Obesity

Obesity is a chronic disease characterised by excessive body fat that impairs health. It arises from a sustained positive energy balance shaped by genetic, hormonal, behavioural, and environmental factors, and it raises the risk of type 2 diabetes, cardiovascular disease, and several cancers. It is increasingly understood as a biologically defended state rather than a simple consequence of personal choice.

Definition

Obesity is an abnormal or excessive accumulation of body fat that presents a risk to health; in adults it is commonly defined operationally by a body mass index of 30 kg/m2 or higher, while recognising that body-mass index is an imperfect surrogate for adiposity and its distribution.

Scope

The topic covers the definition and measurement of obesity, its multifactorial causes, the pathophysiology that links excess fat to metabolic disease, its epidemiology, and the evidence base for prevention and intervention. It is presented as a reference overview and does not provide individualised diagnostic or treatment advice.

Core questions

• How is obesity defined and measured, and what are the limits of body mass index?
• Why is obesity a chronic, relapsing disease rather than a transient state?
• How does excess adipose tissue drive insulin resistance and cardiometabolic disease?
• What is the evidence for lifestyle, pharmacological, and surgical approaches at the population level?

Key concepts

• Body mass index and its limitations
• Visceral versus subcutaneous adiposity
• Positive energy balance and defended body weight
• Adipose tissue as an endocrine and immune organ
• Metaflammation (chronic low-grade inflammation)
• Obesity as a chronic relapsing disease

Mechanisms

Obesity develops when energy intake chronically exceeds expenditure, but the susceptibility to that imbalance is strongly modulated by biology: hypothalamic circuits defend a body-weight set point, and weight loss provokes hormonal and thermogenic adaptations that promote regain. Expanded adipose tissue, particularly visceral fat, becomes dysfunctional and infiltrated by immune cells, generating a chronic low-grade inflammatory state (metaflammation) and releasing free fatty acids and adipokines that impair insulin signalling. This links excess adiposity mechanistically to insulin resistance, type 2 diabetes, dyslipidaemia, hypertension, and cardiovascular disease.

Clinical relevance

Obesity is among the most prevalent chronic diseases and a major driver of cardiometabolic morbidity, so understanding its definition, biology, and evidence base is central to the health sciences. This entry summarises that knowledge for educational reference and is not a basis for individual diagnosis, drug selection, or dosing decisions.

Epidemiology

The prevalence of obesity has risen sharply over recent decades across high-, middle-, and low-income countries, affecting both adults and children, and it is a leading modifiable contributor to type 2 diabetes, cardiovascular disease, and certain cancers, with substantial associated health-system burden.

Evidence & guidelines

Evidence spans randomised lifestyle-prevention trials, controlled-outcome studies of bariatric surgery, and integrative reviews of obesity management. Landmark data include lifestyle and metformin prevention of type 2 diabetes in high-risk adults and long-term outcome studies of surgical weight loss; this entry describes the evidence landscape rather than issuing recommendations.

History

For much of the twentieth century obesity was framed primarily as a behavioural problem. The discovery of leptin and hypothalamic appetite circuits in the 1990s, together with evidence that energy expenditure adapts to defend body weight, reframed obesity as a chronic disease with strong biological underpinnings. Outcome studies of bariatric surgery and lifestyle-prevention trials subsequently shaped the modern evidence base, and major bodies now classify obesity as a disease.

Debates

Is body mass index an adequate definition of obesity?

Body mass index is a convenient population measure but does not distinguish fat from lean mass or capture fat distribution, prompting debate over complementary measures and over diagnosing obesity by health impairment rather than a single threshold.

Why is intentional weight loss so often regained?

Biological defence of a body-weight set point through appetite and energy-expenditure adaptations helps explain the high rate of weight regain, supporting the view of obesity as a chronic, relapsing condition.

Key figures

• George Bray
• Lars Sjostrom
• Gokhan Hotamisligil
• Michael Schwartz

Seminal works

• morton-2006
• sjostrom-2007
• bray-2016

Frequently asked questions

Is obesity a disease or a lifestyle issue?

Major medical bodies classify obesity as a chronic disease; while energy balance and behaviour matter, susceptibility is strongly shaped by genetic, hormonal, and environmental factors, and the body biologically defends an elevated weight.

How does carrying excess fat lead to diabetes and heart disease?

Dysfunctional, especially visceral, adipose tissue produces chronic low-grade inflammation and releases factors that impair insulin signalling, linking excess fat to insulin resistance, type 2 diabetes, and cardiovascular disease.