What Causes Obesity?
Clinician's Perspective:
What can't be changed (non-modifiable risks)
• Genetic Predisposition: Research indicates that 40% to 70% of the variation in Body Mass Index (BMI) is attributable to genetic factors, rather than lifestyle choices alone.
• Time in the Womb: The intrauterine environment (the environment inside the womb) can trigger epigenetic changes (modifications that affect how genes work) that program a child’s metabolic future.
What can be changed (modifiable risks)
• Hormonal Disruption: Sleep deprivation of fewer than six hours per night alters the balance of leptin (the hormone that signals fullness) and ghrelin (the hormone that signals hunger).
• Chemical Influence: Exposure to Endocrine Disrupting Chemicals (EDCs) found in modern plastics can interfere with adipogenesis (the process by which new fat cells are formed).
• The Microbiome Factor: A decrease in the diversity of the gastrointestinal microbiome (the community of bacteria living in the gut) is consistently observed in individuals with higher levels of adiposity (body fatness).
• Pharmacological Weight Gain: Several classes of common medications, including corticosteroids and certain antidepressants, are documented to cause significant metabolic shifts and weight gain.
Historically, humans evolved to survive energy scarcity by efficiently storing calories. These ancient biological systems are now clashing with a modern, "obesogenic" environment.
Obesity is no longer viewed as a simple lack of willpower, but as a complex interplay between our evolutionary blueprint and contemporary surroundings.
Non-Modifiable Factors
The Genetic Blueprint
Researchers have identified over 100 genetic loci that influence how our bodies process energy. While DNA remains stable, "polygenic obesity" occurs when multiple small genetic variations make a person more susceptible to weight gain in environments where food is abundant. 40% to 70% of the variation in Body Mass Index (BMI) is attributable to genetic factors.
Early Years
The biological trajectory can begin in the womb. Fetal over-nutrition or under-nutrition can cause epigenetic modifications —biological "tags" that dictate genes switching on or off without changing DNA. These can permanently shift how offspring regulate glucose and store fat. In infants, every additional month of breastfeeding reduces obesity risk by 4%, while early introduction of solids (before 4 months) can increase it. The speed of weight gain in infancy and high birth weight are also established risk factors that set a long-term metabolic pace.
Ethnicity
Data reveals significant disparities in obesity prevalence among different groups. In Singapore, data from the National Population Health Survey (the most current data released into 2025) reflects an ethnic disparity in obesity prevalence. The Malay community continues to have the highest prevalence of obesity at 24.1%, followed by the Indian community at 15.2%, and the Chinese community at 9.3%
Modifiable Factors
Lifestyle: Diet, Physical Activity, and Sleep
The availability of inexpensive, ultra-processed foods drives overconsumption. Data suggests high-sugar diets induce hyperinsulinemia, shifting calories into fat cells and leaving the body feeling "starved" and hungry, a vicious circle leading to obesity.
Our diet changes the Gut Microbiome (the community of bacteria living in our gut). Modern diets low in fiber lead to dysbiosis (An imbalance in the gut microbial community). Abundance of bacteria group called Firmicutes, can increase energy harvesting from food and cause systemic inflammation. However, as of now, we don't have evidence that modifying the microbiome in humans can lead to any meaningful, sustainable weight loss.
Rapid urbanization has promoted sedentary jobs and passive transport. Increased physical activity remains a primary tool to decrease obesity risk, particularly for those with high genetic risk scores.
Short sleep duration (under 6–7 hours) elevates cortisol and disrupts satiety (the sensation of feeling full), which may lead to overeating.
Medical Conditions and Pollutants
Hypothyroidism (condition where there is insufficient levels of thyroxine) and Cushing Syndrome (a condition with abnormally high levels of steroids) are conditions that may lead to weight gain. Medications such as Antidepressants and steroids can alter metabolism or appetite. Endocrine Disrupting Chemicals in plastics and pesticides mimic hormones have been purported to affect body hormone systems potentially increasing the risk of development of obesity.
Socioeconomic Status
In high-income countries, deprivation is a significant driver. It has been reported that children in the most deprived areas are nearly three times more likely to experience severe obesity (4.6%) than those in the least deprived areas (1.3%). This may be because access to healthier lifestyle options often incur significant costs in high-income countries.
Conclusion
Obesity is a multifaceted condition where "calories in vs. calories out" is only one part of a much larger puzzle. While our genetic and intrauterine foundations provide the blueprint, modern environmental disruptors—from poor diet, lack of physical activity and sleep, to socioeconomic inequality—determine how those genes are expressed. Addressing the global rise in obesity requires shifting focus from individual blame to broad, culturally tailored health interventions that account for both biological susceptibility and environmental reality.
Evidence Strength: This review synthesizes a wide array of high-quality observational and mechanistic studies to provide a comprehensive overview of the multifaceted drivers of weight gain. Final Rating: ★★★★☆
Source: Read the full study
