Trends

The prevalence of obesity changed relatively little during the 1960s and 1970s, but it increased sharply over the ensuing decades—from 13.4% in 1980 to 34.3% in 2008 among adults and from 5% to 17% among children during the same period.^1–2,9–10 The prevalence of extreme obesity also increased during 1976–1980 and 2007–2008, and approximately 6% of U.S. adults now have a BMI of 40 kg/m2 or higher.¹⁵

The United States is not alone in experiencing an obesity epidemic. Similar increases in the prevalence of obesity have been reported in developed countries such as England and in countries where obesity was formerly rare.^16–17 For example, the prevalence in China among preschool-aged children living in urban areas has increased eightfold—from 1.5% in 1989 to 12.6% in 1997.¹⁸

Disparities

The burden of obesity is disproportionately borne by some racial and ethnic groups. For example, among 40- to 59-year-old women, about 52% of non-Hispanic blacks and 47% of Hispanics are obese; for non-Hispanic whites, the prevalence is 36%.¹⁹ These differences also are seen among children and teenagers. For example, obesity is much more common among non-Hispanic black teenagers (29%) than among Hispanic teenagers (17.5%) or non-Hispanic white teenagers (14.5%).¹⁹

The burden of obesity and the severity of related health conditions vary among different population groups. While obesity is a public health crisis within the general population, it is even more prevalent in persons with mental illness^20–21 with some reports indicating 83% of people with serious mental illness being overweight or obese.²² This puts people with mental disorders in double jeopardy. Not only are they dealing with a mental disorder that often leads to social isolation, a sedentary lifestyle and physical inactivity – all risk factors for obesity - but they are also vulnerable to the chronic diseases associated with being overweight, mood instability and low self-esteem.²³ People with serious mental illness have shortened life-spans, on average living only until 53 years of age.²⁴ Their deaths are not brought on by their mental illness, but rather from other chronic health conditions, including obesity. Many people are not aware that rapid weight gain is one of the most common and alarming side effects of psychiatric medications for both children and adults.²⁵ Youth aged 4 to 19 being treated with antipsychotic medications gained more than 7% of their total body weight in 12 weeks.²⁶ Clearly persons with mental illness are a vulnerable population at high-risk for obesity and obesity-related disorders.

Measuring Overweight and Obesity

Obesity is generally defined as excess body fat. However, since excess body fat is difficult to measure directly, obesity is often defined as excess body weight as measured by BMI. bmi, which is calculated as weight in kilograms divided by height in meters squared, is used to express weight adjusted for height. Although BMI has limitations as a measure,²⁷ it has been a useful indicator of overweight and obesity. For example, several studies have found that adults with a high BMI are at increased risk for various diseases⁶ and children who have a high BMI are likely to have relatively high levels of body fatness.²⁸ Adults who have a BMI of 25.0 to 29.9 are considered to be overweight, those with a BMI ≥ 30 are considered obese, and those with a BMI ≥ 40 are considered extremely obese.^6,29

Figure 1Adult BMI Classification

Figure 2Children BMI Classification

In overweight and obese adults and children, other health risk factors (such as blood pressure, blood sugar, and blood fats) should be assessed, as recommended by published guidelines. Because excess body fatness in the abdomen can be a marker for increased health risk even at a lower BMI, measurement of waist circumference is recommended in overweight and obese adults.³⁰ BMI does not distinguish between lean tissue and body fat, and some growing children or athletic children and adults will have a BMI in the overweight or obese range without having an excess of body fat.³¹ However, most children and adults with a BMI in the range considered obese will also have excess body fat.

Assessing if a child is at a healthy weight is complex. While BMI is often utilized, clinical assessment and other markers should be considered when determining a child’s overall health and development. Among children, the marked BMI changes that occur with growth and development make it necessary to specify a high BMI relative to children of the same sex and age. The 2000 CDC growth charts are³² used for this purpose in the United States. Children and adolescents with a BMI at or above the sex-and age-specific 95th percentile of this reference population are often considered obese, and those with a BMI between the 85th and 94th percentiles are often considered overweight.³³ Although these cut points are not diagnostic criteria, elevated BMI among children most often indicates increased risk for future adverse health outcomes and/or development of disease.

Health Consequences

Obesity in early life has been found to increase the risk for various diseases in adulthood, including diabetes and heart disease, in part because obese children are likely to become obese adults.^34–35 Several studies also have found short-term effects of excess weight during childhood—for example, high BMI levels among children and teenagers are associated with childhood development of atherosclerosis.^36–38

The growing U.S. obesity epidemic is reflected in the tripling, since 1980, of the number of Americans who have diabetes.³⁹ Approximately 8% of U.S. adults have type 2 diabetes,⁴⁰ a disease that increases the risk for cardiovascular disease, stroke, kidney disease, blindness, lower-limb amputation and other problems. Obesity is the most important risk factor for type 2 diabetes.⁴¹ Although type 2 diabetes has traditionally been viewed as developing among middle-aged (or older) adults, type 2 diabetes is now occurring in early life.⁴² Although the rate of type 2 diabetes in children has increased, it is very low⁴² (one quarter of 1 percent), but more than 75% of children and adolescents with type 2 diabetes are obese.^43–44 The poor glycemic control of many adolescents with type 2 diabetes,⁴⁵ along with the increased duration of diabetes diagnosed in early life, may increase the risk of subsequent complications.

Causes of Obesity

In addition to consuming too many calories and not getting enough physical activity, genes, metabolism, behavior, environment, and culture can also play a role in causing people to be overweight and obese. Identifying determinants of and supporting changes in behaviors and in the environment are likely to be the most effective actions to combat obesity. Key modifiable risk factors are physical activity, sedentary behavior and diet. Physical activity plays several important roles in the prevention and control of obesity, and it is essential for health at any weight. Increased physical activity and decreased sedentary behavior are associated with lower rates of obesity, and it reduces the risk for many of the diseases associated with obesity, such as diabetes and heart disease.

A healthy diet is also important. Beginning early in life, breastfeeding is a relatively short-term intervention which has significant long-term potential for maintaining a lower BMI.⁴⁶ At any stage of life, increased consumption of excess calories from fats and added sugars in foods that are energy dense, such as fast food, is associated with obesity. These foods are relatively higher in calories than nutrients that are needed for health. Sugar-sweetened beverages, such as soda, contribute to excess calorie intake from added sugars or displace more nutritious foods in the diet. Some evidence suggests that the body may not compensate for the calories consumed with these beverages.⁴⁷ In contrast, consumption of fruits and vegetables in place of high calorie foods may reduce the risk for obesity and help sustain weight loss because the body’s sense of fullness at meals is partly regulated by volume. Fruits and vegetables contain few calories and are bulky foods, so they have a low caloric density and are more filling than fast foods.

The amount of time spent watching television is another association with obesity in both children and adults. The association with obesity may be mediated in part by the effects of television time on food consumption. The more time children spend watching television, the more likely they are to eat while doing so and the more likely they are to eat the high-calorie foods that are heavily advertised on television.⁴⁸

Stress is another contributing factor to overweight and obesity. Studies have shown chronic stress adversely affects blood pressure and cholesterol and may lead persons to increase their caloric intake.⁴⁹ Furthermore stress may limit people’s motivation and ability to adopt positive weight-related behaviors.⁵⁰