While BMI is a very useful indicator of obesity, the distribution of body fat, either peripheral or central, is also of substantial clinical importance. In peripheral or gynecoid obesity, fat deposits are located in subcutaneous tissues in the lower body, mainly in the hips and thighs. Individuals with central-type obesity, also referred to as android or visceral adiposity, have the majority of their fat located in the abdominal area in both subcutaneous and visceral locations.

Measurement of waist circumference has been shown to correlate well with the amount of visceral adipose tissue [10]. A waist circumference more than 88 cm for women and more than 102 cm for men is indicative of substantially elevated cardiovascular risk and need for medical intervention [11]. While the waist-to-hip ratio (WHR) is used preferentially by some investigators [12], data exist to suggest that waist circumference alone is better correlated with visceral adipose tissue, especially in women. Waist circumference provides a gross approximation of abdominal adipose tissue, while WHR indicates the relative accumulation of abdominal fat [13].

Differentiation between the different types of fat distribution has clinical relevance, as visceral central adiposity is associated with a greater risk of metabolic and cardiovascular disorders, including insulin resistance, type 2 diabetes mellitus (T2DM), hypertension, and coronary heart disease [14, 15].

The prevalence of obesity is typically determined from broad-based surveys or population studies. Two major sources of data in the United States are the National Health and Nutrition Examination Survey (NHANES), conducted by the Centers for Disease Control and Prevention (CDC), and the Behavioral Risk Factor Surveillance Survey (BRFSS).

NHANES [16] is a series of studies designed to assess the health and nutrition status of both adults and children throughout the United States. The program was initiated on an intermittent basis in the 1960s and became a continuous study in 1999, evaluating a representative sample of approximately 5,000 individuals. Study participants are evaluated with an extensive questionnaire that includes demographic, socioeconomic, dietary, and health-related questions, as well as a thorough physical exam and laboratory studies.

BRFSS [17] is a much larger survey, evaluating more than 350,000 adults throughout the United States and affiliated territories. Unlike the NHANES, it is limited to a phone survey, focusing on health-related behaviors and healthcare access, particularly as they relate to chronic disease and injury. While advantageous because of its large size, the BRFSS necessarily relies upon self-reported weight (commonly underestimated) and height (commonly overestimated) and thus likely underreports the prevalence of obesity.

NHANES provides some striking statistics regarding the prevalence of obesity in the United States. The 2001–2004 study [18] revealed that 133.6 million adults were either overweight or obese. This constitutes 66%, or roughly two-thirds, of the US population. The most recent NHANES 2-year cycle was completed in 2008 and included 8,082 subjects, 71% of whom were both interviewed and examined. This study [19] showed an overall prevalence of obesity of 32.2% in men and 35.5% in women. The prevalence of both the overweight and obese increased slightly from 2004 to 68.0%.

Comparing data from different NHANES study periods provides valuable information regarding population trends in obesity. From the 1976–1980 study to the 1988–1994 study, obesity prevalence increased by 8.9% for women and 7.9% for men. Between that time and the 1999–2000 study, obesity rates continued to increase, by 8.1% for women and 7.1% for men [19]. The most recent study period available suggests that this increase may be leveling off. The increase from 1999–2000 to 2007–2008 was 2.1% for women (nonsignificant) and 4.7% points for men [19].

Obesity trends are shown in Table 1.2. From 1988 to 2006, the overall prevalence of obesity (BMI greater than 30) increased from 22.9 to 34.2%, an increase of nearly 12% over a period of 18 years. The prevalence of class III obesity (BMI greater than 40), which generally qualifies a patient for bariatric surgery, increased during the same period from 2.9 to 5.9% [19].

Worldwide obesity data are similarly concerning, although the prevalence is generally not as high as that in the United States. In 2005, according to the WHO [1, 5], approximately 1.6 billion adults (age higher than 15) were overweight and at least 400 million were obese. Projections suggest that by 2015, approximately 2.3 billion adults may be overweight (BMI greater than 25) and more than 700 million obese (BMI greater than 30) [1, 5, 7].

Multiple approaches have been used in the past to describe and report overweight and obesity rates in children. These systems have been deemed necessary since children are still growing in both height and weight, and boys and girls mature at different rates. The CDC [20] uses growth charts based on national surveys that indicate weight for age and BMI for age. Data sources for these charts include the National Health Examination Surveys (NHES) I and II, and the NHANES I, II, and III. These data were obtained over a substantial time span, starting in 1963 (NHES I) and extending through 1994 (NHANES III). Such charts are primarily used clinically to measure growth.

Commonly used references for childhood obesity are the 85th and 95th percentiles of BMI for ages 6–19 based on data from the NHANES I (1971–1974). Use of these values has been endorsed by a WHO expert committee in 1995 [21]. Gender-specific BMI above the 95th percentile is considered to represent excessive weight (Table 1.3). BMI at 14 years of age has been shown by Laitinen et al. [22] to be an important predictor of adult obesity.

The most recent NHANES data available for children and adolescents is the 2007–2008 study. Earlier studies had categorized heavy children and adolescents into two groups: “at risk of being overweight or obese” (BMI for age between the 85th and 95th percentile) and “overweight” (BMI for age at the 95th percentile or higher). The 2007–2008 study used the simplified nomenclature of “overweight” and “obese” to describe children between the 85th and 95th percentile BMI for age and above the 95th percentile, respectively.

For children aged 2–19 years, 31.7% had a BMI above the 85th percentile and 16.9% were above the 95th percentile [23]. BMI above the 97th percentile was noted in 11.9% overall. Age ranges were broken down into 2–5 years, 6–11 years, and 12–19 years. The incidence of BMI above the 85th percentile was 21.2% for the youngest group, 35.5% for the middle group, and 34.2% for the oldest group. The incidence of BMI above the 95th percentile was 10.4% for the youngest group, 19.6% for the middle group, and 18.1% for the oldest group. Overall, no statistically significant differences by gender were noted at any of the three BMI cutoff points of 85, 95, and 97%.

From 1976 to 2006, the obesity prevalence tripled in children 6–10 years old [23]. However, this trend may have leveled off in recent years. Data regarding prevalence of high BMI for age were compared from the 1999–2000 NHANES through the 2007–2008 studies [23, 24]. No statistically significant trend for BMI above the 85th or 95th percentile was noted for 6- to 19-year-olds during this time period, although a slight increasing trend was identified for those above the 97th percentile.

Some correlation is noted between childhood obesity and low socioeconomic status. In 2009, the CDC [25] reported that the incidence of obesity in children of low-income families was higher than that in the general population: 14.6% vs. 12.4%, respectively.

Longitudinal trends in the adolescent population were reported in a 2009 study. The National Longitudinal Study of Adolescent Health [26] followed more than 20,000 US teenagers in grades 7–12 through their early 30s. Obesity, defined as BMI above the 95th percentile on the 2000 CDC growth charts, was found in 13.3% of adolescents in 1996. This increased to 36.1% overall by 2008. Substantial differences were noted among different subpopulations, with the highest obesity prevalence found in non-Hispanic black females (54.8%).

The obesity epidemic observed in the United States is echoed in many countries around the world. As in the USA, the prevalence of obesity in Europe has tripled since 1980 [27]. As of 2007, the worldwide prevalence of overweight people ranges from 32 to 79% in men and from 28 to 78% in women as per WHO estimates [1]. Worldwide, an estimated 1.7 billion adults are overweight and 310 million are obese [28]. Overall, class III obesity (BMI greater than 40) afflicts 4.7% of the world’s population.

Worldwide, the highest prevalences of obesity are found in Albania, Bosnia, and the UK; the lowest rates are noted in Turkmenistan and Uzbekistan [29]. Obesity levels are relatively low in China (with a prevalence under 5%) and high in Samoa (with a prevalence of 75%) [29]. In England, from 1993 to 2004, the prevalence of obesity in men increased from 13.6 to 24% and in women from 16.9 to 24.4% [30]. A total of 9.3 million people were obese in England in 2004.

In 2008, Kelly et al. estimated the prevalence of overweight people in the world in 2005 at 23.2% overall, 22.4% in women and 24% in men. The prevalence of obesity was 9.8% overall, 7.7% for men and 11.9% for women. These estimates suggest that obesity afflicts approximately 396 million people worldwide [31]. The WHO global database calculates the prevalence of overweight people (BMI greater than 25) and the prevalence of obesity (BMI greater than 30) [32].

The WHO data [29] suggests that significant gender differences in obesity exist in about one-third of countries studied—the prevalence of obesity was noted to be higher in men than in women in 14 of the 36 evaluated countries. Disparity also exists in the pediatric obesity rates—about 22 million children under the age of five are overweight. In Europe, the highest prevalences of overweight school-age children were found in Portugal (32% in children 7–9 years), Spain (31% in children 2–9 years), and Italy (27% in children 6–11 years) [29]. Throughout the world, an estimated 10% of all children are overweight [28].