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Overview

What Is Obesity?

Obesity is a chronic disease involving excess body fat that may affect health and well-being. It can affect people of any age, background, or body shape. Body size alone does not determine whether someone is healthy, and obesity should not be viewed as a personal failure. Health professionals consider a person’s overall health, medical history, and individual circumstances.

40.3%

U.S. adults had obesity in 2021–2023, according to CDC data.

21.1%

U.S. children and adolescents ages 2–19 had obesity in 2021–2023, according to CDC data.

1 in 8

People worldwide were living with obesity in 2022, according to WHO estimates.

How It's Diagnosed

How Is Obesity Diagnosed?

Clinicians often use body mass index, or BMI, as a screening measure calculated from weight and height. BMI can help identify patterns that may be associated with health risks, but it does not directly measure body fat or overall health. Clinicians do not use BMI alone to determine a person’s health or treatment needs. They interpret it alongside medical history, examination findings, and the person’s goals and circumstances.

CategoryAdult BMI rangeClinical context
Overweight25.0–29.9A screening range that may be associated with increased health risk in some people.
Class 1 obesity30.0–34.9A screening category; health risk varies with other conditions and measurements.
Class 2 obesity35.0–39.9A screening category that may indicate higher risk for some obesity-related conditions.
Class 3 obesity40.0 or higherA screening category sometimes called severe obesity; individual assessment is still needed.

Causes

What Causes Obesity?

Body weight is affected by the balance between energy taken in through food and energy used by the body, but this process is not simple. Appetite, metabolism, genetics, sleep, stress, medications, and the food and activity environment interact in different ways for each person. These influences can make weight gain more likely and can make obesity difficult to prevent or reverse through willpower alone.

BIOLOGICAL FACTORS

Genetics, appetite signals, metabolism, sleep, stress, age, and certain medical conditions can influence hunger, energy use, and how the body stores fat.

ENVIRONMENT AND SOCIAL FACTORS

Food availability, work and school routines, medication access, income, transportation, safe places to move, and chronic stress can shape eating and activity options.

Risk Factors

Risk Factors for Obesity

  • Genetics and family history can affect appetite, metabolism, body-fat distribution, and the likelihood of developing obesity.
  • Age-related changes in muscle, metabolism, hormones, and activity can alter obesity risk over time.
  • Insufficient or disrupted sleep can affect appetite hormones, energy, and eating patterns.
  • Chronic stress may influence sleep, appetite, food choices, and body-weight regulation.
  • A diet high in energy-dense foods or sugary drinks may increase risk, especially when nutritious choices are difficult to access.
  • Low physical activity can reduce energy use and may contribute to changes in muscle and metabolic health.
  • Some medicines, including certain steroids and medicines for mental health or seizures, can promote weight gain.
  • Medical conditions such as hypothyroidism, polycystic ovary syndrome, and hormonal disorders may affect weight or make weight management harder.
  • Pregnancy-related changes can affect body weight and may require individualized support before, during, or after pregnancy.
  • Social and environmental barriers, including limited income, food insecurity, unsafe neighborhoods, and limited healthcare access, can increase risk.

Complications

Health Complications of Obesity

  • Metabolic health: Obesity can increase the risk of insulin resistance and type 2 diabetes.
  • Heart and blood vessels: It is associated with high blood pressure, abnormal cholesterol, heart disease, and stroke.
  • Breathing and sleep: Excess body fat can contribute to obstructive sleep apnea and other breathing problems.
  • Bones and joints: Added strain and inflammation may increase the risk of osteoarthritis and mobility problems.
  • Liver health: Obesity can contribute to fatty liver disease and related liver inflammation.
  • Cancer: Obesity is associated with a higher risk of certain cancers, although risk varies by cancer type and person.
  • Reproductive health: It may affect fertility, pregnancy, menstrual cycles, and pregnancy-related complications.
  • Mental health: Weight stigma, chronic health problems, anxiety, and depression may affect emotional well-being.

Complications can occur at different body sizes, and risk is influenced by factors beyond BMI, including genetics, age, fitness, sleep, and existing medical conditions. Treatment can improve blood pressure, blood sugar, sleep, mobility, and quality of life even when a person does not reach a particular target weight.

Treatment & Management

Obesity Treatment and Long-Term Management

FOUNDATIONAL CARE

Nutrition support, regular physical activity adapted to ability, sleep care, stress support, and treatment of related conditions can improve health and support sustainable goals.

BEHAVIORAL SUPPORT

Behavioral therapy can help with eating patterns, activity, emotional triggers, routines, and problem-solving while keeping goals personalized and realistic.

Medications and Metabolic or Bariatric Surgery

Prescription anti-obesity medications may be considered for people who meet clinical criteria and need more intensive support. Metabolic or bariatric procedures may also be options for some people with obesity after an individualized discussion. Decisions should include expected benefits, risks, contraindications, monitoring, access, and personal preferences. These treatments work best with ongoing medical follow-up and nutrition, activity, and behavioral support.

Personalized care

Treatment should fit the person

Weight regain can occur and is not a personal failure. A clinician can help choose safe, sustainable goals and monitor related conditions over time.

Prevention

Can Obesity Be Prevented?

  • Choose a balanced eating pattern that includes a variety of nutritious foods when these choices are available and affordable.
  • Build regular movement into daily life in ways that match ability, health, and personal preferences.
  • Aim for adequate, consistent sleep and seek help for ongoing sleep problems.
  • Use supportive strategies for stress, such as counseling, social connection, relaxation, or enjoyable activities.
  • Limit sugary drinks when possible and choose water or unsweetened drinks more often.
  • Create family-based routines around meals, sleep, movement, and positive health behaviors without focusing on blame or weight stigma.
  • Seek early clinical support when weight, blood pressure, blood sugar, sleep, or mobility changes raise concerns.

Prevention is most effective when families and communities can access affordable nutritious foods, safe places to move, healthcare, and supportive policies. Individual habits matter, but no single behavior can prevent obesity in every person. In America and elsewhere, reducing obesity risk requires both personal support and healthier environments.

Outlook and Prognosis

Outlook and Prognosis for Obesity

Obesity is a chronic condition that may improve, recur, or change over time. Outcomes depend on a person’s health conditions, biology, access to treatment, environment, and sustained support. Long-term follow-up can help adjust treatment, address complications, and support health goals as circumstances change.

Next Steps After Diagnosis

Next Steps After an Obesity Diagnosis

  • Ask how BMI, waist measurement, and other findings were interpreted in the context of your overall health.
  • Review related conditions, laboratory results, and medicines that may affect weight or health.
  • Agree on patient-centered goals that focus on health, function, and quality of life.
  • Discuss treatment options, including nutrition and behavioral support, medicines, and procedures when appropriate.
  • Ask whether referrals to specialists, a dietitian, a behavioral health professional, or another service would help.
  • Schedule follow-up to monitor health conditions, treatment effects, goals, and needed adjustments.

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