14 The Epidemiology of Obesity

Carol A. Braunschweig

Abstract

It is estimated that globally more than 1 billion adults are overweight and at least 300 million are obese. This epidemic evolved over many decades in the industrialized countries; however, today it is occurring in a little as 10 years in underdeveloped countries. In the United States, the prevalence of overweight increased significantly from 56% in 1994 to 65.2% in 1999–2000 with individuals at the higher end of the body mass index getting larger at a faster rate than the remaining population in both men and women and in all race/ethnic groups. Women in all age groups have higher obesity preva- lence than men. Alarmingly, this epidemic is no longer restricted to adults. Childhood obesity has increased more than threefold in the United States in the past three decades with the majority of the increase occurring within the past 10 years. Initially, obesity in children was primarily isolated to those from lower economic households; however, recently this association has been weakening as greater numbers of middle and upper income children have become obese. Obesity is associated with many diseases and tremendously increases health care costs. Although there is consensus that this epidemic is increasing throughout the developed world, the causes for the surge remain unknown and pose challenges for clinicians, researchers and policy makers.

Key Words: Overweight; obesity; NHANES; surveys; trends; BMI.

1. INTRODUCTION

The fundamental basis for studying the epidemiology of obesity centers on the hypo- thesis that there are causal and preventative factors that can be identified through systematic investigation of different populations, or subgroups within a population, in different places or times. Epidemiological studies allow the quantification of the magnitude of the expo- sure–disease relationship in free living human populations and thus provides the possibil- ity of altering this risk through individual or population-based interventions.

This chapter describes the current distribution patterns for adult and childhood obe- sity worldwide, with specific emphasis on populations in the United States. The modi- fying influences of gender, race/ethnicity, age, and socioeconomic status on obesity rates are discussed. A final section is devoted to central adiposity’s role in obesity- related diseases and a comparison of obesity rates using waist circumference standards rather than the traditional measure of body mass index (BMI). Knowledge of current obesity distributions and the secular changes that have occurred better position clini- cians to select the most appropriate populations for future research investigations and design interventions and governmental policies and programs tailored to meet the unique requirements of those individuals and populations with the greatest needs.

From: Nutrition and Health: Adipose Tissue and Adipokines in Health and Disease Edited by: G. Fantuzzi and T. Mazzone © Humana Press Inc., Totowa, NJ

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2. DEFINITION

Obesity is defined as an unhealthy excess of body fat that increases the risk of mor- bidity and premature mortality. Unfortunately, accurate measures of actual fat mass require sophisticated technologies that are not typically available in clinical settings.

The BMI (weight in kg/height in m

) has become the method of obesity classification used by the World Health Organization (WHO) and the National Institutes of Health (NIH) (1). This index provides a simple measure of the relation between height and weight that correlates with percentage body fat in young and middle-aged adults (2).

Increased morbidity and mortality from hypertension, stroke, coronary artery disease, type 2 diabetes, sleep apnea, and some cancers increases as BMI rises above 25 kg/m

, and a sharp increase occurs when BMI exceeds 30 kg/m

(1) (Fig. 1).

The obesity classification categories based on BMI are similar for men and women and are provided in Table 1.

3. WORLDWIDE EPIDEMIC

It is estimated that, globally, more than 1 billion adults are overweight and at least 300 million are obese (3,4). Obesity rates range from 5% in rural China, Japan, and cen- tral Africa to over 75% in urban Samoa. In the nonindustrialized countries obesity gen- erally follows an economic gradient, with the least economically developed having the least obesity (approx 1.8%). As economics expand, obesity prevalence increases from 4.8% in “developing” countries, to 17.1% in “economy in transition,” to 20.4% in

“developed market economy” countries (4).

Improvement of a country’s economy results in shifts from high fertility and mortal- ity rates with high prevalence for malnutrition, poor sanitation, and infectious diseases to more urbanization and chronic diseases associated with urban–industrial lifestyles, including obesity, type 2 diabetes, and cardiovascular diseases (5).

the differences in the prevalence and rate of change for overweight and obesity for various countries at different stages of economic development.

As illustrated in Fig. 2B, the US prevalence for overweight and obesity is increasing by approx 0.5% annually, whereas rates in Mexico, Brazil, and Morocco are two to five times higher (e.g., Mexico 2.4%/yr, Brazil 1%/yr). In the developed countries the changes in diet and physical activity that lead to their current levels of obesity occurred over many decades, whereas in the developing countries of Asia, North Africa, the Middle East, Latin America, and in parts of sub-Saharan Africa, these changes are occurring in as little as 10 to 20 years, in economies that are at far lower levels than those of the industrialized countries. These changes have profound implications for their future health care needs (6).

4. OBESITY EPIDEMIC IN THE UNITED STATES

Prevalence rates for overweight and obesity in the United States are generated from

the National Health and Nutrition Examination Survey (NHANES) program. These cross-

sectional surveys have been conducted since 1960 on a large representative sample of the

US population and provide national estimates for overweight and obesity in children and

adults from 2 to 74 yr of age. The National Health Examination Survey (NHES I,

1960–62) was followed by three NHANES surveys (NHANES I, 1971–74; NHANES II, 1976–80; NHANES II,I 1988–94). In 1999 NHANES became a continuous survey; the 1999–2000 data were first published in 2002.

Data for these surveys reveal dramatic increases in overweight or obesity between NHANES II and III, which were continued at an even greater rate in 1999–2000. The prevalence of overweight increased significantly, from 56% in NHANES III to 65.2% in 1999–2000. These changes are summarized in Table 2 (7).

When changes in weight are partitioned into overweight (BMI 25–29.9) and obese (BMI v 30), it can be see that the prevalence for overweight increased very little (<2%);

however, the prevalence for obesity doubled, from 15.1 to 31.1%, between NHANES III and 1999–2002. Within the obesity category, the prevalence of self-reported BMI greater than 40 increased from 0.5 to 2% of the US population and those with a BMI greater than 50 quadrupled, from 0.5 to 2% between 1986 and 2000 (8). Individuals at the higher end of the BMI spectrum appear to be getting larger at a faster rate than the remaining population.

Fig. 1. J-shaped curve of all-cause mortality compared with body mass index. From ref. 1.

Table 1

Classification of Overweight and Obesity by BMI

Obesity class BMI (kg/m²)

Underweight < 18.5

Normal 18.5–24.9

Overweight 25.0–29.9

Obesity I 30.0–34.9

II 35.0–39.9

III

•

Fig. 2. Obesity patterns and trends across the world, adults aged 20 yr and older. (A) Prevalence rates;

(B) obesity trends (the annual percentage point increase in prevalence). With permission from ref. 6.

5. GENDER AND OBESITY

Obesity classification and treatment recommendations are the same for men and women (Table 1) (1); however, at any given BMI women throughout the world are more likely to have greater amounts of adipose tissue than men (9) and have higher rates of obesity owing to biological differences (10). In the United States, proportionate increases in obesity preva- lence have occurred for both men and women 20 to 74 yr of age since 1960, and women have had higher prevalence rates in all age groups in each of the national surveys (Table 3).

Overall, in both men and women, there has been a general shift of the top half of the BMI distribution toward higher BMI levels, with the heaviest subgroup of the popula- tion becoming much heavier in NHANES III than II (11). For 20- to 49-yr-old men changes in the lower portion of the BMI distribution was less than 1 BMI unit, whereas there was a fairly large shift at the upper end among 50- to 74-yr-old men. For women a gradual increase across the entire BMI distribution occurred. Collectively these findings suggest that the entire spectrum is changing, with the most marked changes occurring at the upper end of the distribution.

6. ETHNICITY/RACE AND OBESITY

Obesity and extreme obesity prevalence increased in all sex and racial/ethnic groups between NHNAES III and 1999–2000. Differences in obesity rates by ethnicity/race were found to vary by gender (12). Specifically, no difference in obesity rates were found among African American, Hispanic, or Caucasian men; however, among women, African Americans had a higher prevalence of overweight and obesity (77 and 50%, respectively) than Caucasians (57 and 30%, respectively), and Mexican American women had a prevalence that was intermediate between these two (72% overweight and 40% obese). Significant increases in extreme obesity occurred between NHANES III and 1999–2000 in all men and women and for non-Hispanic black women, with simi- lar trends in all other groups (Table 4).

Interestingly, although there has been a significant increase in the prevalence of obesity in the past 15 yr, the mean BMI measurements have changed much less. Between NHANES III and 1999–2002 the mean BMI increased 1.7 and 2.5 BMI units in non- Hispanic white and black women respectively, representing a 6.6% and 8.8% increase in rate of change, whereas overall obesity prevalence for women during that same time period increased 7.8 and 10.8% respectively, representing a 34% and 38.2% increase in

Prevalence and Trends of Overweight and Obesity Among Adults in the United States, 1960–2002

Overweight Overweight but not

Survey year or obese % (SE) obese % (SE) Obese % (SE)

1960–1962 44.8 (1.0) 31.5 (0.5) 13.3 (0.6)

1971–1974 47.7 (0.7) 33.1 (0.6) 14.6 (0.5)

1976–1980 47.4 (0.8) 32.3 (0.6) 15.1 (0.5)

1988–1994 56.0 (0.9) 32.7 (0.6) 23.3 (0.7)

1999–2002 65.2 (0.8) 34.1 (0.8) 31.1 (1.0)

rate of change, respectively (Tables 4 and 5). These findings further support the observation that overall shifts in obesity prevalence are occurring among the heavier end of the BMI spectrum.

7. OBESITY AND SOCIOECONOMIC STATUS

Obesity has frequently been reported in the past to occur disproportionately among economically disadvantaged populations in the United States (15–17); however, more recently, substantial variation in obesity rates between income and ethnic groups over time have emerged (14,18). Using level of education attainment to categorize socioeconomic status (SES), Zhang and Wang found a gradual decrease in the association between SES (determined according to level of education) and obesity between 1971 and 2000, par- ticularly among women, and a disproportionate increase in the obesity prevalence in the high-SES groups (18) (Fig. 3).

Table 3

Prevalence of Obesity in Men and Women in the United States, 1960–2002^a

Men Women

20–74 20–39 40–59 60–74 20–74 20–39 40–59 60–74 Survey yr yr (%) yr (%) yr (%) yr (%) yr (%) yr (%) yr (%) yr (%)

1960–1962 10.7 9.8 12.6 8.4 15.8 9.3 18.5 26.2

1971–1974 12.1 10.2 14.7 10.5 16.6 11.2 19.7 23.4

1976–1980 12.7 9.8 15.4 13.5 17.0 12.3 20.4 21.3

1988–1994 20.6 14.9 25.4 23.8 25.0 20.6 30.4 28.6

1999–2002 27.7 23.7 28.8 35.8 34.0 28.4 37.8 39.6

aAdapted from ref. 7.

Table 4

Prevalence of Obesity (BMI > 30) and Extreme Obesity (BMI > 40) in Different Racial Groups in the United States, 1988–2002^a

Prevalence of obesity (BMI > 30)

Men Women

Non- Non- Mexican Non- Non- Mexican

Hispanic Hispanic American All Hispanic Hispanic American Survey yr All white (%) black (%) (%) (%) white (%) black (%) (%)

1988–1994 20.2 20.3 21.1 23.9 25.4 22.9 38.2 25.3

1999–2002 27.6 28.2 27.9 27.3 33.2 30.7 49.0 38.4

Prevalence of extreme obesity (BMI > 40)

Men Women

1988–1994 1.7 1.8 2.4 1.1 4.0 3.4 7.9 4.8

1999–2002 3.3 3.3 3.4 2.9 6.4 5.5 13.5 5.7

aAdapted from refs. 12,13.

Chang and Lauderdale (14) compared obesity rates in non-Hispanic blacks, whites, and Mexican Americans by income using the poverty–income ratio (PIR) (grouped into four categories, 1 low, 4 high) between NHANES I and the 1999–2000 NHANES. They found that obesity increased over this interval at all levels of income in all ethnic/ racial groups. In African American and Caucasian women the lowest obesity prevalence occurred in the highest income category; however, among African American and Mexican American men, obesity prevalence was highest in the highest income category (28.3 and 36.2%, respectively) and lowest in the lowest two income categories (category 1 in African Americans, 22.8%; and category 2 for Mexican Americans, 19.3%).

Additionally, the rate of increase in obesity was greater among the more affluent than the poor. Among African American women the rate of increase for obesity was lowest among the lowest PIR (14.5%) and highest in the middle-income category (27%);

among non-Hispanic white women the lowest increase (13.0%) was found in the

BMI in 18- to 64-Yr-Old Non-Hispanic White and Black Men and Women Between NHANES I (1960–1962) and 1999–2002^a

Women Men

Non-Hispanic Non-Hispanic Non-Hispanic Non-Hispanic

white black white black

mean BMI mean BMI mean BMI mean BMI

NHANES I 24.3 26.9 25.4 25.4

NHANES II 24.5 26.9 25.3 25.3

NHANES III 25.9 28.5 28.5 26.4

1999-2002 27.6 31.0 27.9 27.3

aAdapted from ref. 14.

Fig. 3. Secular trends in the association between SES and obesity among US adults, 1971–2000. With permission from ref. 18.

highest income category (PIR category 4) and the highest increase was found in PIR category 2. The significant “catch-up” in obesity prevalence that has occurred in high-SES groups implies that obesity is a much more equal-opportunity condition than previously suspected. These findings suggest that social–environmental factors that affect all SES groups may be the primary contributors to the increase in obesity that has occurred over the past 30 yr.

8. CHILDHOOD OBESITY

There has been a threefold increase in childhood obesity in the United States in the past three decades (12). Coinciding with this increase has been an increase in prevalence for type 2 diabetes, hypertension, gallbladder disease, hyperlipidemia, orthopedic com- plications, sleep apnea, and nonalcoholic steatohepatitis in children. Obese children are predisposed to adult obesity and have increased risks for adult obesity-related diseases (19). Approximately 30% of obese adults became obese during childhood, and 80% of obese adolescents become obese adults (20).

No single cut-point for obesity classification can be used for children because between birth and approx 20 yr of age, height and weight—and thus, BMI—vary by age. To address this children (age 2–11) and adolescents (age 12–19) are classified as at risk of overweight or overweight if they are at or above the 85th or the 95th percentile, respectively, for their age and gender using the 2000 Centers for Disease Control and Prevention Growth Charts (21). The prevalence and trends between 1971 and 2002 of overweight in 6- to 11-yr-olds and 12- to 19-yr-olds are presented in Tables 6 and 7.

The majority of the increase in overweight prevalence among children has occurred within the past 10 yr, and the most recent data indicate that this trend continues. In 1999– 2000, 29.9% of 6- to 19-yr-old children were at risk of overweight and 15% were overweight; in 2000–2001 these levels increased to 30.5% at risk of overweight and 16.5% overweight (13).

Gender and racial–ethnic differences are presented in Table 7. Distinct from adults, where women at all ages have higher rates for obesity, very little difference in over- weight prevalence exists between boys and girls. Ethnic differences for overweight do exist in children. African American and Mexican American children are significantly heavier than Caucasian boys and girls. Similar to observations in adults, a pronounced upward shift (at least 2 BMI units) in the upper end of the BMI distributions in all age/gender groups occurred during this time period; however, very little change occurred in the lower end of the BMI distributions, and median BMI remained virtually unchanged. The heaviest groups of children are much heavier in NHANES III than they were in previous surveys (11).

The secular trends of the impact of SES on obesity in children have received very

limited study. In Canada the prevalence of overweight has been reported to vary

inversely by SES (22); however, the data used in their analysis spanned 1981 to 1996

and thus do not reflect the most recent decade. A recent report using the 1971–2002

NHANES surveys examined trends in obesity prevalence and SES status categorized by

level of parental education attained (less than high school, high school, college, or

higher) (Table 8) (23). The report found an overall weakening of association between

obesity risk and SES category that varied by racial/ethnic groups.

In NHANES I the overweight prevalence was 7.1, 6.3, and 3.8% in low-, medium-, and high-SES non-Hispanic white girls, compared with 8.2, 14.8, and 1.9% in non- Hispanic black girls; in 1999–2002 these figures became 17.9, 10.6, and 10.6% vs 24.5, 18.7, and 38.0%, respectively. Although the prevalence for overweight remains higher among African American than among Caucasian children, the highest rates of increase in overweight were found in the highest SES groups for both racial/ethnic groups. The disproportionate protection that once was conferred by being within the higher SES cat- egory is eroding among all racial/ethnic groups. Although the cause for this weakening secular trend remains uncertain, these findings should be considered when predicting future trends in obesity prevalence and guide the development of programs and govern- mental policies designed for obesity prevention and management.

9. ADIPOSE TISSUE DISTRIBUTION

The previous sections have presented obesity data based on BMI classifications. The primary assumption of BMI is that it is an independent index of body fat—i.e., it

Prevalence of Overweight (

v

Non-Hispanic Non-Hispanic Mexican

Age All white black American

Boys

6–19 16.8 14.3 17.9 25.5

2–5 9.9 8.2 8.0 14.1

6–11 16.9 14.0 17.0 26.5

12–19 16.7 14.6 18.7 24.7

Girls

6–19 15.1 12.9 23.2 18.5

2–5 10.7 9.1 9.6 12.2

6–11 14.7 13.1 22.8 17.1

12–19 15.4 12.7 23.6 19.9

aAdapted from ref. 13.

Table 6

Prevalence of Overweight (

v

Survey yr Children 6–11 (%) Adolescents 12–19 (%)

1971–1974 4.0 6.1

1976–1980 6.5 5.0

1988–1994 11.3 10.5

1999–2002 15.8 16.1

aAdapted from ref. 7.

assumes that after adjusting for body weight-for-stature, all individuals with the same BMI will have the same fatness, regardless of their age, gender, ethnicity, or disease his- tory. Many investigations have demonstrated that this assumption is not correct for var- ious populations. For example, it is widely recognized that body fatness increases with age, is greater in females than in males, and differs among ethnic groups (Asian Indians and Chinese have greater percentage of fat at any BMI). Thus, categorization of an indi- vidual’s overweight or obesity status should not be done without consideration of other factors that could have influenced their BMI measurement.

Centrally located adipose tissue as assessed by waist circumference (WC) was first associated with heightened risk for obesity-related disease risks approx 50 yr ago (24).

Since that time numerous studies have confirmed the effect of excessive central fat dis- tribution on increased risks for insulin resistance (25), type 2 diabetes (26), cardiovas- cular disease (27,28), hypertension (29), and inflammation (30).

Women tend to gain adipose tissue peripherally, and men centrally. Because of the more peripheral distribution in women, they tend to have lower disease risks and less disease for a given weight than men. Visceral fat accumulation is thought to increase risks more than subcutaneous fat. Differentiation between subcutaneous and visceral fat depots requires analysis with computerized tomography or magnetic resonance imag- ing. These are expensive tests and are not readily available in many clinical settings.

The NIH and WHO recommend waist circumference (measured midway between the lower rib and the iliac crest) cut-points for central obesity classification at v88 cm for women and v102 cm for men (1,3). Age-, gender-, and race/ethnicity-specific median WC for men and women in NHANES I, II, and III were reported by Okosun et al. (31).

Changes in WC distributions that occurred between 1960 and 1962 and 1999 and 2000 for men and women are illustrated in Fig. 4.

Waist circumference shifted to the right (higher values) in both men and women between 1960 and 2000, although women tended to have greater shifts then men. This trend paral- lels the changes that occurred for BMI over this time period. No specific differences in WC were found between African-American men or women in NHANES I (1960–62), followed by an increasing gradient in both genders (Fig. 5). In subsequent surveys Caucasian men had significantly larger WC than African-American men, whereas African-American women had larger WC than Caucasian women.

The prevalence of abdominal obesity stratified by BMI across 1960–2000 is provided in

obese, and more than two-thirds of overweight women also had central obesity. At BMIs greater than 35 the additional risk conferred by measuring WC is lost, as virtually all of

Odds Ratios for Overweight in 2- to 19-Yr-Old Children in Low and High SES Compared With Middle SES Categories

NHANES I NHANES II NHANES III 1999–2000 OR (95% CI) OR (95% CI) OR (95% CI) OR (95% CI) Low SES 0.82 (0.48–1.41) 1.15 (0.79–1.69) 1.24 (0.73–2.09) 1.04 (0.82–1.33) High SES 0.66 (0.43–1.0) 0.58 (0.34–1.0) 0.42 (0.23–0.76) 0.99 (0.68–1.43)

these individuals exceed the WC cut-points. The overall age-adjusted prevalences of abdominal obesity in men were 12.7, 29, and 38.3% in 1960–62, 1988–94, and 1999–2000 respectively; among women, these rates were 19.4, 38.8, and 59.9%, respectively. These secular trends in abdominal obesity are similar in direction but greater in magnitude than those for generalized obesity (men, 13.1, 23.4, and 31.0%; women, 15.6, 25.8, and 34.3%).

The NIH clinical recommendations for the need to institute weight-loss therapy have been divided into two levels. Action level I (men WC f 94 cm, women f 80 cm) indicates the need for lifestyle modifications and level II (men WC > 102 cm, women > 88 cm) indicates that professional help is needed.

Overall there has been a shift in abdominal obesity across the entire population, includ- ing those with normal BMI, since 1960, and abdominal obesity rates have increased faster than generalized obesity. The higher health risk for diabetes and cardiovascular disease from excessive central fat, independent of generalized obesity, suggests that public health strategies that address this issue are urgently needed. Studies focused on the identification of factors that are causing increases in the entire population are needed to guide interven- tions focused on the entire population rather than solely on the heaviest individuals.

Fig. 4. Distribution of waist circumference in US adults from 1960 to 2000. With permission from ref.32.

10. CONCLUSIONS

Obesity becomes a disease when excess body fat reaches levels associated with

increased morbidity and mortality. In 2000 the WHO designated obesity as the major

unmet public health problem worldwide (3). It is probable that the increase in obesity

that has occurred is the result of the “modern lifestyle” that encourages increased

caloric consumption and discourages physical activity. All recent studies suggest that

this epidemic continues to expand into younger, more diverse populations that were pre- viously thought to be somewhat impervious to its development. A complex web of prob- lems can be expected to emerge when this epidemic is superimposed on the aging US population, ultimately predisposing large segments of society to reduced independence, increased health care costs, prolonged medical care, and ultimately shorter, less produc- tive lives. Partnerships between public and private sectors are needed to develop both population-based environmental approaches and individual strategies for successful prevention and treatment.

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Table 9

Prevalence of Abdominal Obesity Stratified by Gender and BMI Across NHANES Studies^a

1960–1962 1988–1994 1999–2000 p-value BMI category men Prev (%) Prev (%) Prev (%)

<25 0.3 0.8 1.5 <0.001

25–29.0 12.1 25.4 29.6 <0.001

>30 65.4 88.3 93.5 <0.001

BMI category women

<25 0.8 11.9 11.5 <0.001

25–29.9 15.9 70.3 69.0 <0.001

>30 75.4 98.1 98.7 <0.001

aAdapted from ref. 32.

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