Defining and Measuring Obesity

When someone weighs significantly more than what is considered healthy for their height, medical professionals classify them as overweight or obese. Historically, this has been assessed using the body mass index (BMI).

body mass index (BMI)

BMI is calculated by dividing an individual’s weight in kilograms by their height in meters squared. According to the Centers for Disease Control and Prevention (CDC):

Classification	BMI Range
Normal weight	18.5–24.9
Overweight	25–29.9
Obese	30 or higher
Severely obese	40 or higher

Health Consequences of Obesity

Being significantly overweight or obese increases the risk for numerous health conditions, including:

• Cardiovascular disease and stroke
• Type 2 diabetes
• Liver disease
• Sleep apnea
• Certain cancers (colon, breast, and others)
• Infertility
• Arthritis and joint problems

According to the CDC, approximately 40% of U.S. adults have obesity, and nearly 10% have severe obesity. This represents a significant public health concern, as obesity-related conditions contribute to millions of deaths globally each year.

Causes of Obesity

Obesity results from a complex interaction of factors:

Biological Factors

• Genetics — Twin studies show strong heritability of body weight; hundreds of genes influence appetite, metabolism, and fat storage
• Hormonal regulation — As discussed in the previous section, hormones like leptin, ghrelin, and GLP-1 powerfully regulate hunger and satiety
• Metabolic differences — Individual variation in metabolic rate affects how efficiently the body uses energy

Environmental Factors

• Food environment — Availability of inexpensive, high-calorie, highly palatable foods
• Portion sizes — Restaurant and packaged food portions have increased dramatically over decades
• Physical activity — Sedentary lifestyles, mechanized transportation, and urban design limit activity
• Sleep and stress — Both affect hormones that regulate appetite and metabolism

Socioeconomic Factors

• Income and education — Obesity rates are higher in lower-income populations
• Food access — “Food deserts” in some neighborhoods limit access to fresh, healthy foods
• Time constraints — Working multiple jobs or long hours limits time for cooking and exercise
• Neighborhood safety — Unsafe neighborhoods discourage outdoor physical activity

Understanding obesity as resulting from this complex web of factors—rather than simply “poor choices”—is essential for developing effective interventions and reducing stigma.

Weight Loss Approaches

Lifestyle Interventions

The foundation of obesity treatment remains diet and exercise. However, research shows that lifestyle interventions alone produce modest results—typically 5-10% weight loss—and most people regain weight over time. This isn’t due to lack of willpower; as discussed earlier, powerful biological mechanisms resist sustained weight loss.

Pharmacological Treatment: GLP-1 Medications

A revolution in obesity treatment began with the approval of GLP-1 receptor agonist medications for weight management. These include:

• Semaglutide (Wegovy, Ozempic)
• Tirzepatide (Zepbound, Mounjaro) — a dual GLP-1/GIP agonist

These medications work by mimicking the satiety hormone GLP-1, reducing hunger signals in the brain and slowing gastric emptying. Clinical trials showed impressive results:

• Semaglutide: ~15-17% total body weight loss (e.g., ~40 pounds for someone starting at 250 lbs)
• Tirzepatide: ~20-22% total body weight loss (e.g., ~50 pounds for someone starting at 250 lbs)

These results far exceed what most people achieve through diet and exercise alone (typically 5-10%). However, real-world results often differ significantly from clinical trials. A 2025 study of over 50,000 patients found that those prescribed GLP-1 medications lost an average of only 12 pounds over two years—far less than the 40-50+ pounds seen in trials.^[1]

Key considerations:

• Weight typically returns when medication is stopped
• Long-term effects are still being studied
• Cost and insurance coverage remain barriers for many patients

Bariatric Surgery

Bariatric surgery involves modifying the gastrointestinal system to reduce food intake and/or nutrient absorption. Common procedures include:

• Sleeve gastrectomy — Removes approximately 80% of the stomach
• Gastric bypass (Roux-en-Y) — Creates a small stomach pouch and bypasses part of the small intestine

Bariatric surgery remains the most effective treatment for severe obesity, with patients typically losing 25-30% of body weight and maintaining significant weight loss for 10+ years. Recent studies comparing surgery to GLP-1 medications found surgery produced approximately five times more weight loss at two years (Gloy et al., 2013; ASMBS, 2025).

Surgery is typically recommended when:

• BMI is 40 or higher, OR
• BMI is 35 or higher with obesity-related health conditions, OR
• BMI is 30 or higher with inadequate response to other treatments

Combining Approaches

Increasingly, treatment involves combining approaches—lifestyle changes plus medication, or medication before or after surgery. The goal is matching the right intervention to each individual’s needs and circumstances.

Prader-Willi Syndrome (PWS) is a genetic disorder that illustrates the powerful biological control of hunger. PWS results in:

• Persistent, intense feelings of hunger
• Reduced metabolic rate
• Cognitive and emotional difficulties

Children with PWS often require 24-hour supervision to prevent excessive eating. The syndrome is currently the leading genetic cause of morbid obesity in children.

Figure 3. Eugenia Martínez Vallejo, depicted in this 1680 painting, may have had Prader-Willi syndrome. At just eight years old, she weighed approximately 120 pounds.

PWS demonstrates that hunger is not simply a matter of willpower—it is controlled by biological systems that, when disrupted, can produce overwhelming drive to eat. Research shows that individuals with PWS have hypothalamic abnormalities, consistent with the hypothalamus’s central role in regulating hunger.

While there is no cure for PWS, careful weight management can significantly increase life expectancy. Advances in growth hormone treatment and other interventions continue to improve quality of life for affected individuals (Cassidy & Driscoll, 2009).