What Causes Obesity?
Updated on March 31, 2026
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Obesity is a chronic disease that occurs when a person has excess body fat, resulting in a body mass index (BMI) of 30 or higher. A variety of factors play a role in obesity risk, and understanding the potential underlying causes of obesity is essential to help individuals lose weight and improve their overall health.
“The causes and risk factors for obesity are multifactorial: genetics, environment, lifestyle, and even socioeconomic status,” says Mir Ali, MD, a bariatric surgeon and the medical director of MemorialCare Surgical Weight Loss Center at Orange Coast Medical Center in Fountain Valley, California.
Below are the most common causes of obesity.
Diet Quality
When we eat, our bodies turn food into the energy that’s used to fuel everyday functions and physical activity. Unused energy is stored as fat. If you regularly take in significantly more energy than your body uses each day, you may be at risk for developing obesity.
Whole foods like vegetables, fruits, whole grains, dairy, fish, nuts, and lean meats are both more nutrient dense and more likely to keep you feeling full longer than processed, packaged foods, such as bagged snacks, processed meats, and desserts.
“Processed foods are high in sugar and fat and low in fiber, which makes it easy to overeat,” says Lisa Young, PhD, RDN, a New York City–based nutrition consultant and author. She also points to larger portion sizes in general as a contributor to overeating.
Highly processed foods are formulated to be palatable, often leading to the rapid eating and overconsumption of energy-dense, nutrient-poor foods, which is linked to weight gain. These foods then displace minimally processed, more nutritious options in person’s diet.
According to data from the National Health and Nutrition Examination Survey, 53 percent of an average U.S. adult's calories are supplied by ultra-processed foods, versus nearly 62 percent for children age 1 to 18.
Lifestyle
Lifestyle factors, including lack of exercise, excessive time sitting during the day, and insufficient quality sleep, can influence obesity risk.
Regular physical activity allows the body to use the energy it takes in through food. If your activity level aligns with your calorie intake, then you may have a reduced risk for obesity. Conversely, people who spend most of the day sitting, whether due to a sedentary job or lack of movement in their leisure time, may not get the recommended amount of physical activity and therefore have an increased obesity risk.
The U.S. Department of Health and Human Services recommends the following activity guidelines:
- Children ages 3 to 5 should be physically active throughout the day.
- Children and adolescents ages 6 to 17 should get one hour or more of moderate to intense physical activity each day.
- Adults should get at least 150 minutes of moderate to intense physical activity throughout the week.
Not getting enough quality sleep may also contribute to increased weight gain and obesity. Sleeping less than seven hours each night can affect hormones that regulate hunger, increasing appetite and making it more difficult to recognize your body’s satiety signals.
Genetics
Genetics can play a role in determining whether someone develops obesity by affecting metabolism, appetite, cravings, and body fat distribution. How much genetics influence obesity risk varies from person to person, but often genes are just one piece of the puzzle, with behavior and environment also being contributing factors.
When obesity is caused by a specific disorder, such as Bardet-Biedl syndrome or Prader-Willi syndrome (both of which are rare genetic conditions), genes do play a direct role.
Environment
Your environment — including where you live, work, and play, and the people you interact with on a regular basis — can impact your obesity risk.
For example, if you work in an office and live in a place where you have to drive rather than walk to get around, you may be less likely to get regular physical activity. On the flip side, high neighborhood walkability (how easy it is to get around an area on foot) is associated with lower rates of obesity.
Environmental substances like air pollution and chemicals found in items you use in your home can also play a role. In fact, exposure to air pollution in adolescence is linked to an increased obesity risk.
Perfluoroalkyl substances (PFAS), synthetic chemicals used in various packaging and household items, are considered potential obesogens (compounds that disrupt the body’s endocrine system). PFAS accumulate in the body over time, and PFAS exposure is correlated with an increased obesity risk, notably affecting children and pregnant people.
Social Determinants
Social determinants of health are circumstances beyond your control that impact your life and surrounding community. Socioeconomic status is a social determinant that can contribute to obesity. Low-income communities often lack access to healthy food sources, leading to higher consumption of highly processed foods. They also tend to lack access to safe places to exercise outdoors.
There appears to be a link between education and obesity: People with college degrees are less likely to have obesity.
Stress
Stress can contribute to obesity by prompting changes in your eating patterns and hormones. While short periods of stress won’t necessarily lead to weight gain, long-term (or chronic) stress can increase your risk for obesity. One factor at play is cortisol, a stress hormone that can increase appetite and signal your body to store more fat.
Other Medical Conditions
Childhood trauma and mental health diagnoses like anxiety and depression are linked to an increased risk of obesity.
Other medical conditions associated with weight gain, which could potentially lead to obesity, include the following:
- Bardet-Biedl syndrome
- Cushing’s syndrome
- Polycystic ovary syndrome
- Prader-Willi syndrome
- Underactive thyroid
Certain Medications
Some medications can cause weight gain as a side effect, increasing appetite as a result of disrupting the hormones that regulate hunger. Over time, these medications may cause weight gain and, ultimately, contribute to obesity:
- Antipsychotics
- Beta-blockers (a type of high blood pressure medication)
- Glucocorticoids (steroid hormones used to reduce inflammation and immune response)
- Insulin (a hormone that helps regulate blood sugar
- Some antidepressants
- Some forms of birth control
- Some medications used to treat chronic pain
If you suspect that a medication you’re taking is causing weight gain, talk to your healthcare provider about your concerns. They may be able to recommend an alternative that doesn’t impact your weight.
The Takeaway
- Obesity is a complex, chronic disease influenced by a combination of dietary choices, such as high consumption of ultra-processed foods, and lifestyle factors like physical inactivity and poor sleep.
- Genetic predispositions can also affect a person’s metabolism and appetite, while environmental factors, including neighborhood walkability and exposure to certain chemicals and pollutants, further impact one’s obesity risk.
- Social determinants, such as low socioeconomic status and limited access to healthy foods, alongside chronic stress, can trigger hormonal changes that lead to increased fat storage.
- Underlying medical conditions, such as hypothyroidism, and side effects from common medications like antidepressants and beta-blockers, can also contribute to the development of obesity.
FAQ
What is the real cause of obesity?
Obesity is caused by a chronic imbalance between the amount of energy you consume and the amount of energy your body uses, although this imbalance can be driven by a complex combination of factors.
What is the worst food for weight gain?
Ultra-processed foods tend to be the main culprit of unwanted weight gain due to their high energy density, poor nutrition profile, and formulations that encourage overeating and disrupt the body’s appetite regulation.
What foods should you eat to avoid obesity?
Prioritize whole and minimally processed foods that contain plenty of protein, dietary fiber, and other essential nutrients. Whole grains, fruits and vegetables, lean meats and fish, dairy, and nuts and seeds can all be included in a balanced diet.
Can obesity be reversed?
Yes, obesity can be reversed through healthy, sustained weight loss. Successfully managing obesity-related health conditions can also help reverse obesity.
Resources We Trust
- Cleveland Clinic: This Is How to Lose Weight the Right Way — and Keep It Off
- Harvard Health Publishing: Hidden Causes of Weight Gain
- Mayo Clinic: Weight Loss: 6 Strategies for Success
- National Heart, Lung, and Blood Institute: Overweight and Obesity: Causes and Risk Factors
- Centers for Disease Control and Prevention: Risk Factors for Obesity
EDITORIAL SOURCES
Everyday Health follows strict sourcing guidelines to ensure the accuracy of its content, outlined in our editorial policy. We use only trustworthy sources, including peer-reviewed studies, board-certified medical experts, patients with lived experience, and information from top institutions.
Resources
- How Overweight and Obesity Impacts Your Health. Centers for Disease Control and Prevention. December 4, 2025.
- Putting Processed Foods Into Perspective. Harvard Health Publishing. April 1, 2024.
- Dicken SJ et al. Ultra-processed Food and Obesity: What Is the Evidence? Current Nutrition Reports. January 31, 2024.
- Ultra-Processed Food Consumption in Youth and Adults: United States, August 2021–August 2023. National Center for Health Statistics. August 2025.
- Mageit M et al. The Association Between Physical Activity and Obesity: A Systematic Review. Cureus. December 12, 2025.
- What Are the Risks of Sitting Too Much? Mayo Clinic. March 26, 2025.
- Physical Activity Guidelines for Americans 2nd Edition. U.S. Department of Health and Human Services. 2018.
- Causes and Risk Factors. National Heart, Lung, and Blood Institute. March 24, 2022.
- Ahmed SK et al. Obesity: Prevalence, Causes, Consequences, Management, Preventive Strategies and Future Research Directions. Metabolism Open. June 14, 2025.
- Risk Factors for Obesity. Centers for Disease Control and Prevention. November 14, 2025.
- Wang ML et al. Higher Walkability Associated with Increased Physical Activity and Reduced Obesity among U.S. Adults. Obesity. December 12, 2022.
- de Bont J et al. Ambient Air Pollution and the Development of Overweight and Obesity in Children: A Large Longitudinal Study. International Journal of Obesity. February 24, 2021.
- Lewis N et al. Per- and Polyfluoroalkyl Substances (PFAS) as Emerging Obesogens: Mechanisms, Epidemiological Evidence, and Regulatory Challenges. Physiologia. December 16, 2024.
- Social Determinants of Health. Office of Disease Prevention and Health Promotion.
- Anekwe CV et al. Socioeconomics of Obesity. Current Obesity Reports. July 6, 2020.
- Obesity and Severe Obesity Prevalence in Adults: United States, August 2021–August 2023. Centers for Disease Control and Prevention. September 2024.
- You Guessed It: Long-Term Stress Can Make You Gain Weight. Cleveland Clinic. March 1, 2023.
- Chu WW et al. Adverse Childhood Experiences and Development of Obesity and Diabetes in Adulthood—A Mini Review. Obesity Research and Clinical Practice. March-April 2021.
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