What is obesity class three?

Class III Obesity: Definition and Health Implications

Class III obesity is defined as a body mass index (BMI) of 40 kg/m² or greater, also referred to as severe, extreme, or morbid obesity. 1

Definition and Classification

Body Mass Index (BMI) is the standard measurement used to classify obesity, calculated as weight in kilograms divided by height in meters squared (kg/m²). The established categories are:

• Underweight: BMI < 18.5 kg/m²
• Normal weight: BMI 18.5-24.9 kg/m²
• Overweight: BMI 25-29.9 kg/m²
• Class I obesity: BMI 30-34.9 kg/m²
• Class II obesity: BMI 35-39.9 kg/m²
• Class III obesity: BMI ≥ 40 kg/m² 1

Health Implications of Class III Obesity

Class III obesity carries the highest health risks among all BMI categories:

• Mortality Risk: Compared to normal-weight individuals, those with Class III obesity have significantly higher mortality rates (856.0 vs 346.7 per 100,000 person-years in men; 663.0 vs 280.5 in women) 2

• Life Expectancy Reduction: Class III obesity is associated with substantial reductions in life expectancy:

  • BMI 40-44.9: 6.5 years of life lost
  • BMI 45-49.9: 8.9 years of life lost
  • BMI 50-54.9: 9.8 years of life lost
  • BMI 55-59.9: 13.7 years of life lost 2

• Cardiovascular Risk: Class III obesity is associated with extremely high risk for cardiovascular disease, with heart disease being the largest contributor to excess mortality 2, 1

• Other Health Risks: Individuals with Class III obesity have significantly increased risks for:

  • Type 2 diabetes
  • Certain cancers (including colon, breast, endometrial)
  • Sleep apnea
  • Osteoarthritis
  • Nonalcoholic fatty liver disease
  • Mental health conditions (anxiety, depression)
  • Gallbladder disease 1, 3

Epidemiology

The prevalence of Class III obesity has increased dramatically:

• In the United States, the prevalence increased from 0.78% in 1990 to 2.2% in 2000 4
• More recent data shows continued increases, with Class III obesity increasing by 37.9% between 2016 and 2022 (from 2.9% to 4.0% of the population) 1
• Class III obesity is more prevalent among women than men, and particularly high among Black women (6.0%) 4
• Lower educational attainment is associated with higher rates of Class III obesity 4

Clinical Assessment

When evaluating patients with Class III obesity:

• Calculate and document BMI at each visit 5
• For BMI ≥ 40 kg/m², excess adiposity can be pragmatically assumed without further confirmation 6
• Assess for obesity-related health conditions that increase cardiovascular risk 5
• Evaluate for functional limitations in mobility and activities of daily living 6

Treatment Considerations

Individuals with Class III obesity require intensive intervention:

• High-intensity behavioral interventions (>1 session/month for first 3 months) 1, 5
• Pharmacotherapy options including GLP-1 agonists (semaglutide, liraglutide), tirzepatide, phentermine-topiramate, naltrexone-bupropion, and orlistat 5
• Bariatric surgery consideration, which can result in 25-30% weight loss at 12 months 5
• Aim for initial weight loss of 5-10%, which can significantly improve health outcomes 5

Important Caveats

• Use person-first language (e.g., "person with obesity" rather than "obese person") to avoid stigmatization 5
• While BMI is useful for population-level screening, it has limitations as an individual measure of health 6
• Long-term follow-up is essential as weight regain occurs in ≥25% of participants at 2-year follow-up 5
• Class III obesity should be approached as a chronic, progressive disease requiring ongoing management 3

Class III obesity represents the highest risk category of obesity and requires prompt, comprehensive medical intervention to reduce mortality risk and improve quality of life.