Obesity and overweight have emerged as major global health challenges, characterized by excessive accumulation of body that impairs health.

The etiology of obesity is multifactorial, encompassing complex interactions between genetic predispositions, behavioral patterns, environmental influences, and physiological factors.

Energy Imbalance and Lifestyle Factors

At its core, obesity results from a sustained energy imbalance where calorie intake exceeds energy expenditure. Modern lifestyles contribute heavily to this imbalance. Increased consumption of energy-dense, processed foods rich in sugars and unhealthy, combined with widespread physical inactivity due to sedentary occupations, extensive screen time, and transportation conveniences, fosters excessive caloric retention.

Even subtle lifestyle elements influence weight gain. For instance, inadequate sleep disrupts metabolic regulation, elevating hunger hormones and appetite. Chronic psychological stress elevates cortisol levels, promoting lipid storage and cravings for high-calorie “comfort” foods. These behavioral and physiological factors potentiate the cycle of overeating and weight gain.

Genetic and Epigenetic Influences

Genetic factors play a major role in determining susceptibility to obesity. Dozens — and likely hundreds — of genes influence how we regulate appetite, store lipid, and use energy. Twin and family studies consistently show that genetic makeup accounts for a large fraction of the variation in BMI. Variants in hormone‑signaling pathways, such as those involving leptin and ghrelin, can disrupt normal hunger and fullness cues, leading some individuals to require more food before feeling satiated.

Moreover, epigenetic changes triggered by early-life influences — including prenatal nutrition and environmental exposures — can “program” metabolic systems in ways that predispose individuals to lipid accumulation later in life. This combination of inherited genetic risk and epigenetic programming underscores why obesity is such a complex, multifaceted condition.

Environmental and Socioeconomic Factors

The external environment plays a critical role in shaping obesity risk. Urban design that lacks safe recreational spaces, sidewalks, and parks limits opportunities for physical activity. Food environments in lower‑income areas often favor cheaper, calorie‑dense, nutrient-poor options, pushing dietary choices toward patterns that promote weight gain. Socioeconomic status further compounds this risk. Individuals in lower‑income communities may face limited access to healthy food stores, educational resources, and health-promoting infrastructure, making healthy behaviors harder to sustain. Cultural norms and social networks also influence what and how people eat, as well as attitudes toward physical activity. These structural and social factors create a complex, interlocking system that drives obesity risk in disadvantaged populations.

Medical Conditions and Medications

Certain health conditions and pharmacological treatments contribute to weight gain. Endocrine disorders, such as hypothyroidism and Cushing syndrome, disrupt metabolic balance, favoring lipid accumulation. Neurological conditions and chronic pain may limit a person’s ability to exercise, indirectly promoting obesity. Various medications—including corticosteroids, some antidepressants, antipsychotics, and antiepileptics have side effects that increase appetite, alter metabolism, or encourage fluid retention, culminating in weight gain if not managed.

Hormonal and Metabolic Dysregulation

Obesity is often linked to abnormalities in hormonal regulation and metabolic pathways. Adipose tissue itself acts as an endocrine, secreting adipokines such as leptin, adiponectin, resistin, and inflammatory cytokines. In obese individuals, dysregulated secretion of these factors leads to chronic low-grade inflammation, insulin resistance, and alterations in glucose and lipid metabolism. This systemic imbalance perpetuates further lipid storage and can complicate weight loss efforts, creating a self-sustaining pathological state.

Fatima Cody Stanford, M.D., M.P.H. (Massachusetts General Hospital / Harvard Medical School): “Obesity is the only entity, illness, process, disease … where we accuse the patient of doing something wrong.”

Obesity and overweight arise from an intricate nexus of factors that include but are not limited to energy imbalance, genetic predisposition, environmental constraints, socioeconomic influences, medical conditions, and hormonal disturbances. Effective approaches to addressing obesity require comprehensive strategies encompassing lifestyle modification, environmental redesign, medical management, and public health policies tailored to individual and community needs.