Understanding Obesity: Causes, Epidemiology, And Health Consequences

Prevalence of Obesity in the United States

Obesity has emerged as the most pressing nutritional problem facing the developed world. This trend has occurred over a relatively short period of time; in the United States, it appears to have begun in the last quarter of the 20th century. The epidemic in children followed shortly thereafter. The most recent data (1999–2000) from national surveys in the United States suggest that almost two thirds of the adult population is overweight, and almost one third is obese (Flegal et.al, pp. 1724). In children, current estimates (1999–2000) put the prevalence of overweight at 15%, a threefold increase over the past 30 years (Ogden et.al, pp. 1729). Although this epidemic has spared no subgroup of the population and has been documented in individuals of all ages and racial/ethnic and socioeconomic subgroups, the problem is greatest in minority populations and among persons living in poverty. Obesity is a global public health problem, affecting virtually every region of the world with the exception of sub-Saharan Africa.

The World Health Organization (2000) defines obesity as a condition of abnormal or excess accumulation of adipose tissue (body fat) to an extent that an individual’s health may be impaired. Because the precise measurement of adipose tissue requires invasive laboratory measures, in the population context, a simpler measure on which to base an obesity definition is required. Although imperfect, the Body Mass Index (BMI), defined as weight in kilograms divided by height in meters squared, has been adopted by consensus in the United States by the National Institutes of Health (1998) and the Centers; for Disease Control and Prevention (CDC) and internationally by WHO (2000). Consensus definitions of overweight and obesity have been set at 25 (overweight) and 30 (obesity), with severity classes of obesity defined as follows: overweight, 25.0 to 29.9; Class I obesity, 30.0 to 34.9; Class II obesity, 35.0 to 39.9; and Class III obesity, 40.0+. The WHO (2000) terminology differs slightly, but the cutoff points are the same.

In growing children, in whom weight and height are both changing (and at different rates), the definition of obesity is inherently more complicated. Although no universally agreed on standard exists for assessing overweight and obesity in children and adolescents, there is a growing consensus that BMI should be adopted as an indirect measure of adiposity for children and adolescents, as well (Barlow & Dietz, pp. 223). Because BMI varies substantially by age and gender during childhood and adolescence, the specific BMI cutoffs used to classify obesity must be gender- and age-specific and must be referenced against a standard. In the United States, the standard used is the CDC Revised Growth Reference (Barlow & Dietz, pp. 228). Internationally, several standards (Cole et.al, pp. 27; Ogden et al., pp. 1728), including one based on a pooled international sample (Cole et.al, pp. 1241), are also in use.

Several periods in development have been proposed as critical periods in the development of persistent obesity and its comorbid consequences. These include the prenatal period (when intrauterine exposures may influence adiposity), early childhood, and adolescence. Some evidence suggests that breast-feeding may protect against later obesity. Likelihood of persistence in adulthood of obesity from childhood is related both to age at onset and severity.

Definition of Obesity

Childhood obesity has a number of immediate, intermediate, and long-term health consequences (Must & Strauss, pp. S3). These include classic cardiovascular risk factors, such as high blood pressure, abnormal blood lipid levels, and impaired glucose tolerance. Respiratory conditions include sleep-disordered breathing. In addition, early menarche and menstrual abnormalities are linked to overweight. Of particular concern is the emergence of type 2 diabetes, once considered an adult-onset disease, as a disease of childhood. The psychological impact may represent one of the most damaging effects of obesity given that stigmatization and social isolation may result in lower self-esteem and depression. In a recently replicated classic study, children were asked to rank order a series of drawings of children with various handicaps (crutches, wheelchair, missing a hand, facial disfigurement, obesity) based on which child they would “like best” (Latnerm & Stunkard, pp. 456). The obese child was ranked last irrespective of the ranking child’s sex, race, socioeconomic status, living environment, and own disability. In another study, “ratings of quality of life for children with obesity were similar to those of children undergoing chemotherapy for cancer” (Schwimmer et.al, 2003, pp. 1814).

Although obesity in adulthood that has been present from childhood may carry an additional burden due to increased severity, much adult obesity arises through adult weight gain. The health consequences of obesity present in adulthood are enormous, both in magnitude and impact on quality of life. In developing its clinical guidelines, the NIH report identified an extensive list of health conditions for which obesity increased risk. These include hypertension, type 2 diabetes, coronary heart disease, stroke, gallbladder disease, osteoarthritis, sleep apnea and respiratory problems, many cancers, and depression. The number of deaths per year in the United States attributable to obesity has been estimated at about 300,000.

In all persons, child or adult, obesity arises due to energy imbalance: When energy intake exceeds energy expenditure, most of the excess calories are stored as adipose tissue. To give rise to obesity, energy imbalance must occur over a long period of time and likely reflects a combination of factors. Individual behaviors, environmental factors, and heredity, singly and in combination, contribute to the development of obesity. The rapidity with which the obesity epidemic emerged rules out simple genetic explanations. The current environment in the United States has been characterized as obesogenic, meaning that it promotes high energy intakes and low energy expenditures—the energy imbalance that gives rise to weight gain. Modern industrialized societies provide abundant, relatively inexpensive food; modern life is organized to reduce energy expenditure at work and at home, through technology and urbanization. For a species that evolved to store fat in times of plenty in order to survive in leaner times, many individuals are genetically susceptible to gain weight in the current environment.

Excessive energy intake is a primary risk factor for the development of childhood and adult obesity, although the specific aspects of intake responsible are controversial. Dietary factors, such as diet composition, energy density, fat intake, fruit and vegetable consumption, snacks, sugar-rich foods, and soft drinks, have all been identified in association with obesity. Increased consumption of fruits and vegetables can help reduce the intake of dietary fat and calories because they are naturally low in fat and energy density compared with other foods. Despite current recommendations that individuals over the age of 2 years consume 2 to 4 servings of fruits and 3 to 5 servings of vegetables daily, children and adolescents eat an average of only 3.6 servings of fruits and vegetables per day, and fried potatoes account for a large proportion of those servings. A number of other dietary variables, including soft drinks, snacking, portion sizes, and infant feeding have also been linked to childhood obesity. Studies suggest that the increased consumption of sugar-sweetened soft drinks, snack foods (which are often high in fat or/and sugar), and large-sized portions of foods have contributed to the increase in energy intakes.

Reduced physical activity may be the most important factor in explaining the increase of obesity over the past two decades. Physical activity among U.S. youth is in decline, with nearly half of young people aged 12 to 21 reporting that they do not engage in vigorous physical activity regularly and one fourth reporting no vigorous physical activity. Whereas leisure time physical activity has increased in men and remained constant in women over the past four decades in the United States, activity associated with work and home life has declined over the same period. Sedentary behavior and “inactivity,” such as watching television and playing video/computer games, also are contributory factors. Gortmaker et al. (1996) reported an adjusted-odds ratio (OR) for obesity of 8.3 for adolescents who watched TV more than 5 hours per day compared with those who watched 0 to 2 hours. Compelling evidence comes from intervention studies, which show that reducing TV viewing time can help prevent childhood obesity. Adults in a trial of maintenance following weight loss sustained their losses best when physical activity was high and television viewing was low (van Baak et.al, pp. 210). TV watching may promote obesity by reducing physical activity, lowering metabolic rate, and increasing energy intake. The latter may occur due to the fact that TV viewing may be associated with snacking and may moderate eating habits generally through greater exposure to advertising of foods high in added sugars and fat or by conveying mixed messages about lifestyle and health in the content of advertisements. TV viewing is a major source of inactivity among Americans. Screen time, a summary measure of time spent viewing television and videos and engaged in computer-based activity, represents the largest proportion of nonsleep, nonschool time for youth.


Obesity has emerged as the major nutritional problem facing the pediatric and adult populations worldwide. The etiology of obesity is multifactorial and includes individual risk factors, genetic influences, and environmental effectors. The severity of the problem, in terms of immediate and long-term health consequences to physical and psychological health, suggests it is a problem that will dominate the public health agenda in the 21st century.

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