Obesity: causes, measurement, health effects, prevention and treatment

Definition and measurement

Body mass index (BMI) is the most widely used screening tool to categorize weight status in adults. BMI equals weight in kilograms divided by height in metres squared; this simple ratio provides thresholds commonly used in clinical practice. A BMI under 18.5 is typically regarded as underweight, 18.5–24.9 as a normal range, 25–29.9 as overweight, and 30 or above as obese. For many adults the BMI is a useful population-level indicator, but it has known limits: it does not distinguish fat from muscle and may misclassify very muscular people.

Because of these limitations, clinicians may use additional measures such as waist circumference, waist-to-hip ratio, body fat percentage, or imaging in particular circumstances. Growth charts and age- and sex-specific percentiles are used to assess excess weight in children rather than the adult BMI categories; consult pediatric guidelines when evaluating younger people (children), and use adult standards only for fully grown individuals (adults).

Causes and contributing factors

Obesity arises from a complex interaction of factors that influence energy intake, energy expenditure, and how the body stores fat. Common contributors include:

• Persistent energy imbalance: consuming more calories than are burned.
• Dietary patterns high in energy-dense, nutrient-poor foods and sugar-sweetened beverages.
• Physical inactivity and sedentary lifestyles.
• Genetic predisposition and family history affecting appetite and metabolism.
• Environmental and social influences such as food access, urban design, and socioeconomic status.
• Certain medications and medical conditions that alter weight.

Health consequences and importance

Excess body fat increases the risk of multiple chronic conditions. Well-established associations include type 2 diabetes, coronary heart disease, stroke, hypertension, and several types of cancer. Obesity can also contribute to musculoskeletal problems (for example, osteoarthritis), sleep-disordered breathing, liver disease, and reduced quality of life. Psychological and social harms, including stigma and reduced mental well-being, are common and may hinder efforts to change health behaviors.

Prevention, management and treatments

Prevention strategies emphasize healthy eating, regular physical activity, supportive environments, and policies that make healthier choices easier at the population level. Individual management typically begins with behavioral interventions: diet modification, increased physical activity, and structured lifestyle programs. When lifestyle approaches do not achieve sufficient benefit, medical options include pharmacotherapy for weight loss and, for people with severe obesity or obesity with serious complications, bariatric (weight-loss) surgery. Decisions about treatment are individualized and consider risks, benefits, and patient preferences.

Long-term maintenance is often the greatest challenge; sustainable changes, social support, and monitoring help reduce weight regain. Public health initiatives aimed at food systems, transport, education, and income inequality are important components of broader prevention efforts.

History, trends and notable distinctions

Global prevalence of obesity has risen notably over recent decades in many regions, prompting greater attention from clinicians, researchers, and policymakers. The term "epidemic" is used to describe this rapid rise, though patterns vary by country and community. Important distinctions in assessment include the different methods used for children versus adults, and the recognition that BMI is one of several tools rather than a definitive measure of individual health. Addressing obesity requires both clinical care for affected individuals and societal changes to reduce risk across populations.

Further resources and clinical guidance can be found through organizations and specialist services; for basic background on weight categories see data and explanations of BMI and related concepts (BMI details). Additional reading on public health approaches and treatment options is available from medical societies and health authorities (overview, disease classification, epidemic context, adult standards, child growth charts).