What Is Obesity?

Obesity is identified by the World Health Organization as the presence of abnormal or excessive body fat that can negatively impact health. In the U.S., more than a third of adults are affected by this condition, based on data from the Centers for Disease Control and Prevention (CDC). The CDC uses the body mass index (BMI)—a figure calculated by dividing a person’s weight in kilograms by the square of their height in meters—to evaluate obesity. A BMI of 30 or above falls within the obesity range. However, while BMI is a useful preliminary screening metric, it has its drawbacks. The WHO emphasizes that BMI is a basic tool and does not account for deeper individual variations.

Why Is Obesity Classified as a Disease?

The WHO first recognized obesity as a disease back in 1948. In the United States, the American Medical Association (AMA) followed suit in 2013. This classification is crucial because it reframes obesity from being seen as a lifestyle choice to being treated as a complex health condition. Historically, obesity has been shrouded in stigma, with many—healthcare professionals included—believing it is simply a result of overeating and inactivity. But recognizing it as a disease reveals that its origins are multifactorial, involving genetics, environmental influences, and biological mechanisms. Moreover, obesity plays a role in the development of over 236 medical conditions, including 13 cancers.

Acknowledging obesity as a disease paves the way for serious research into prevention and treatment strategies. It validates the need for structured interventions such as behavioral therapy, medical treatment, dietary planning, and sometimes surgery. Importantly, it signals that sustained weight management usually requires more than just willpower or lifestyle tweaks. It also underscores the need for compassionate and respectful treatment—people with obesity should be cared for just as seriously as those with heart disease or hypertension. In fact, the United States Preventive Services Task Force (USPSTF) advises that adults be screened for obesity and, if identified, offered intensive intervention programs.

What Causes Obesity?

Obesity results when excess fat accumulates in the body to a point that it disrupts normal health. Numerous factors can contribute to this condition:

• Genetic Influence: Research suggests that 40–70% of obesity risk may be inherited. Thousands of genes are tied to obesity risk, and certain gene mutations can directly cause the condition. However, a genetic predisposition does not mean a person is destined to become obese. 
• Dietary Factors: Not all calories have the same effect on the body. High intake of processed carbohydrates and unhealthy fats plays a more significant role in promoting fat accumulation. 
• Hormonal Imbalances: Insulin resistance—a condition where the body struggles to regulate blood sugar—frequently appears in individuals with obesity, highlighting the role of disrupted metabolic pathways. 
• Prescription Drugs: Many medications are linked to weight gain, including those used for treating depression, diabetes, seizures, allergies, high blood pressure, and sleep issues. 
• Environmental Conditions: Family habits, cultural background, and one’s neighborhood or access to healthy foods can heavily influence weight gain. 
• Sleep Patterns: Both insufficient and excessive sleep are associated with obesity. Additionally, disorders like sleep apnea—often worsened by obesity—disrupt quality rest, creating a cycle. 

Diseases Associated with Obesity

Current research indicates that obesity can shorten life expectancy by as much as eight years. It is linked to over 236 health conditions, including several types of cancer, metabolic and cardiovascular diseases, and mental health issues.

• Cancer: Obesity-related inflammation and hormonal changes create conditions that support cancer development. Fat cells can release inflammatory substances and hormones that fuel abnormal cell growth. A healthier lifestyle, including better nutrition and increased activity, can reduce these risks. Thirteen cancers closely tied to obesity include: 
  • Postmenopausal breast cancer
    
  • Colorectal cancer
    
  • Esophageal adenocarcinoma
    
  • Gallbladder cancer
    
  • Kidney cancer
    
  • Liver cancer
    
  • Meningioma
    
  • Multiple myeloma
    
  • Ovarian cancer
    
  • Pancreatic cancer
    
  • Thyroid cancer
    
  • Stomach (upper) cancer
    
  • Uterine cancer 
• Type 2 Diabetes: A significant majority of individuals with type 2 diabetes also experience obesity. Insulin resistance—an early sign of future diabetes—often emerges in those with high body fat, leading to elevated blood sugar levels and ultimately, diabetes. 
• Sleep Apnea: Affecting around 20% of Americans, this condition involves pauses in breathing during sleep. Risk increases with higher BMI and larger neck circumference. Excess fat restricts airflow by collapsing airway passages and disrupting breathing patterns, leading to fatigue and other serious health issues. 
• Hypertension: High blood pressure is directly affected by excess weight. The Framingham Heart Study showed that up to 28% of hypertension cases could be traced to being overweight or obese. Reducing weight often brings blood pressure down, lowering the need for medications. 
• Depression: People with obesity face a significantly higher risk of depression, with some studies indicating a 20–44% increase in likelihood. Symptoms like low energy, irregular sleep, and overeating are often shared between these conditions. 

Obesity Facts

In 2014, obesity cost the U.S. economy $1.42 trillion—accounting for over 14% of healthcare expenditures—and contributed to approximately 320,000 deaths. A person with a BMI over 40 who manages to lose just 5% of their weight can save over $2,000 annually in healthcare-related costs.

Obesity is now the most widespread chronic illness in the country. According to the CDC, nearly 93.3 million adults in the U.S. were living with obesity between 2015 and 2016. Within a few decades, half of all American adults are projected to be affected.

Education plays a role too—higher levels of education are linked with lower obesity risk. Individuals with very high BMIs may lose as much as 8 years off their life expectancy, an impact similar to heavy smoking.

Frequently Asked Questions

What treatment options are available for managing obesity?

Obesity is best treated with a multi-layered, evidence-informed strategy, consisting of four foundational elements: dietary management, physical activity, behavior modification, and medication. Treatment plans are personalized. For example:

• Nutrition may involve structured meal plans, reduced portions, and avoiding certain food categories.
  
• Exercise programs can begin with basic activities such as walking and progress as the individual becomes more active.
  
• Behavioral therapy includes mindful eating practices and identifying non-hunger cues for eating.
  
• Medications approved specifically for obesity may be introduced by specialists when lifestyle changes alone are not sufficient. 

How is obesity categorized?

Obesity is divided into three categories based on BMI:

• Class 1: BMI from 30 to 34.9
  
• Class 2: BMI from 35 to 39.9
  
• Class 3: BMI over 40 

Although widely used, BMI does not consider body fat distribution or differences across ethnicities, which can limit its effectiveness in assessing true health risks.

How quickly should weight loss occur?

A healthy rate of weight loss is about 1 to 2 pounds per week. Though it can feel slow considering the effort involved, gradual and sustained loss is more likely to last. While some structured programs can safely accelerate this process, rapid diets rarely offer lasting results. The goal is to adopt sustainable habits that support long-term well-being, ideally under medical supervision.