What Is Obesity? BMI, Classes, and Why It’s a Chronic Disease

What Is Obesity?

Obesity is defined by the World Health Organization (WHO) as abnormal or excessive fat accumulation that presents a risk to health.

Many people mistakenly view obesity as a cosmetic issue or a sign of lacking self-control. However, major medical organizations including the WHO, American Medical Association, and Cleveland Clinic now recognize obesity as a chronic, progressive disease resulting from multiple environmental and genetic factors.

Obesity means weighing more than what is healthy for a given height. It's a serious medical condition that increases the risk of life-threatening diseases including diabetes, heart disease, and some cancers.

Understanding BMI: The Screening Tool for Obesity

Body Mass Index (BMI) is the most widely used screening measure to assess whether a person has a healthy weight for their height. It's calculated as:

BMI Categories for Adults (20 and older)

Important note: BMI is a screening tool, not a diagnostic tool. It doesn't directly measure body fat or health. Your healthcare provider should evaluate other factors like waist circumference, blood pressure, blood glucose, and cholesterol levels.

Waist Circumference and Risk

Waist size is another important indicator of obesity-related health risks:

• Women: Waist > 35 inches (89 cm) = increased risk

• Men: Waist > 40 inches (102 cm) = increased risk

People with "apple-shaped" bodies (waist bigger than hips) have higher risk for heart disease and type 2 diabetes.

The Three Classes of Obesity

Obesity is subdivided into three classes based on BMI, with increasing health risks as the class number rises:

Class 1 Obesity (BMI 30–34.9)

• Low-risk obesity

• Early stage of obesity

• May have few or no symptoms

• Risk of complications begins to increase

Class 2 Obesity (BMI 35–39.9)

• Moderate-risk obesity

• Higher likelihood of health problems

• May experience breathing difficulties, joint pain, or fatigue

• Medical intervention often recommended

Class 3 Obesity (BMI 40+)

• High-risk or severe obesity (also called "morbid obesity" or "extreme obesity")

• Significantly increased risk of serious complications

• May have difficulty with daily activities

• Often requires comprehensive medical treatment including medication or surgery

Why Is Obesity a Chronic Disease?

Recent research has established that obesity is not simply about "eating less and moving more." Here's why it's classified as a chronic disease:

1. Complex Physiology

• The body has a weight-regulation system that defends a higher body-weight set point once obesity develops

• Hormones like leptin and insulin regulate appetite and fat storage

• The body actively resists weight loss through metabolic adaptations

2. Multiple Contributing Factors

Obesity results from a combination of:

• Genetic factors (family history, metabolism)

• Environmental factors (access to healthy food, sedentary lifestyle)

• Physiological factors (hormonal imbalances, medications)

• Psychological factors (stress, depression, eating disorders)

• Socioeconomic factors (income, education, food deserts)

3. Chronic and Progressive

• Like diabetes, hypertension, and asthma, obesity is long-term and requires ongoing management

• Without treatment, it tends to worsen over time

• Relapse is common, similar to other chronic diseases

4. Fundamental Cause: Energy Imbalance

The fundamental cause of obesity is an imbalance between calories consumed and calories expended:

• Consuming more calories than you burn leads to fat storage

• Modern environments promote high-calorie foods and sedentary behavior

5. Requires Medical Treatment

• Lifestyle changes alone often aren't enough due to biological defenses

• Medical treatments include prescription medications, behavioral therapy, and bariatric surgery

• Long-term monitoring and support are essential

Major Health Complications

The risk of developing heart disease, stroke, and kidney problems increases significantly if you have obesity plus additional risk factors like high blood pressure, high cholesterol, or high blood glucose.

Prevention and Management

Many causes of overweight and obesity are preventable and reversible.

Lifestyle Changes

• Reduce calories from fats and sugars

• Increase fruit, vegetables, legumes, whole grains, and nuts

• Physical activity: 150 minutes/week for adults, 60 minutes/day for children

• A starting goal of losing 5% to 10% of current weight significantly reduces disease risk

Medical Treatment

• Consult your healthcare provider for personalized weight-loss programs

• Prescription medications for chronic weight management

• Bariatric surgery for severe obesity (Class 2–3)

• Ongoing monitoring and support for long-term success

1. https://www.ncbi.nlm.nih.gov/books/NBK279167/
2. https://www.drkmh.com/blogs/what-is-obesity-and-why-does-it-matters/
3. https://medlineplus.gov/ency/patientinstructions/000348.htm
4. https://www.kokilabenhospital.com/blog/obesity-a-global-epidemic/
5. https://my.clevelandclinic.org/health/diseases/11209-weight-control-and-obesity