Definition of Morbid/Severe Obesity by BMI
A BMI of 40 kg/m² or greater defines morbid (severe) obesity in adults, while a BMI of 35-39.9 kg/m² with obesity-related comorbidities also qualifies as morbid obesity. 1, 2
BMI Classification System
The American Heart Association and World Health Organization classify BMI into the following categories:
- Underweight: BMI < 18.5 kg/m²
- Normal weight: BMI 18.5-24.9 kg/m²
- Overweight: BMI 25.0-29.9 kg/m²
- Obesity class I: BMI 30.0-34.9 kg/m²
- Obesity class II: BMI 35.0-39.9 kg/m²
- Obesity class III (severe/morbid obesity): BMI ≥ 40 kg/m² 1, 2
Alternative Definitions of Severe Obesity
While BMI ≥ 40 kg/m² is the primary definition of morbid/severe obesity, there are additional considerations:
- BMI ≥ 35 kg/m² with significant obesity-related comorbidities (e.g., controlled hypertension, diabetes, hyperlipidemia) also qualifies as severe obesity 3
- For medication treatment purposes, a BMI ≥ 27 kg/m² with comorbidities may qualify for pharmacotherapy 3
Prevalence and Trends
The prevalence of severe obesity has increased significantly:
- Between 2000 and 2010, the prevalence of BMI > 40 kg/m² increased by 70% 4
- In 2010, approximately 6.6% of adult Americans (15.5 million people) had a BMI > 40 kg/m² 4
Health Implications of Severe Obesity
Severe obesity is associated with significantly increased health risks:
- Higher mortality rates, particularly when BMI exceeds 40 kg/m² 1, 5
- Increased healthcare expenditures (81% greater than normal-weight adults) 6
- Longer hospital stays for inpatients 5
- Higher risk of death among surgical critically ill patients (7.4 times increased odds) 7
- Multiple comorbidities including cardiovascular disease, type 2 diabetes, sleep apnea, gastrointestinal disorders, and psychological disorders 1, 8
Special Considerations in Children and Adolescents
In children and adolescents, severe obesity is defined differently:
- BMI at or above the 99th percentile for age and sex 1
- BMI ≥ 120% of the 95th percentile for age and sex 1
- For children 16 years and older, an absolute BMI ≥ 35 kg/m² may also qualify as severe obesity 1
Clinical Implications
The recognition of severe obesity is important for:
- Determining eligibility for more intensive interventions, including bariatric surgery
- Recognizing the substantially increased health risks compared to lower BMI categories
- Implementing appropriate clinical protocols for patients with severe obesity, particularly in hospital settings 5, 7
Understanding these BMI thresholds helps clinicians accurately identify patients with severe obesity who may benefit from more aggressive weight management interventions to reduce mortality and morbidity risks.