example registration and annual reviews) and appropriate cancer screening; WHO published a global guideline on the use of GLP-1 medicines in treating obesity in adults in December 2025, supporting their safe, equitable inclusion as one component of multimodal care. ^[1]

Childhood obesity is an early marker of adult disease. ^[1]

• Use age- and sex-specific centiles; do not use adult BMI cut-offs. ^[3]
• Lifestyle and behavioural programmes first; involve the family.
• Weight-management medicines are not generally recommended under 12 years of age. ^[3]
• Surgery is not generally recommended for children or young people; it may be considered in exceptional circumstances after physiological maturity and a comprehensive psychological, educational, family and social assessment, by a specialist paediatric multidisciplinary team. ^{[3] [4]}
• ASMBS paediatric thresholds: BMI above 120% of the 95th centile (class 2) with a major comorbidity, or above 140% of the 95th centile (class 3). ^[4]
• Semaglutide 2.4 mg is licensed from age 12 years with obesity and weight above 60 kg under the SPC; tirzepatide has no paediatric weight-management licence. ^{[6] [7]}

• Weight-loss medicines should not be used in pregnancy. ^{[3] [6] [7]}
• Tirzepatide is contraindicated in women of childbearing potential not using contraception; use a non-oral or barrier method for 4 weeks on initiation and after each dose escalation. ^{[6] [7]}
• Stop tirzepatide at least 1 month and semaglutide at least 2 months before a planned pregnancy. ^{[6] [7]}
• High-dose folic acid before conception; screen for gestational diabetes; avoid excessive gestational weight gain.
• Bariatric surgery is usually deferred until after childbearing; where already done, pregnancies are generally safer with specialist follow-up and close micronutrient monitoring.

BMI should be interpreted cautiously after 65 years; consider comorbidities and functional capacity. ^[3] Older adults are at risk of inadequate micronutrient intake; consider a broad multivitamin supplement. ^[3]

Lower BMI thresholds identify risk earlier in South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean populations, and referral thresholds for surgery and medicines are usually reduced by 2.5 kg/m2. ^{[3] [4]}

• BMI and waist measurements with a tape are free and enough to screen; bioimpedance is not needed. ^[3]
• Build a tiered, local "universal to specialist" pathway rather than an expensive technology stack.
• Exercise on prescription costs little; walking groups and community interventions are effective first steps.
• Orlistat (three times daily with meals) is cheaper than newer GLP-1R agonists and requires no injection; many other drugs may not be affordable.
• Bariatric surgery needs specialist centres, suitable equipment (scales, beds, theatre tables, hoists) and long-term follow-up capacity; refer early and keep a shared-care model with primary care after discharge. ^[3]
• Nutritional follow-up after surgery (especially micronutrient monitoring and supplementation) must be planned where laboratory capacity is limited.

Type 2 diabetes mellitus; metabolic syndrome; dyslipidaemia; obstructive sleep apnoea; metabolic dysfunction-associated steatotic liver disease; endocrine causes of weight gain; malnutrition; postoperative care and complications.