Overweight and obesity are significant and increasing public health challenges in both economically developed and developing regions of the world, with 33.0 % of the world’s adult population (1.4 billion people) overweight or obese [1]. In 2008, more than 1.4 billion adults, and more than 40 million children under the age of five were overweight in 2010. It is estimated that if recent trends continue, by 2030 up to 57.8 % of the world’s adult population (3.3 billion people) could be either overweight or obese [8]. The prevalence of overweight and obesity is higher in economically developed countries compared with economically developing countries [8]. Close to 35 million overweight children are living in developing countries and 8 million in developed countries [1]. Although overweight and obesity is more common in economically developed countries, the much larger population of developing countries results in a considerably larger absolute number of individuals affected. The prevalence of overweight and obesity is also on the rise in developing countries, particularly in urban settings. This is in part due to promotion of unhealthy “fast foods” in these countries in the last two decades. Many developing nations are now facing a “double burden” of disease as is seen in much of Asia, Latin America, the Middle East, and Africa. While they continue to deal with the problems of infectious disease and undernutrition, they are experiencing a rapid upsurge in noncommunicable disease risk factors such as obesity and overweight. It is not uncommon to find undernutrition and obesity existing side by side in the same community and the same household. It is estimated that up to 20 % of Chinese urban children are obese with increase in childhood obesity rates at 8 % per year [9]. Children in developing countries are more vulnerable to inadequate prenatal, infant, and young child nutrition. At the same time, they are exposed to high-fat, high-sugar, high-salt, energy-dense, micronutrient-poor foods, which tend to be lower in cost. Additionally, urbanization and mechanization, higher rates of television viewing, and increasing pressure among children in developing countries to perform scholastically have led to a sharp decline in physical activity. Interaction of these factors with changing dietary patterns results in sharp increases in childhood obesity and metabolic syndrome. This complex interaction of genetic factors such as variation in DNA sequence or expression and epigenetic factors including in utero environment, behavior, lifestyle, ethnic variability in body composition, and values/perceptions has led to an increase the prevalence of obesity and chronic diseases associated with it such as diabetes and cardiovascular disease.

The strongest data on obesity prevalence rates over time in the United States come from the results of the National Health and Nutrition Examination Surveys (NHANES). The NHANES program of the National Center for Health Statistics (NCHS), Centers for Disease Control and Prevention, includes a series of cross-sectional nationally representative health examination surveys beginning in 1960 [10–14]. In each survey, a nationally representative sample of the US civilian non-institutionalized population was selected using a complex, stratified, multistage probability cluster sampling design. In the 2009–2010 survey, household interview and a physical examination were conducted for each survey participant including height and weight measured as part of a comprehensive set of body measurements by trained health technicians, using standardized measuring procedures and equipment excluding pregnant women and persons missing a valid height or weight measurement [2]. Age was based on age at the interview and grouped into 20–39 years of age, 40–59 years of age, and 60 years and older. Race and ethnicity were self-reported and for purposes of this report were classified as non-Hispanic white, non-Hispanic black, Mexican-American, other Hispanic, and other. Data for 2009–2010 were analyzed overall, including all race/ethnicity groups, and separately for non-Hispanic white, non-Hispanic black, all Hispanic participants (including both Mexican-American and other Hispanic participants), and Mexican-American participants.

Results from the 2009–2010 National Health and Nutrition Examination Survey (NHANES), using measured heights and weights, indicate that an estimated 33.0 % of US adults aged 20 and over are overweight, 35.7 % are obese, and 6.3 % are extremely obese [2]. In 2009–2010, the age-adjusted mean BMI was 28.7 (95 % CI, 28.3–29.1) for men and also 28.7 (95 % CI, 28.4–29.0) for women [2]. The age-adjusted prevalence of obesity was 35.5 % among adult men and 35.8 among adult women [2]. One of the national health objectives for the Healthy People 2020 initiative is to reduce the prevalence of obesity among adults by 10 % to 30.5 % [15]. Figure 1.1 shows the trends in overweight and obesity among adults from 1960 to 2010 [13]. Data for adults suggests a steady prevalence of obesity from the 1960s through the 1980s, with a steady increase in obesity between the late 1980s and today in the United States, with the estimated age-adjusted prevalence moving upward from a previous level of 23.0 % in 1988–1994 to approximately 36.0 % in 2009–2010 [13]. It is interesting to note in this figure that the rate of overweight has been more or less stable, but there have been significant increases in the rates of obesity, with obesity rates having very recently overtaken the rate of prevalence of overweight in the adult population.

Among children, results from the 2009–2010 NHANES, using measured heights and weights, indicate that an estimated 16.9 % of children and adolescents aged 2–19 years are obese (Fig. 1.2) [14]. Just as with adults, the prevalence of overweight has increased over time. In the same time period, the rates of childhood obesity in children and adolescents aged 2–19 increased from approximately 6.5 % to about 17 % [14]. Recent projections based on NHANES predict that if the current trends continue, more than half (51.1 %) of US adults are likely to be obese and 86.3 % are likely to be overweight or obese by 2030 [16]. In children, at the current rate, the prevalence of overweight is likely to nearly double by 2030 [16].

Data suggests that those who are obese may be gaining weight at a more rapid pace than those who are not. Data from the Behavioral Risk Factor Surveillance System (a random-digit telephone survey of the household population of the United States) shows that it is not just that more Americans are becoming obese, but that it is the most severe obesity that is increasing the most in relative terms. From 2000 through 2005, the prevalence of obesity (self-reported) increased by 24 %, the prevalence of a self-reported BMI greater than 40 increased by 50 %, and the prevalence of a BMI greater than 50 increased by 75 % [17]. The greatest relative increase has been in the proportion of individuals with a BMI greater than 50 kg/m². The most recent NHANES data also confirm this trend: the percentage of the population with a BMI greater than 40 has increased from 0.9 % in the 1960s to approximately 6 % at the current time [14].

Obesity rates are high in most age groups. Obesity rates, in general, increase with age until approximately 75 years of age, when rates decline. The decline in obesity rates in the elderly could be attributed to a decrease in lean body mass and a tendency to gain fat in the older patient, which plateaus as the older patient establishes a new weight set point. In addition there is increasing mortality from obesity-related conditions with age; a significantly higher all-cause mortality has been noted in obese individuals compared to normal weight subjects, with one study predicting that mortality was likely to occur 9.44 years earlier for those who were obese (BMI, ≥30) [18].

Increasing BMI and increasing obesity prevalence are affecting the entire adult population with no group being immune [19, 20]. Increasing rates of obesity are seen across men and women all ethnic groups, of all ages, and of all educational and socioeconomic levels. Still racial, ethnic, and socioeconomic disparities are seen in the prevalence of obesity and some subgroups in the population are affected to a greater extent than others.