The obesity epidemic in the United States is a complex public health challenge driven by environmental, economic, biological, and behavioral changes. Over four in ten U.S. adults currently live with obesity, a rate that has climbed significantly over the past decades. This severity is also reflected in younger populations, with nearly 20% of U.S. children and adolescents aged 2 to 19 having obesity. Understanding the causes requires examining the systemic forces that have altered the relationship between Americans and food, physical activity, and their environment.
Changes in the American Food Supply and Consumption
The composition of the American diet has shifted due to the widespread availability and consumption of ultra-processed foods (UPFs). These industrially manufactured products often contain little whole food ingredients and now account for approximately 55% of the total calories consumed by Americans. UPFs are engineered to be hyper-palatable, combining high levels of sugar, sodium, and unhealthy fats to encourage overconsumption.
This shift is compounded by “portion distortion,” where the size of a standard serving has dramatically increased over the last few decades. Marketplace food portions began growing in the 1970s, often exceeding federal dietary guidance standards. For example, a typical bagel from 1980 was about half the size and calorie count of an average bagel sold today.
The expansion of serving sizes, particularly in the fast-food and restaurant industries, trains consumers to eat more without corresponding satiety signals. Studies show that people unknowingly eat significantly more food when served a larger portion, demonstrating that portion size directly influences total caloric intake. This increased energy consumption, driven by both food type and volume, is a primary factor in the energy imbalance underlying weight gain.
The Role of Sedentary Behavior and Physical Environment
The modern American lifestyle is characterized by a decline in daily energy expenditure, creating an environment that discourages physical movement. This reduction in activity occurs across leisure, occupational, and domestic settings. Jobs that once required manual labor have largely been replaced by desk-based, low-activity occupations, lowering the calories burned during the workday.
The rise of digital technology has simultaneously fueled an increase in sedentary screen time for both adults and adolescents. Between 2007 and 2016, the estimated total sitting time for adults increased from 5.5 to 6.4 hours per day. Recreational computer use outside of school or work also rose significantly across all age groups during this period.
The physical design of communities further contributes to this “obesogenic environment,” particularly the reliance on car-centric infrastructure. Many communities lack safe sidewalks, bike lanes, and mixed-use zoning, forcing residents to drive for most errands and commutes. Research indicates a direct correlation between this infrastructure and body weight. Each additional hour spent driving per day is associated with an estimated 6% increase in the likelihood of obesity, while high walkability is associated with lower obesity rates.
Economic and Systemic Policy Drivers
Government policies and economic structures shape the food landscape and drive obesity rates. Agricultural subsidies in the United States overwhelmingly favor commodity crops like corn, soy, and wheat over fruits and vegetables. This policy lowers the market price of ingredients used in calorie-dense, ultra-processed foods, such as high-fructose corn syrup and soy-based oils. Consequently, unhealthy food options become the cheapest and most abundant choices for consumers.
This economic reality creates significant disparities in food access, particularly in low-income areas. These neighborhoods often become “food swamps,” defined by an overabundance of fast-food outlets and convenience stores offering high-calorie, nutrient-poor options. Studies suggest that the presence of a food swamp may be a stronger predictor of obesity rates than the mere absence of grocery stores, known as a food desert.
Aggressive food marketing further exploits these disparities by disproportionately targeting vulnerable populations. Food companies spend billions promoting unhealthy products to Black and Hispanic youth. Black youth and adults, for example, view up to 21% more commercials for unhealthy foods and beverages than their white peers. This targeted advertising, often utilizing cultural cues and celebrity endorsements, exacerbates existing inequalities in diet-related disease risk.
Biological and Genetic Predispositions
While environmental factors set the stage, individual biology dictates the response to the modern food environment. Obesity has a clear genetic component, with hundreds of genes influencing appetite regulation, metabolism, and fat storage. This predisposition makes certain individuals more susceptible to weight gain when exposed to an energy-dense, sedentary environment.
The interaction between genes and the environment is often mediated by epigenetics—modifications to gene expression without changing the underlying DNA sequence. Environmental factors, such as poor diet and physical inactivity, can alter these epigenetic “switches,” influencing metabolism and increasing obesity risk. These changes can be transmitted across generations, affecting the metabolic health of offspring.
Certain common prescription medications also contribute to weight gain by interfering with metabolic processes or appetite signals. Classes of drugs like antipsychotics, antidepressants, and some diabetes medications frequently list weight gain as a side effect. These medications can stimulate appetite, reduce the metabolic rate, or alter how the body stores glucose, complicating weight management for individuals treating chronic health conditions.
The American obesity epidemic is not a simple problem of individual willpower but a systemic outcome of intertwined forces. Solutions must be multi-layered, addressing changes in the food supply, the physical environment, economic drivers, and the complex biological susceptibility of the population. Systemic adjustments across these areas are necessary to reverse long-term trends and mitigate the rising health burden.