Overweight: definition, measurement, causes, health effects and social aspects

How overweight is measured and its limitations

The most widely used screening tool is the body mass index (BMI), a ratio of weight to height. For adults, a BMI of about 25 or greater is typically classified as overweight and a BMI of 30 or greater as obese. These cutoffs were standardised internationally in the late 1990s. Other measurements include waist circumference, waist-to-hip ratio and assessments of body composition that estimate percentage of body fat. Simple anthropometric measures are useful for large studies and routine checks, but they do not perfectly reflect body composition, fitness, or the distribution of fat—factors that influence health.

Causes and prevalence

Overweight arises from a long-term imbalance between energy intake and energy expenditure, but its causes are multifactorial. Contributing factors include diet quality, physical activity, genetics, age, certain medications, socioeconomic conditions, and environmental influences that affect food availability and lifestyle. Stress, sleep patterns and medical conditions can also play roles. Animals and children can also become overweight when caloric intake exceeds needs.

• Environmental and social drivers: urbanisation, food marketing and reduced physical activity.
• Biological drivers: heredity, metabolic differences and some endocrine disorders.
• Behavioural drivers: long-term eating patterns and sedentary behaviour.

Globally, a substantial proportion of adults are affected; estimates commonly cited place the number of overweight adults in the hundreds of millions to more than a billion, and prevalence has risen in many regions over recent decades.

Health consequences and notable research findings

Being overweight is associated with higher risks for several chronic conditions, including cardiometabolic diseases such as type 2 diabetes, hypertension, dyslipidaemia and some cancers, and can worsen outcomes for other health problems. The magnitude of risk grows with greater excess weight. However, important nuances exist. Some people with higher BMI appear to be metabolically healthy, and some studies have observed an "obesity paradox" in which modestly higher BMI is not always linked to increased mortality in certain groups. For example, systematic reviews and pooled analyses have reported varying relationships between BMI categories and overall mortality, suggesting that BMI alone is an imperfect predictor of individual prognosis. Clinicians therefore consider additional measures of metabolic health, fitness and fat distribution when assessing risk.

Research findings are often reported with caveats: associations do not prove causation, and population averages do not determine individual outcomes. A range of studies—large observational cohorts and meta-analyses—inform guidelines, and readers should interpret single results cautiously. For more on research methods and summaries, see study reviews and public health resources that aggregate evidence.

Management, prevention and social considerations

Clinical advice commonly recommends lifestyle changes to achieve weight loss when appropriate: modest calorie reduction, improved diet quality, regular physical activity and behavioural support. In some cases, medications or surgery are considered for people with significant obesity or obesity-related illness. Many people attempt diets and supplements, but sustained weight loss is challenging and regains are common; maintenance strategies and long-term support improve outcomes. Alternative and complementary products such as herbal supplements are marketed for weight control, but their effectiveness and safety vary.

Beyond health, cultural attitudes shape how overweight people are treated. Weight bias and stigma—sometimes called fatphobia—can lead to discrimination and psychological harm. Addressing overweight at a population level therefore requires both medical approaches and attention to social determinants of health, access to nutritious food and safe places to be active.

For further context on physiology and measurement, see summaries about the human body, reviews of health outcomes, and introductory guides to anthropometric methods (body fat estimates).