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Definition of obesity

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Here is a thorough breakdown of the definition of obesity, drawing from authoritative medical textbooks and updated 2025 frameworks.

Definition of Obesity

Core Definition

Obesity is a degree of excess adiposity (body fat accumulation) that predisposes to adverse health consequences. It is considered a chronic disease of body weight regulation - analogous to how diabetes is a disease of glucose regulation - rather than simply a result of poor willpower or lifestyle choices.
  • Goldman-Cecil Medicine, p. 3533

Standard BMI-Based Classification (WHO/NIH)

Body Mass Index (BMI = weight in kg / height in m²) is the most widely used screening tool:
CategoryBMI (kg/m²)
Underweight< 18.5
Normal18.5 - 24.9
Overweight25.0 - 29.9
Obesity Class I30.0 - 34.9
Obesity Class II35.0 - 39.9
Obesity Class III (Severe/Morbid)≥ 40.0
In children and adolescents, obesity is defined as BMI at or above the 95th percentile for age and sex (not a fixed BMI cutoff).
  • Swanson's Family Medicine Review, p. 2597
  • The Harriet Lane Handbook, p. 1541
Morbid obesity (Class III) can also be defined as being 100 lb above ideal body weight, or twice ideal body weight.

Important Caveats to BMI

  1. Asian populations - Obesity risk begins at a lower BMI (23-24 kg/m²), as metabolic complications appear at lower body weights compared to other ethnic groups.
  2. Central/Abdominal obesity - Waist circumference (WC) and waist-to-hip ratio (WHR) add important information. Persons with central obesity face higher cardiometabolic risk even at a "normal" BMI.
  3. Muscle mass - Some individuals with high BMI (25-29.9) have increased muscle mass rather than fat; these people do not carry the same elevated health risks.

2025 Lancet Commission Update: "Clinical Obesity"

A significant 2025 update from the Lancet Diabetes & Endocrinology Commission proposed a new framework distinguishing:
  • Preclinical obesity - Excess body fat + metabolic risk markers, but no end-organ dysfunction yet
  • Clinical obesity - Excess adiposity with measurable functional impairment or organ-level disease (e.g., heart failure, type 2 diabetes, sleep apnea)
This framework aims to move beyond BMI alone and instead use waist-based adiposity measures combined with actual clinical consequences. Research shows that up to 1 in 4 people with a normal BMI may qualify as clinically obese under this definition.

Why It Matters Clinically

Comorbidity risk rises sharply once BMI exceeds 30. Key associated conditions include:
  • Coronary artery disease and myocardial infarction (obese patients ~250% more likely)
  • Type 2 diabetes mellitus
  • Hypertension and left ventricular hypertrophy
  • Obstructive sleep apnea
  • Osteoarthritis
  • Non-alcoholic fatty liver disease
  • Cholelithiasis and certain cancers
  • Swanson's Family Medicine Review, p. 2617-2634
In summary: Obesity is formally defined as BMI ≥ 30 kg/m² in adults and ≥ 95th BMI percentile in children. The 2025 Lancet Commission has broadened this to a clinical framework that incorporates abdominal adiposity and functional consequences, better reflecting the true disease burden.
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