Obesity in adults is defined as a body mass index of 30 or above. Body mass index (BMI) is determined using a person's weight and height to calculate an estimate of the amount of fat in that person's body. For children and adolescents, the Centers for Disease Control and Prevention (CDC) defines obesity as "a BMI at or above the ninety-fifth percentile for children and teens of the same age and sex." Obesity is not measured against a reference population. Obesity places stress on the body and is associated with higher risks for other health problems, such as heart disease, diabetes, liver disease, high blood pressure, infertility, gallbladder disease, depression, several forms of cancer, stroke, and sleep apnea. During pregnancy, obesity and excessive weight gain increase the likelihood of miscarriage, gestational diabetes, preeclampsia, and complications during labor. As stated by the American College of Obstetricians and Gynecologists, children born to obese mothers face an increased likelihood of birth defects and may encounter related issues throughout their development and into adulthood.
According to the World Health Organization (WHO), rates of worldwide obesity nearly tripled between 1975 and 2016, particularly among children and adolescents. Using data from the CDC's 2017–2020 National Health and Nutrition Examination Survey, the public policy advocacy organization Trust for America's Health determined in The State of Obesity 2022 that obesity affects 41.9 percent of US adults and 19.7 percent of children ages two to nineteen. Obesity rates have risen substantially since 1990, when no state had an obesity rate over 15 percent. In 2021, forty-one states had adult obesity rates above 30 percent. Obesity rates in the United States are highest among non-Hispanic Black adults (49.9 percent) and adults of Hispanic ethnicity (45.6 percent), as well as among adults living in rural areas (34.2 percent).
Though rising obesity rates indicate a national public health issue, health care professionals discourage a singular focus on weight or body mass, stressing instead the importance of monitoring other health indicators. Many body positivity activists contend that a person can more effectively address health concerns by accepting their size and concentrating on improving overall health. Alternatively, some health care professionals have warned against normalizing obesity, as doing so can deter obese people from seeking medical help.
Body Mass Index
Though health care professionals, fitness experts, and nutritionists commonly use BMI to appraise how a person's body size relates to overall health, BMI, like all measurements, has limitations. Relying too much on BMI can result in misleading conclusions because bodies store fat differently depending on a person's age, sex, and overall body composition. An adult woman who is five feet, four inches tall (162.56 centimeters) would have a BMI of 30 at 175 pounds (79.38 kilograms), placing her in the obese category. In comparison, an adult woman who is five feet, seven inches tall (170.18 centimeters) and the same weight of 175 pounds would have a BMI of 27 and would be categorized as overweight rather than obese. People with a BMI between 25 and 29.9 are considered overweight, while people with a BMI under 18.5 are considered underweight.
Critics warn against the use of BMI as a measurement of an individual's relative health. BMI, they note, does not provide an accurate measurement of body fat. Muscle is denser than fat, with the volume of a pound of muscle measuring about 80 percent the volume of a pound of fat. This means that many athletes in good health would classify as overweight based on BMI alone. The BMI rating system also fails to consider where fat is concentrated in a person's body. For example, though excess fat around the midsection is associated with more health complications than fat in other areas, this could go unnoticed in a person with a healthy BMI.
A BMI in the healthy range also does not guarantee that a person is protected from the physical health concerns associated with obesity. Other indicators and metrics may better determine whether a person is unhealthy. Further, when scientists first developed the BMI, they based their conclusions on people of European background, leading to inaccurate BMI-based risk assessments for patients of different backgrounds. For example, researchers have discovered that an Asian person who has the same the BMI as a white person has a much higher likelihood of developing diabetes.
Causes and Risk Factors
Obesity most commonly develops when the body consumes more energy, typically measured in calories, than it expels through normal functioning and physical exercise. However, researchers have noted that obesity results from a combination of factors. Researchers at the National Heart, Lung, and Blood Institute at the National Institutes of Health (NIH) have found that risk factors for obesity include lifestyle and behavioral habits, unhealthy environments, race and ethnicity, use of certain medications, and genetics. A lack of sleep has also been known to slow metabolism and contribute to excessive weight gain.
Men and women store fat differently, which can contribute to different susceptibilities to obesity. Transgender people experience higher rates of overweight or obesity than do their cisgender counterparts. In a study published in the International Journal of Obesity in 2021, researchers from the United States and Australia noted that transmasculine individuals experienced higher incidence of weight gain than transfeminine individuals, both before and during gender-affirming hormone therapy. The study's authors stressed the importance of monitoring weight gain during treatments that carry their own powerful physical effects.
Environmental contributors to obesity include a lack of access to affordable healthy foods and a lack of safe places to exercise. Communities with limited food options, commonly referred to as food deserts, experience higher rates of obesity than people in areas with more diverse options. Likewise, obesity rates are higher in food swamps, or areas where retailers selling fast food and junk food outpace those offering healthier options. Sedentary work, including most office work and jobs with low physical demands, contributes to a general lack of exercise, and long-term unemployment has also been linked to obesity. In addition, researchers have noted that eating habits and family food culture can affect the likelihood of obesity. Generally, families that eat together without the television on have lower rates of obesity than families that do not eat together or eat with the television on.
Obesity can also create serious complications when patients experience an additional illness or medical condition. Because obesity impairs immune function, its presence as a comorbidity prevents the body from healing itself and fighting disease. During the COVID-19 pandemic, obesity appeared as a frequent factor contributing to severe infection, hospitalization, and death. The CDC advised individuals with obesity to take additional steps during the pandemic to protect their health, such as minimizing stress, engaging in regular physical activity, and getting proper sleep.
Prevention Strategies and Related Challenges
Managing behavioral and environmental factors can help mitigate excessive weight gain and prevent obesity. Health care professionals recommend increasing physical activity, reducing the consumption of unhealthy foods, improving sleep, and reducing stress as means to combat obesity on the individual level. Certain social and economic factors, such as lack of access to affordable and nutritious foods, can present challenges for overcoming obesity. A poor diet can also negatively affect how the brain reacts to food consumption, causing increased cravings for unhealthy foods high in sugar and fat content.
Negative social bias against people who are obese and overweight can commonly result in what has been termed body shaming, also referred to as fat shaming. While some people cite body shaming as a potential factor in motivating people to lose weight, evidence suggests that body shaming can have the opposite effect. Alternatively, increased social acceptance of diverse body types, the development of community initiatives that encourage self-advocacy, and a focus on well-being have demonstrated better outcomes than programs that focus only on weight loss.
Effects of Obesity
The CDC reports that obesity-related conditions, such as heart disease, stroke, and type 2 diabetes, are among the leading causes of preventable death in the United States. According to the WHO, about 2.8 million people die each year from complications related to obesity and excess weight. Experts predict dramatic increases in the prevalence of obesity-related diseases and health care costs as well as significant losses in economic productivity by 2030.
To accommodate larger numbers of people with obesity, businesses and public infrastructure will incur additional expenses. For example, hospitals will need to invest in equipment such as larger beds and toilets; schools and offices may require larger seats and desks. In a study published in the Journal of Managed Care and Specialized Pharmacy in 2021, researchers estimated that the additional medical costs related to obesity in US adults exceeded $260 billion annually. The study linked obesity to raised costs for both inpatient and outpatient treatments as well as prescription drugs.
People classified as obese also often face biases from health care providers and are at an increased risk of suffering from depression and anxiety. Weight stigma among medical and health providers can have potentially fatal consequences. When people with obesity visit a doctor, they are often advised to lose weight and exercise regardless of the complaint that brought them in for a visit. In some instances, this approach has prevented patients from receiving accurate diagnoses of life-threatening conditions. In a study published in Obesity Reviews in 2022, researchers tested several strategies that health care providers could incorporate to reduce weight stigma when treating patients. The most effective approaches involved educating providers and patients about the complexities behind body weight and its effects on overall health, providing empathy and building relationships, and focusing on health and well-being issues beyond weight.
In addition to bias in health care, many obese people experience employment discrimination and encounter weight bias that limits their earning potential. Discrimination in the hiring process results in obese people losing career opportunities and experiencing longer periods of unemployment between jobs. In the workplace, weight discrimination can result in lower wages for obese and overweight workers, particularly women. An analysis by researchers at the University of Florida and the London Business School estimated that, compared to the annual incomes of women of average weight, the incomes of women who were obese or overweight were lower and the incomes of women who were perceived as thin were higher. While some city governments maintain laws against employment discrimination on weight, as of 2022, Michigan remained the only state that explicitly prohibited weight discrimination by law. Emerging in the 1960s alongside other social justice movements, fat acceptance activists have organized to seek expanded civil and political protections as well as social acceptance for overweight and obese individuals.
Weight-Loss Medications and Surgery
Weight-loss surgery or medication can assist some people with obesity who wish to lose weight but are not able to do so effectively through diet and exercise alone. Physicians caution, however, that both surgical and pharmaceutical weight-loss interventions carry significant side effects and can lead to adverse events. For example, the once-popular diet medication fen-phen, short for fenfluramine/phentermine, was linked to heart-valve disease and pulmonary hypertension and pulled off the market in 1997. Some weight-loss medications approved by the Federal Drug Administration (FDA) have particularly bothersome or dangerous side effects such as loose stools following high-fat meals, birth defects, and increased suicidality. Health care professionals increasingly believe that weight loss cannot be accomplished by medication alone, and they urge patients to follow instructions precisely and remain in regular contact with their doctors.
Weight loss, or bariatric, surgery has become increasingly popular in the United States, with about 256,000 procedures performed in 2019, up from an average of 16,000 performed annually in the early 1990s, according to the American Society for Metabolic and Bariatric Surgery. One of the most well-known forms of bariatric surgery is gastric bypass surgery, where a portion of the stomach is sectioned off into a small pouch and reconnected to the lower part of the small intestine to reduce the amount of food absorbed by the body. After surgery, the patient's stomach is so small that it can hold only a few ounces of food or liquid at a time. Candidates for gastric bypass must make significant lifestyle changes regarding diet and physical activity both before and after the surgery, and those who do typically lose between 65 and 70 percent of their excess weight in the first year following surgery. Recovering from bariatric surgery, however, can be difficult and painful. Patients must follow strict guidelines to ensure they receive proper nutrients and hydration, and even moderate overeating can cause severe pain and nausea. Other forms of bariatric surgeries, including laparoscopic gastric band surgery and sleeve gastrectomy, also require significant lifestyle and diet changes.