What Is Obesity? Causes, Measurement, and Modern Treatment

Defining Obesity: Beyond BMI

Body mass index (BMI)—calculated as weight in kilograms divided by height in meters squared—is the most widely used tool for classifying body weight at a population level. Overweight is defined as a BMI of 25–29.9 kg/m2, and obesity as a BMI of 30 kg/m2 or above, with Class I (30–34.9), Class II (35–39.9), and Class III (40 or above, sometimes called severe or morbid obesity). Despite its ubiquitous use, BMI has significant limitations as an individual diagnostic tool.

BMI does not distinguish between fat mass and lean mass—a muscular athlete may be classified as overweight or obese despite minimal body fat. Conversely, individuals within a 'normal' BMI range can have high proportions of visceral (abdominal) fat and metabolic abnormalities—a condition sometimes called 'metabolically obese, normal weight.' BMI also fails to account for ethnic differences in body composition; South Asian populations, for example, carry greater metabolic risk at lower BMI values, leading some guidelines to set lower thresholds (23 for overweight, 27.5 for obesity) for Asian populations. More precise measures include waist circumference, waist-to-hip ratio, and body fat percentage measured by DEXA, bioelectrical impedance, or hydrostatic weighing.

Types of Adipose Tissue

Not all body fat is metabolically equivalent. Subcutaneous fat—stored beneath the skin, particularly in the thighs, buttocks, and hips—has relatively benign metabolic effects and may even serve some protective functions. Visceral fat—stored deep in the abdominal cavity surrounding the liver, pancreas, and intestines—is highly metabolically active and closely linked to insulin resistance, inflammation, and cardiovascular risk. Visceral fat releases free fatty acids directly into the portal circulation (which drains to the liver), promoting hepatic fat accumulation and impairing insulin signaling.

Ectopic fat refers to fat deposited within or around organs where it does not normally accumulate: in the liver (hepatic steatosis, or 'fatty liver'), in skeletal muscle (intramyocellular lipid), around the heart (pericardial fat), and in the pancreas. Pancreatic fat is particularly relevant to type 2 diabetes, as it impairs beta-cell function. Brown adipose tissue (BAT) is a thermogenic fat that burns energy to generate heat and is found primarily in the neck and supraclavicular regions; its activity is reduced in obesity. Research into BAT activation as a therapeutic target for obesity is ongoing. Beige adipocytes are white fat cells that can be induced to take on brown fat-like thermogenic properties through cold exposure or exercise.

Metabolic Syndrome

Metabolic syndrome is a cluster of interrelated metabolic abnormalities that commonly co-occur with central obesity and dramatically increase the risk of type 2 diabetes and cardiovascular disease. The harmonized international definition requires any three of: central obesity (waist circumference above population-specific thresholds); elevated triglycerides (≥1.7 mmol/L or drug treatment); reduced HDL cholesterol (below 1.0 mmol/L in men, 1.3 mmol/L in women, or drug treatment); elevated blood pressure (≥130/85 mmHg or drug treatment); and elevated fasting glucose (≥5.6 mmol/L or drug treatment for type 2 diabetes).

Metabolic syndrome is present in approximately 25–35% of adults in developed countries and up to 80% of adults with severe obesity. Its pathophysiology centers on insulin resistance and chronic low-grade inflammation—adipose tissue in obesity secretes pro-inflammatory adipokines (including TNF-alpha, IL-6, and leptin) while producing less of the anti-inflammatory adiponectin, creating a systemic inflammatory state that impairs insulin signaling throughout the body.

Causes: A Multifactorial Disease

Obesity results from a sustained imbalance in which energy intake exceeds energy expenditure, causing excess energy to be stored as adipose tissue. However, framing obesity purely as a matter of willpower or individual choices ignores the complex biological, environmental, and social determinants that strongly influence both sides of the energy balance equation. Genetic factors account for 40–70% of BMI variability, with genome-wide association studies identifying over 900 genetic loci associated with BMI, many near genes involved in appetite regulation, reward pathways, and energy metabolism.

The gut microbiome—the trillions of microorganisms inhabiting the intestines—influences energy harvest from food, gut hormone secretion, inflammation, and appetite regulation. Studies show that obese individuals have characteristically altered gut microbiome composition, with animal models demonstrating that transplanting gut microbiota from obese to germ-free mice can transmit an obese phenotype. The modern obesogenic environment—characterized by ubiquitous ultra-processed food, large portion sizes, sedentary occupations, car-dependent transport, and food marketing—creates conditions that override biological satiety signals for a large proportion of the population.

Treatments: Lifestyle, Surgery, and GLP-1 Drugs

Lifestyle interventions—combining caloric restriction, increased physical activity, and behavioral support—remain the foundation of obesity management and can achieve meaningful weight loss of 5–10% in motivated patients. Even modest weight loss of 5–10% produces clinically significant improvements in blood pressure, lipids, blood glucose, and sleep apnea. However, long-term weight maintenance after dietary intervention is challenging, as physiological adaptations (including reduced resting metabolic rate and elevated hunger hormones) conspire to restore lost weight—explaining why the majority of dieters regain weight over 3–5 years.

Bariatric surgery—particularly Roux-en-Y gastric bypass and sleeve gastrectomy—achieves the greatest and most durable weight loss, with average excess weight loss of 60–80% and dramatic improvements in or remission of comorbidities including type 2 diabetes, hypertension, and sleep apnea. The most significant pharmacological advance in decades is the class of GLP-1 receptor agonists—originally developed for type 2 diabetes—which have demonstrated unprecedented weight-loss efficacy. Semaglutide 2.4 mg (Wegovy) achieved an average 15% weight loss in trials, tirzepatide (a dual GIP/GLP-1 agonist, sold as Zepbound) achieved up to 22% average weight loss—approaching bariatric surgery outcomes—and represents a new era of pharmacological obesity treatment that is reshaping the field.