children posing for the camera


Obesity is a chronic disease that affects millions of adults, adolescents, and children in the United States. Obesity results from a combination of genetic susceptibility, biological pathways related to metabolism and body weight regulation, and environmental factors.

Although obesity is often attributed to poor dietary habits and lack of physical activity, the truth is more complicated. The complex processes by which the body converts food into energy can be influenced by chemicals in the environment (over which we have limited control) and genetics (over which we have no control).


According to the World Health Organization, obesity has nearly tripled since 1975. In the U.S., 42% of adults were considered obsess (2017-2018). Among major ethnic groups, only non-Hispanic Asian men (17.4%) and women (17.2%) had rates significantly lower than the average. Some 18.5% of children and adolescents in the U.S. (2015-2016), about 13.7 million, were obese. Childhood obesity is more common among Hispanics (25.8%) and non-Hispanic blacks (22%) than other population groups. The Centers for Disease Control and Prevention (CDC) compiles state-level estimates of obesity prevalence among U.S. adults.

Health Effects From Obesity

People affected by obesity have a higher chance of developing serious health problems, including asthma, type 2 diabetes, high blood pressure, cardiovascular disease, stroke, and many cancers, which together are among the leading causes of preventable or premature death. Children with obesity may be at a greater risk for allergies.

Obesity is one of the signs of metabolic syndrome, along with high blood pressure, blood sugar, and cholesterol levels.

According to the CDC, overweight and obesity are associated with at least 13 types of cancer.

Several studies show that different immunological diseases can be more severe in obese patients. And those patients are resistant to therapies that are effective in lean patients. To gain insight into why this happens, researchers, partially supported by NIH, examined differences in the immune response between obese and lean mice with atopic dermatitis, an itchy skin condition also known as eczema. Their findings suggest a precision medicine approach, which targets the immune dysregulation caused by obesity, is warranted.

How Much Is too Much?

Weight that is higher than what is considered healthy for a given height is described as overweight or obese, according to the Centers for Disease Control and Prevention. Body Mass Index (BMI), a measure of weight for height is an indicator of body fatness.

A BMI score of 18.5 to 24.9 is considered normal. For adults, “overweight” is defined as a BMI of 25 to 29.9, and “obese” is a BMI range of 30 to 34.9. A score of 35 or above is “severely obese.” Note that BMI can be used as a screening tool but is not diagnostic of the health of an individual.

Measurements of body fat distribution—whether fat is carried around the hips or the abdomen—are increasingly being used along with BMI as indicators of obesity. These measurements include the waist-to-hip ratio.

For children and adolescents younger than 20, overweight and obese are based on BMI-for-age growth charts, available from the CDC.

How Is the Environment Linked to Obesity?

little girl holding a water bottle

Increasingly, researchers are questioning whether poor nutrition and lack of exercise are the only factors responsible for the obesity epidemic. Since most of the increase in obesity has occurred in the past 40 years, genetics alone are likely not to blame. Other factors in the environment or behavioral activities may be triggering the current obesity epidemic.

Endocrine disruptors. Many endocrine disruptors are synthetic chemicals. We are exposed to them daily through their use in plastics, pesticides, industrial and household products, flame retardants, and as ingredients in some cosmetics.

One major way chemicals can contribute to obesity is by interfering with our hormones. Hormones are the messengers in the body’s endocrine system, which plays a fundamental role in metabolism—the conversion of food to energy. Some endocrine disruptors have been linked to various diseases, including diabetes.

Obesogens. Obesogens are a broad class of chemicals that disrupt metabolism. Endocrine disruptors are a subset of obesogens. These chemicals do not directly cause obesity, but they may increase our susceptibility to weight gain, especially when the exposures occur during fetal development or early in life.

The most sensitive time for exposure to obesogens is during early development—as a fetus or during the first years of life—when the body’s weight control mechanisms are being developed.

An international group of researchers conducted a comprehensive analysis of many suspected environmental obesogens. Their study provides strong evidence for how smoking, air pollution, and characteristics of the built environment contribute to childhood obesity risk.

Examples of chemical compounds that may be obesogens are:

  • Bisphenol A (BPA), found in many plastic products including food storage containers, has been linked to childhood obesity.
  • Flame retardants are associated with adverse health effects in animals and humans.
  • Pesticides and insecticides, such as DDT, have been linked to obesity and associated disease.
  • Phthalates, chemicals used to make plastics more flexible, are also found in some food packaging, cosmetics, children’s toys, and medical devices.
  • Polychlorinated biphenyls (PCBs), industrial chemicals that were used widely in the past in products such as paints, cements, fluorescent light ballasts, sealants, and adhesives.
  • Tributyltin, widely used as a fungicide and heat stabilizer in polyvinyl chloride (PVC) piping, has been linked to transgenerational obesity.

Obesogens are believed to work in several ways, such as changing:

  • How a person’s fat cells develop, meaning they may increase fat storage capacity or the number of fat cells.
  • How the body regulates feelings of hunger and fullness, or magnify the effects of high-fat and high-sugar diets.

Air pollution. Many different chemicals are found in air pollution, which may be endocrine disruptors and obesogens.

Childhood obesity is associated with multiple environmental factors, including early life exposure to smoking and air pollution. NIEHS research has found links between traffic-related air pollution (TRAP) and negative effects on gut health, which has been linked to the development of type 2 diabetes in overweight and obese adolescents.

Air pollution exposure during childhood might contribute to poor diets. In one study, adolescents with higher exposures to TRAP as children were 34% more likely to eat foods high in unhealthy trans fats, regardless of household income, parent education level, or proximity to fast-food restaurants.

People living in poorer neighborhoods tend to be more exposed to air pollution. An NIEHS-funded study in New York City found that prenatal exposure to air pollution was linked to childhood obesity.

What is NIEHS Doing?

Grant recipients in the NIEHS Obesity and Diabetes program study related environmental topics and exposures during different life stages. A list and descriptions of NIEHS-funded research projects focused on obesity are found on our Who We Fund webpage. This research will help achieve a better understanding of the role of environmental factors in obesity and diabetes, which is necessary for developing prevention strategies.

NIEHS is part of the NIH Obesity Research Task Force, a multi-institute response to the epidemic. In 2018-2019, the Task Force confirmed ongoing challenges in its Strategic Plan.

NIEHS is a partner in the NIH-wide Environmental Influences on Child Health Outcomes (ECHO) study. The seven-year program will investigate how environmental factors affect child health and development.

Further Reading

Stories from the Environmental Factor (NIEHS Newsletter)

Printable Fact Sheets

Fact Sheets

Children’s Health – Why the Environment Matters

Endocrine Disruptors and Your Health

NTP Botanical Dietary Supplements Program


Additional Resources

  • Diet & Nutrition – From NIH and other federal agencies, find resources to help you improve your health and reduce disease risk.
  • National Library of Medicine Medline Plus: Obesity Links and definitions about obesity.
  • NIH Nutrition for Precision Health, powered by the All of Us Research Program This research will explore how different people respond to different diets. Its results will help individuals and their health care providers create healthy, precise, and effective diet plans.
  • NIH Obesity Research – As the nation’s biomedical research agency, the NIH seeks to gain insights into the factors contributing to obesity and to design and test strategies for prevention and treatment. This webpage includes information on the NIH Obesity Research Task Force.
  • NIH Strategic Plan for Nutrition Research – The first NIH-wide strategic plan to advance nutrition research across a wide range of areas.
  • Obesogens: An Environmental Link to Obesity – Read a summary of the science on chemicals linked to obesity in this 2012 Environmental Health Perspectives article.
  • Overweight and Obesity, Healthy People 2030 – Under a cross-agency federal framework, the Healthy People initiative provides science-based, 10-year national objectives for improving the health of all Americans.
  • Physical Wellness Toolkit – Watching what you put into your body, how much activity you get, and your weight are important for keeping your body working properly. Positive physical health habits can help decrease your stress, lower your risk of disease, and increase your energy.
  • The Interplay Between Environmental Chemical Exposures and Obesity: Proceedings of a Workshop (2016) – The National Academies of Sciences, Engineering, and Medicine held a workshop at NIEHS to explore the role that chemical exposures may play in the development of obesity. Participants considered new approaches that go beyond the standard focus on energy intake and expenditure to combat the multifactorial problem of obesity.

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