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Obesity in the United States

A Worrisome Epidemic

Carlos J. Crespo, DrPH, MS; Joshua Arbesman

THE PHYSICIAN AND SPORTSMEDICINE - VOL 31 - NO. 11 - NOVEMBER 2021


In Brief: In the past decades, obesity has reached epidemic proportions in the United States, even among children, adolescents, and young adults. The prevalence of obesity is higher among non-Hispanic black (36%) and Mexican American women (33%) than among non-Hispanic white women (22%). Various explanations for increased obesity have been proposed, including decreases in exercise and occupational and recreational physical activity and an increase in sedentary lifestyles. Television watching is directly related to obesity and energy intake among children age 8 to 16 years and may be a target for intervention. Physicians can use these data to help counsel patients about weight management, especially in reducing inactivity and overeating.

The prevalence of obesity continues to increase in the United States for both children and adults. Multiple cultural, environmental, genetic, and behavioral factors have been identified as possible causative agents. Obesity is of great public health concern, because it is directly related to diabetes, hypertension, osteoarthritis, and other chronic conditions. Moreover, obesity, as well as other conditions for which it serves as a major risk factor, is highly prevalent in all groups of the population.1-4 The number of adults who are classified as extremely obese (body mass index [BMI] > 40 kg/m2) tallies in the millions.5-7 Physicians must know the scope and root causes of the problem so they can appropriately intervene.

Epidemiology of Obesity

The National Institutes of Health's National Obesity Education Initiative has proposed a classification for disease risk relative to normal weight and waist circumference (table 1).8 These standards have been supported by the World Health Organization. In adults, preobesity or overweight is defined as a BMI between 25 and 29.9 kg/m2, and obesity is defined as BMI greater than or equal to 30. The obese classification for adults was based on various studies, reference population criteria, and more recently, data relating morbidity and mortality to weight.8,9

TABLE 1. National Institutes of Health Obesity Education Initiative Classification of Categories of Obesity
 Category BMI
(kg/m2)
 Obesity
Class
 Disease Risk Relative to Normal Weight and
Waist Circumference

 Underweight <18.5 - Men: ≤102 cm (≤40 in.)
Women: ≤88 cm (≤35 in.)
 Men: >102 cm (>40 in.)
Women: >88 cm (>35 in.)
 
 Normal 18.5 to 24.9 -   
 Overweight 25 to 29.9 - Increased High 
 Obesity 30 to 34.9 1 High Very high 
  35 to 39.9 2 Very high Very high 
 Extreme obesity ≥40 3 Extremely high Extremely high 

BMI = Body mass index

The terminology used in childhood obesity research is not standardized in the literature. Classifications of the different categories—at risk of overweight, overweight, or obese—among children are based on a statistical definition from the 2021 Centers for Disease Control and Prevention (CDC) growth charts for the United States. Therefore, because outcome-based criteria for children are lacking, a statistical approach has been proposed as the most practical choice.10-13 Some researchers use the term "at risk for overweight" to refer to children between the 85th and 95th percentile and overweight to children in the 95th percentile,14,15 while others prefer to use the terms overweight for children at or exceeding the 85th percentile and obese for children who exceed the 95th percentile to more closely match the terminology used for adults.16-19

Regardless of the classification system, overweight children often become overweight adults and are therefore more likely to experience a higher morbidity associated with excess body weight.20 Although BMI is not a perfect measure in children because it covaries with height, it has been validated against measurements of body density.18 Obese children (95th percentile) do tend to become overweight adults, and therefore obesity may have a better positive predictive value than overweight (85th percentile).10,21 For this paper, overweight is defined as the 85th percentile and obesity as the 95th percentiles, based on sex- and age-specific groups of children.16,19

The Problem Among Children

The most recent data from the National Health and Nutrition Examination Survey (NHANES) show that the prevalence of overweight and obesity in children, adolescents, and young adults ages 2 to 19 years old has increased dramatically since the 1960s. The prevalence of overweight in these age-groups ranges from 20.6% among children 2 to 5 years old to 30.4% among adolescents 12 to 19. Moreover, recent findings from 1999-2021 NHANES show that the problem continues to worsen in all the age-groups (figure 1).15 Another critical problem is that obesity disproportionately affects minority populations.

Upward trends. Between NHANES II (1976-120210) and NHANES III (120218-1994), obesity increased in children 4 to 5 years old but not in younger children.22 On the other hand, among Mexican American children 1 to 2 years old, the prevalence of obesity is double that of non-Hispanic white children of the same age. Non-Hispanic black girls also exhibited a higher prevalence of obesity than non-Hispanic white girls younger than 2 years old.22 It is therefore not surprising to observe a greater prevalence of obesity among minority adolescents and adults. In fact, the prevalence of obesity is highest among Mexican American boys and African-American girls; it is double the prevalence observed among age- and sex-matched non-Hispanic white children.

Predisposing factors. It is unclear what specific or combination of cultural, lifestyle, genetic, or environmental factors account for differences.11,23-25 Public health officials have tried to identify modifiable risk factors to implement policies that will decrease obesity in minority populations.26-28

The recent Healthy People 2010 report28 identified television viewing as an important public health target to reduce the percentage of children who watch 2 or more hours a day. We have found that television watching is positively related to adiposity29 and that obesity also increases with more television viewing.16,30 Almost 40% of African-American boys and girls watch 4 or more hours of television, while 15% of non-Hispanic whites watch similar amounts. Concomitant increases in obesity and energy intake in relation to hours of television watching have been documented (figure 2).16,30 The lowest prevalence of obesity is seen among children who watch 2 hours or less of television. Although television watching is not entirely a surrogate measure for physical inactivity, it does provide a good estimate of the number of hours that children spend being inactive and correlates very well with energy intake.16,29,31

Gregg and Narayan32 studied 2,205 black and white girls who were between the ages of 9 and 10 years old and reported racial variation in the relationship between self-esteem and adiposity, with the magnitude of the effect somewhat less in black girls. Whether these racial differences extend to college students needs to be studied further. Jeffery24 suggested that obesity in US minority groups is best understood as a variation on a larger cultural theme: the creation of an environment in which highly palatable foods are accessible to all at low cost, and physical activity is not required.24

Obesity's Perception Among College Students

Female college students are more likely to think of themselves as being overweight than their male counterparts. However, the percentage of college males and females who were overweight, based on BMI, was not drastically different.33 A larger percentage of college students who had a BMI less than 25 kg/m2 considered themselves as overweight; this was not so for black male college students. These results revealed that not only gender differences but cultural differences may play a role in who considers themselves as overweight.28,34,35

To better understand the nation's physical suitability for joining the armed forces, Nolte et al36 examined the percentage of persons between 17 and 20 years old who would not be eligible for military services because of excess body weight. On average, 15% to 20% of those in this age-group would not qualify for service, based on current military guidelines. Moreover, among women, almost 45% would not qualify for the US Army, while 35% would not qualify for the Marine Corps.36 Height and weight acceptance criteria of the armed forces are not necessarily based on BMI (see table 1), and thus, more research is needed to better determine how they establish who qualifies and what criteria they use for their cutoff points.

The Consequences of Obesity in US Adults

From 1960 to 120210, the prevalence of obesity among adults in the United States was relatively stable; however, recent findings from NHANES showed that 3 of 10 US adults are obese (figure 3).8,37 In addition to increasing mortality from all causes, obesity is closely linked to hypertension, type 2 diabetes mellitus, dyslipidemia, gallbladder disease, osteoarthritis, coronary heart disease, stroke, and sleep apnea and other respiratory problems. Also, increasing evidence suggests that it is a risk factor for endometrial, breast, prostate, and colon cancer.2,7,38,39

Obesity disproportionately affects more women and minorities than non-Hispanic white men.37,40,41 The prevalence of obesity in non-Hispanic black (36%) and Mexican American women (33%) is greater than among non-Hispanic white women (22%). Unfortunately, data have not been collected systematically to address this issue across different ethnic groups in the United States.

Data from NHANES III and NHANES 1999-2021 show that overweight was more common among men (59.1%, 95th confidence interval [CI], 57.4-61.4) than among women (50.3%, 95th CI, 48.6-52.8), yet more women (24.5%, 95th CI, 22.7-26.6) were obese than men (19.4%, 95th CI, 18.1-20.9). These results indicate that more men have a BMI greater than or equal to 25 kg/m2, but more women have a BMI greater than or equal to 30 kg/m2.

Obesity is lowest among adults between ages 20 to 29 and among those 80 and older (< 20%). Several explanations may account for the differences in the age distribution of obesity. First, it may be possible that the prevalence observed among those 20 to 29 will not change in subsequent years, thus producing a "cohort effect." Similarly, the cohort of men 60 to 69 years old with the highest prevalence of obesity (> 35%) may reveal that, 20 years from now, obesity is highest among those 80 and older. In fact, between 120218 and 1994, the highest prevalence of obesity was among those 50 to 59 years old, and now the highest is among those 60 to 69 years old. The highest percent increase in the prevalence of obesity between 120218 to 1994 and 1999 to 2021 was observed among those 60 to 69 years old for both men and women (13% increase).

An explanation for the lower prevalence of obesity among persons 80 and older might be that those who were obese died before age 80 and were not available for the study, producing a "survivor effect." Thus, those persons with healthier body weight tended to live longer, and obese individuals were less likely to survive past age 60.37 These estimates of overweight parallel the excess burden of other chronic diseases such as type 2 diabetes, heart disease, hypertension, breast cancer, and other disorders that occur during this period in life.42-45

Obesity, Education, and Social Class

Some researchers have suggested that the prevalence of obesity is related to social class.11,23,46 Extreme obesity (BMI >= 40 kg/m2) is most prevalent among African-American women (15.1%), while the prevalence of extreme obesity in the rest of the other racial and ethnic groups was less than 5.5%.

Other important indicators of social class are education and income, and these factors seem to influence obesity differently. Education is typically related to behaviors (eg, diet, exercise, smoking), whereas income is related to things purchased (eg, health insurance, prescription medication).47 To better understand how education and income are related to obesity, the prevalence of obesity was tallied in different strata of social classes using nine mutually exclusive categories of education and income (figure 4).47,48 Among women, the highest prevalence of obesity was among those who had less than a high school education and earned less than $20,000 a year. Among men, the prevalence of obesity was highest among those in the highest income category but who had less than high school education. Invariably, obesity is lowest among those who have more than a high school education, regardless of income.

Poverty and lower educational attainment are consistently associated with obesity, independent of ethnicity, and therefore affect more persons in minority populations than in white populations.48 Thus, minorities may be at higher risk for obesity because of their increased poverty rate and lower educational attainment. Our understanding of how and why obesity develops should involve the integration of social, behavioral, environmental, cultural, physiologic, metabolic, and genetic factors.

Excess Calories, Physical Activity, and Obesity

Lack of physical activity and excess calorie consumption are some of the reasons epidemiologists suggest obesity has increased in the last 20 years.16,29,30,49,50 A panel of experts met at the 1999 American College of Sports Medicine to try to understand the role of exercise in preventing and treating obesity.51 The evidence reviewed suggested that exercise is beneficial in preventing weight gain and is also an important adjunct modality in treating obesity. Barely 24% of the general population, however, engages in physical activity for 30 minutes a day, 5 or more days a week as is recommended by the US surgeon general. It is unclear if overweight persons accurately report their exercise habits, and if those who report exercising do so at the recommended frequency, intensity, and duration.

About two thirds (67% of men and 62% of women) of overweight adults reported that they use physical activity to lose weight, but only about one fifth (22% of men and 20% of women) met the recommended guidelines of being physically active for 30 minutes most days of the week. Thus, overweight adults are just as inactive as the general population. Decreases in physical activity combined with excess energy intake are almost certainly responsible for the increase in overweight and obesity at the population level.51,52 However, participation in physical activity, either occupational or recreational, was not tracked systematically until 120215, when the National Health Interview Survey obtained baseline data for the Healthy People 1990 report. In that survey, roughly 24% of the population reported participating in no leisure-time physical activity. These estimates have been confirmed in other national surveys,53 and the estimates have not changed dramatically since.27,28,54

Proactive Clinician Choices

The prevalence of overweight and obesity at the population level highlights the fact that obesity is rife in all segments of the population, especially among minorities, women, and children. Among US children, television watching correlates with obesity, and increased calorie intake has been observed among those children who watched the most. Because obesity is so prevalent, physicians will undoubtedly encounter patients who are overweight or obese and children who are at risk of overweight. These encounters provide an opportunity to assess lifestyle practices, such as daily participation in leisure-time, occupation, and transportation physical activities. They also can help target interventions, for example, suggesting that children watch less television, or at least do something during commercials.55 Also important are culturally sensitive approaches to healthy eating.

Paramount for the clinician is that overweight and obesity are precursors to other serious chronic conditions and are an emerging risk factor for other life-threatening diseases. These disorders tend to disproportionately affect minority populations, which underscores the need to improve the cultural competence of today's clinician.

The Fat and the Lean

Obesity is highly prevalent and rising in the United States. Contributing factors likely include reduction in all types of physical activities and increases in calorie intake and smoking cessation.56 The data presented here reveal the accelerating pace of obesity between 1960 and 2021, a stark contrast from the stable rates observed between 1960 and 120210. These dramatic increases were observed in both adults and children. Future research needs to address the interaction between genetic, social, cultural, and environmental factors.

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Dr Crespo is an associate professor in the department of social and preventive medicine at the State University of New York in Buffalo. Mr Arbesman is a student in premedical sciences at Brandeis University in Waltham, Massachusetts. Address correspondence to Carlos J. Crespo, DrPH, MS, SUNY, 270 Farber Hall, 3435 Main St, Buffalo, NY 14214-3000.

Disclosure information: Dr Crespo and Mr Arbesman disclose no significant relationship with any manufacturer of any commercial product mentioned in this article. No drug is mentioned in this article for an unlabeled use. The work was supported by a grant from US Army Medical Research DAMD 170210252.


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