OBESITY

✓ Medically ReviewedDISEASEID: MDBT0269🚨 TRIAGE: ROUTINEICD-10: E66

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Definition

CLINICAL DEFINITION

Obesity is a chronic, complex disease defined by excessive adiposity that can impair health. In adults, it is defined by a body mass index (BMI) of greater than or equal to 30.00 kg/m², with three severity levels recognised: Class 1 (30.0–34.9 kg/m²), Class 2 (35.0–39.9 kg/m²), and Class 3 (≥40.0 kg/m²).

IN PLAIN ENGLISH

Obesity is a medical condition in which a person has too much body fat, enough to harm their health. It is usually measured with body mass index (BMI); in adults, a BMI of 30 or higher means obesity. Obesity raises the risk of many other diseases, including diabetes, heart disease, and sleep apnoea.

COMMON MISCONCEPTION / WHAT IT DOES NOT MEAN

Obesity is NOT simply a cosmetic issue or a failure of willpower; it is a chronic, multifactorial disease. It is also distinct from overweight (BMI 25.0–29.9 kg/m² in adults), although both categories carry increased health risks.

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Word Breakdown

obes-

=From Latin obesus, meaning ‘fat, stout, or having eaten well’

-ity

=Condition or state (from Latin -itas)

Literal Meaning: The state or condition of being excessively fat

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Abbreviations

BMI=Body Mass Index — the primary anthropometric measure used to classify overweight and obesity in clinical practice.

EOSS=Edmonton Obesity Staging System — a clinical staging tool that incorporates obesity-related comorbidities and functional status.

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Synonyms & Variants

Adiposity›Excess adiposity›

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Category-Specific Context

Obesity is classified as a Disease because it is a defined pathological condition with recognised diagnostic criteria (BMI ≥30 kg/m² in adults), a recognised spectrum of severity (Class 1–3), and a well-established set of associated complications. It is not merely a sign or symptom, although it is identified through anthropometric measurement.

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Clinical Context & Usage

CLINICAL SETTINGS

Primary CareEndocrinology Outpatient ClinicCardiology ClinicBariatric Surgery ClinicGeneral Medicine WardSleep Medicine Clinic

USAGE & DOCUMENTATION NOTES

In clinical documentation, obesity is typically recorded with its BMI class and associated comorbidities, for example ‘Class 2 obesity (BMI 37.2 kg/m²) with type 2 diabetes mellitus’. Clinicians document weight trajectory, waist circumference, and complications. In referral letters, the term is often qualified as ‘obesity with metabolic complications’ to justify specialist input.

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Primary Causes & Pathophysiology

PATHOPHYSIOLOGY

Obesity arises from a chronic positive energy balance sustained by complex interactions between genetic, epigenetic, neuroendocrine, behavioural, and environmental factors. Excessive caloric intake relative to energy expenditure leads to hypertrophy and hyperplasia of adipocytes, particularly in visceral depots. Visceral adipose tissue is metabolically active, releasing free fatty acids, adipokines (e.g., leptin, adiponectin), and pro-inflammatory cytokines (e.g., tumour necrosis factor-alpha, interleukin-6). This promotes insulin resistance, dyslipidaemia, and a chronic low-grade inflammatory state. Neuroendocrine dysregulation of appetite and satiety, involving leptin–melanocortin signalling and reward pathways, further perpetuates weight gain. Over time, ectopic fat deposition in liver, skeletal muscle, and pancreas exacerbates metabolic dysfunction.

PRIMARY CAUSES

Multifactorial obesogenic environment with polygenic susceptibility

Easy access to energy-dense, ultra-processed foods, reduced physical activity, and sedentary behaviour interact with numerous genetic variants, each with small effect, to promote positive energy balance and excess adiposity.

Monogenic obesity (e.g., leptin–melanocortin pathway mutations)

Single-gene mutations impair hypothalamic regulation of appetite and satiety, leading to severe hyperphagia and early-onset obesity.

Syndromic obesity (e.g., Prader–Willi syndrome)

Genetic syndromes affecting hypothalamic function, hypotonia, and developmental delay produce hyperphagia and reduced energy expenditure.

Endocrine disorders (e.g., Cushing’s syndrome, hypothyroidism)

Excess cortisol promotes visceral fat deposition and appetite; thyroid hormone deficiency reduces metabolic rate and energy expenditure.

Medications

Several drug classes increase appetite, reduce energy expenditure, or promote fat storage (see drug_induced_causes).

Reduced physical activity or immobilisation

Neurological, musculoskeletal, or iatrogenic immobility lowers energy expenditure and promotes weight gain when caloric intake is unchanged.

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Drug-Induced Causes

💊 Second-generation antipsychotics (e.g., olanzapine, clozapine) ↗

Mechanism: Histamine H1 and serotonin 5-HT2C receptor antagonism increases appetite and sedation, while also promoting insulin resistance.

→ Consider switching to a lower-risk antipsychotic (e.g., aripiprazole) where clinically appropriate; monitor weight, waist circumference, and fasting glucose regularly.

💊 Glucocorticoids (e.g., prednisolone, dexamethasone) ↗

Mechanism: Increase appetite and promote hepatic and muscle insulin resistance, favouring visceral fat deposition.

→ Use the lowest effective dose for the shortest duration; monitor weight and glucose; consider steroid-sparing agents when possible.

💊 Beta-adrenergic blockers (e.g., metoprolol, atenolol) ↗

Mechanism: Reduce metabolic rate and may cause fatigue, reducing physical activity; effects vary by agent.

→ Consider weight-neutral alternatives (e.g., carvedilol, nebivolol) when clinically appropriate; encourage physical activity.

💊 Tricyclic antidepressants (e.g., amitriptyline, nortriptyline) ↗

Mechanism: Anticholinergic and antihistaminergic effects increase appetite and sedation, promoting weight gain.

→ Consider alternative antidepressants with weight-neutral profiles (e.g., sertraline, bupropion); monitor weight.

💊 Insulin and sulfonylureas (e.g., glibenclamide, gliclazide) ↗

Mechanism: Promote glucose utilisation and storage, and may increase appetite through hypoglycaemia and weight gain.

→ Optimise glycaemic control with weight-sparing agents (e.g., GLP-1 receptor agonists, SGLT2 inhibitors) where appropriate; adjust insulin regimens cautiously.

💊 Antihistamines (e.g., cyproheptadine, ketotifen) ↗

Mechanism: Histamine H1 receptor antagonism increases appetite and sedation.

→ Avoid long-term use when possible; consider non-sedating alternatives if antihistamine therapy is required.

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Epidemiology

Prevalence:In 2024, 1 in 7 people worldwide (approximately 1.1 billion) were living with obesity. Among adults, 2.6 billion were overweight, including 940 million with obesity, corresponding to 16% of adults globally. Adult obesity prevalence has more than doubled since 1990, and adolescent obesity has quadrupled.

Age Group:Obesity affects all age groups. Prevalence has risen in children, adolescents, and adults. Early-onset obesity carries a higher lifetime risk of severe complications and reduced life expectancy.

Sex Predilection:Equal

Geographic Distribution:Prevalence varies by region, from 32% overweight in the WHO South-East Asia and African Regions to 68% in the Region of the Americas. Once considered a high-income country problem, overweight and obesity are rising in low- and middle-income countries.

Red Flags & Triage ★

TRIAGE LEVEL

🚨 ROUTINE

DO NOT MISS — LIFE-THREATENING PRESENTATIONS

Obesity hypoventilation syndrome — consider in patients with BMI ≥30 kg/m² and daytime hypercapnia; evaluation with arterial blood gases and sleep study is required.
Severe metabolic complications (e.g., diabetic ketoacidosis, hyperosmolar hyperglycaemic state) in patients with obesity and diabetes presenting with acute illness.

URGENT EVALUATION TRIGGERS

Rapid unexplained weight gain or weight loss
New-onset dyspnoea or orthopnoea suggesting heart failure
Severe abdominal pain suggesting pancreatitis or gallbladder disease
Altered mental status in a patient with obesity and diabetes (consider hyperosmolar hyperglycaemic state or diabetic ketoacidosis)

Contraindications & Precautions

SPECIAL PRECAUTIONS

Assess for disordered eating patterns before initiating pharmacotherapy or bariatric surgery.
Monitor for micronutrient deficiencies (e.g.
iron
vitamin B12
vitamin D
folate) after bariatric surgery.
Use orlistat with caution in patients with a history of oxalate nephrolithiasis or chronic malabsorption.

Complications

DOWNSTREAM CLINICAL COMPLICATIONS

Type 2 diabetes mellitus — visceral adiposity and chronic inflammation drive insulin resistance.
Hypertension — sympathetic activation
sodium retention
and visceral fat accumulation contribute to elevated blood pressure.
Obstructive sleep apnoea — pharyngeal fat deposition and reduced airway calibre cause recurrent upper-airway collapse during sleep.
Metabolic dysfunction-associated steatotic liver disease (MASLD) — ectopic fat deposition and hepatic inflammation can progress to cirrhosis.
Coronary artery disease — dyslipidaemia
inflammation
and endothelial dysfunction accelerate atherosclerosis.
Osteoarthritis — mechanical loading and adipokine-mediated joint inflammation damage weight-bearing joints.
Certain cancers (e.g.
endometrial
breast
colorectal) — hormonal and inflammatory mechanisms promote carcinogenesis.
Psychological disorders (e.g.
depression
binge-eating disorder) — stigma
neuroendocrine dysregulation
and disordered eating patterns contribute.

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Reference Ranges

NORMAL RANGEBMI 18.5–24.9 kg/m² (normal weight in adults)

UNIT OF MEASUREMENTkg/m² (BMI); waist circumference in cm

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Confused With (LASA & DDx) ★

CLINICAL DIFFERENTIAL DIAGNOSES (DDx)

Cushing’s syndrome

Key Distinguishing Feature: Centripetal obesity, striae, proximal myopathy, and elevated cortisol on overnight dexamethasone suppression testing.

Hypothyroidism

Key Distinguishing Feature: Weight gain with fatigue, cold intolerance, constipation, and elevated thyroid-stimulating hormone (TSH).

Prader–Willi syndrome

Key Distinguishing Feature: Hyperphagia, hypotonia, hypogonadism, and characteristic dysmorphic features; confirmed by genetic testing.

Monogenic obesity (e.g., leptin deficiency)

Key Distinguishing Feature: Severe early-onset hyperphagia and obesity; genetic testing reveals pathogenic variants in leptin–melanocortin pathway genes.

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Clinical Pearls

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BMI is a surrogate marker of adiposity and does not distinguish between fat mass and lean mass; waist circumference or waist-to-height ratio should be used to assess central adiposity, which drives cardiometabolic risk.

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A 5–10% weight loss is sufficient to produce clinically meaningful improvements in blood pressure, glycaemia, and triglycerides, even if BMI remains in the obesity range.

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Visceral adipose tissue is more metabolically active and inflammatory than subcutaneous fat, explaining why individuals with similar BMI can have different cardiometabolic risk profiles.

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Example Usage & SOAP

CLINICAL SENTENCE

"The patient is a 45-year-old woman with Class 2 obesity (BMI 36.4 kg/m²) and type 2 diabetes mellitus who has not achieved glycaemic targets despite lifestyle modification and metformin."

IN A SOAP NOTE

S: Patient reports fatigue and difficulty losing weight. O: Weight 102 kg, height 1.65 m, BMI 37.5 kg/m², waist circumference 105 cm, blood pressure 138/88 mmHg, HbA1c 8.2%. A: Class 2 obesity with uncontrolled type 2 diabetes mellitus. P: Initiate a GLP-1 receptor agonist, refer to dietitian, and consider sleep study for obstructive sleep apnoea.

DIFFERENTIAL DIAGNOSIS CONTEXT

Differential diagnosis for obesity: (1) Exogenous (dietary/environmental) obesity — history of caloric excess, no syndromic features; (2) Cushing’s syndrome — screen with overnight dexamethasone suppression test if centripetal obesity, striae, or proximal weakness; (3) Hypothyroidism — check TSH; (4) Monogenic or syndromic obesity — consider if severe early-onset hyperphagia, developmental delay, or dysmorphic features are present.

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Classifications & Scoring

GRADING SYSTEM

WHO BMI Classification for Adults

Classifies weight status in adults: normal weight (BMI 18.5–24.9 kg/m²), overweight (25.0–29.9 kg/m²), obesity Class 1 (30.0–34.9 kg/m²), Class 2 (35.0–39.9 kg/m²), and Class 3 (≥40.0 kg/m²).

View Reference ↗

GRADING SYSTEM

Edmonton Obesity Staging System (EOSS)

Stages obesity from 0 to 4 based on the presence and severity of obesity-related risk factors, comorbidities, and functional limitations: Stage 0 (no risk factors), Stage 1 (subclinical risk factors), Stage 2 (established chronic disease), Stage 3 (severe disease), and Stage 4 (end-stage disease).

STAGING SCALE

Edmonton Obesity Staging System (EOSS)

A 5-stage system (0–4) that incorporates metabolic, mechanical, mental health, and social milieu risk factors to guide obesity management beyond BMI alone.

SCORING TOOL

Waist-to-Height Ratio (WHtR)

A simple anthropometric index calculated as waist circumference divided by height. A ratio of ≥0.5 indicates increased central adiposity and cardiometabolic risk across adult populations; it may outperform BMI for predicting metabolic complications.

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Prognosis

SHORT-TERM OUTCOME

With lifestyle intervention and/or pharmacotherapy, a 5–10% reduction in body weight is achievable within 3–6 months and is associated with clinically meaningful improvements in blood pressure, glycaemia, and triglyceride levels.

LONG-TERM PROGNOSIS

Severe obesity (Class 3) is associated with a reduction in life expectancy of approximately 10.5–16.9 years compared with normal weight, largely due to cardiovascular disease, cancer, and diabetes.

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References & Coding

ICD-10 DIAGNOSTIC CODE

E66

LITERATURE CITATIONS & GUIDELINES

[1]World Health Organization. Obesity and overweight — fact sheet. WHO. 2026. https://www.who.int/news-room/fact-sheets/detail/obesity-and-overweight

[2]World Health Organization. ICD-11 for Mortality and Morbidity Statistics: 5B81 Obesity. WHO. 2026. https://icd.who.int/browse11/l-m/en#/http://id.who.int/icd/entity/149403041

[3]Koskinas KC, Van Craenenbroeck EM, Antoniades C, et al. Obesity and Cardiovascular Disease: An ESC Clinical Consensus Statement. Eur Heart J. 2024;45(38):4063–4098. doi:10.1093/eurheartj/ehae508

[4]Apovian CM, Aronne LJ, Bessesen DH, et al. Pharmacological management of obesity: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2015;100(2):342–362. doi:10.1210/jc.2014-3415

[5]Anekwe CV, Ahn YJ, Bajaj SS, Stanford FC. Pharmacotherapy causing weight gain and metabolic alteration in those with obesity and obesity-related conditions: a review. Ann N Y Acad Sci. 2024;1533(1):95–118. doi:10.1111/nyas.15103

MEDICAL DISCLAIMER — This content is intended for educational purposes and qualified healthcare professionals only. Always refer to current clinical diagnostic criteria, institutional protocols, and official guidelines. Medobo is not responsible for clinical decisions made based on this content.

OBESITY

Definition

Word Breakdown

Abbreviations

Synonyms & Variants

Category-Specific Context

Clinical Context & Usage

Primary Causes & Pathophysiology

Drug-Induced Causes

Epidemiology

Red Flags & Triage ★

DO NOT MISS — LIFE-THREATENING PRESENTATIONS

URGENT EVALUATION TRIGGERS

Contraindications & Precautions

SPECIAL PRECAUTIONS

Complications

DOWNSTREAM CLINICAL COMPLICATIONS

Reference Ranges

Confused With (LASA & DDx) ★

Clinical Pearls

Example Usage & SOAP

Classifications & Scoring

WHO BMI Classification for Adults

Edmonton Obesity Staging System (EOSS)

Edmonton Obesity Staging System (EOSS)

Waist-to-Height Ratio (WHtR)

Prognosis

SHORT-TERM OUTCOME

LONG-TERM PROGNOSIS

Related Terms

References & Coding