Obesity --- United States, 1988--2008

David S. Freedman, PhD

National Center for Chronic Disease Prevention and Health Promotion, CDC

Corresponding author: David S Freedman, PhD, Division of Nutrition, Physical Activity, and Obesity, National Center for Chronic Disease Prevention and Health Promotion, 1600 Clifton Road, NE, MS K-26, Atlanta, GA 30333. Telephone: 770-488-6016; Fax: 770-488-6500; E-mail: dxf1@cdc.gov.

The prevalence of obesity in the United States has increased substantially since the 1960s (1). From 1976--1980 to 2007--2008, obesity prevalence increased from 15% to 34% among adults and from 5% to 17% among children and adolescents (2,3). Substantial differences exist in obesity prevalence among racial/ethnic groups, and these differences vary by sex and age group.

To assess differences and trends over time in obesity prevalence and to determine whether these disparities can be attributed to differences in family income, CDC analyzed data from the National Health and Nutrition Examination Survey (NHANES) III (1988--1994) and data collected in NHANES between 1999 and 2008. In 1999, NHANES became a continuous survey, with data releases at 2-year intervals; 2007--2008 is the most recent release for which data were available (4). NHANES samples are selected by using a stratified, multistage cluster design and are representative of the U.S. civilian noninstitutionalized population. CDC examined disparities in obesity prevalence by sex, age, time period, and family income. Disparities were not assessed by education level; disability status; lesbian, gay, bisexual, or transgender status; or geographic region.

Weight and height were measured by using standardized techniques and equipment, and body mass index (BMI) was calculated as kilograms per square meter (kg/m2). Adults aged ≥20 years with BMI ≥ 30.0 kg/m2 obesity were categorized as obese (5). Children aged 2--19 years with BMI-for-age ≥ 95th percentile of the CDC growth charts (6) or BMI ≥ 30.0 kg/m2 were categorized as obese. It is possible for persons aged 18--19 years to have a BMI ≥ 30 kg/m2 (the adult definition of obesity) but to have a BMI-for-age that is less than the CDC 95th percentile. Pregnant women (n = 1,661, approximately 2% of the sample) were excluded from analysis. Data regarding race/ethnicity were self-reported for persons aged ≥16 years or reported by a family member (for persons aged ≤15 years) after being provided a list that included an open-ended response. The analyses described in this report include non-Hispanic whites, non-Hispanic blacks, and Mexican Americans. Because of insufficient numbers, other racial/ethnic groups (including Hispanics who were not Mexican American) were excluded.

During each household interview, respondents were asked to report the total annual income for themselves and for other family members. This information was divided by the poverty threshold for the specific family size to yield the poverty to income ratio (PIR), an indicator of socioeconomic status. The 2009 poverty threshold for a family of four was $22,050 (7); therefore, a PIR of 2.0 indicates that the total income for this family was $44,100. CDC recoded PIR values of >5 as 5.0.

Data from NHANES III for 10,275 persons aged 2--19 years and 16,037 persons aged ≥20 years were analyzed. The number of persons aged 2--19 years in each of the five 2-year cycles (beginning in 1999--2000) ranged from 2,677 to 3,888, and the number of persons aged ≥20 years ranged from 3,746 to 4,707. Because blacks and Mexican Americans were oversampled, 29% of the persons in the sample used for this report were black, and 30% were Mexican American. Analyses used the sampling weights and accounted for the complex sampling design. The statistical significance (p<0.05) of the association between the PIR and the prevalence of obesity was assessed in logistic regression models that included both PIR and age as continuous variables. These analyses were restricted to the 15,277 persons examined in 2005--2008. Approximately 6% (n = 971) of these persons did not have information on family income and were excluded from the analyses of PIR.

Racial/ethnic differences occurred among respondents in the six NHANES for 1988--1994 and 1999--2008 (Figures 1 and 2). Although variability exists in the estimated obesity prevalences in each survey, the racial/ethnic differences have not changed substantially throughout this period. No consistent racial/ethnic differences occurred in obesity prevalence among females aged 2--5 years. However, among females aged >5 years, blacks had the highest prevalence in each of the six surveys, followed by Mexican Americans and whites (who had the lowest prevalence) (Figure 1).

Racial/ethnic differences in obesity prevalence also were observed among males aged 2--40 years (Figure 2), although the trends among older men were less consistent. Among males aged 2--19 years, Mexican Americans had a higher prevalence of obesity than whites or blacks in almost all surveys. However, limited or inconsistent differences were observed in the majority of surveys between white and black males aged 2--19 years. Among men aged 20--39 years, obesity prevalence was lower among white men than among either Mexican-American men or black men, although the patterns were somewhat inconsistent. Among men aged ≥40 years, no consistent racial/ethnic differences were observed.

Data are displayed within narrower age groupings for the most recent 4 years (2005--2008) of NHANES (Table 1). Substantial racial/ethnic disparities occurred in obesity prevalence. With the exception of men aged ≥20 years, whites had a lower prevalence of obesity than did blacks and Mexican Americans.

Among females aged 2--19 years, obesity prevalence was 24% among blacks, followed by 19% among Mexican Americans and 14% among whites. A somewhat similar pattern was observed among women aged ≥20 years, with black women having a substantially higher prevalence of obesity (51%) than Mexican-American women (43%) and white women (33%). Among males aged 2--19 years, Mexican Americans had the highest prevalence of obesity (25%), with similar prevalences observed among whites (15%) and blacks (18%). The higher prevalence of obesity among black men aged ≥20 years (37%) than among whites (32%) and Mexican Americans (31%) is largely attributable to the differences among younger (aged 20--39 years) men. There was little difference in the prevalence of obesity across racial/ethnic groups among older (aged ≥40 years) men.

Differences in obesity prevalence persisted within various categories of family income (Table 2). Black females had a higher prevalence of obesity within most income categories than did white or Mexican-American females. Furthermore, Mexican-American males aged 2--19 years had a higher prevalence of obesity than whites or blacks within each category of family income.

Although family income was inversely associated with obesity prevalence among white females of all ages and white males aged 2--19 years (p<0.05 for linear trend), the associations were not statistically significant among white men and the majority of sex-age categories of blacks and Mexican Americans. Furthermore, a positive association existed between family income and obesity prevalence among black men aged ≥20 years (p<0.05). The prevalence of obesity increased from 29% (PIR < 1.3) to 45% (PIR ≥ 3.5) across the three categories of family income.

The racial/ethnic differences in obesity prevalence did not vary substantially between 1988--1994 and 2007--2008. However, considerable changes occurred over longer periods among persons aged <20 years. For example, an analysis of data collected during 1971--1974 (NHANES I) and NHANES 1999--2000 (8) indicated that among girls aged 6--17 years, the increases in obesity prevalence were greater among blacks than whites during this period. In contrast with the higher prevalence of obesity observed among black women aged 2--19 years (24%) compared with white women aged 2--19 years (14%) during 2005--2008 (Table 1), the prevalence of obesity did not differ between black and white girls aged 2--17 years during the early 1970s (8). From 1971--1974 to 1999--2000, the mean weight of black adolescent girls (aged 12--16 years) increased by approximately 11 kg (24 lbs), whereas the increase among white girls of the same age was 4 kg (9 lbs) (8).

Although racial/ethnic differences in obesity among persons aged ≥20 years also have increased over time, the magnitude of these increases has been less than among persons aged 2--19 years. For example, the mean BMI of black women aged ≥20 years was 2.4 kg/m2 greater than that of white women during 1960--1962 (9), and this difference increased to 3.4 kg/m2 during 2005--2008. Furthermore, 24% of white women and 42% of black women had a BMI ≥27.3 kg/m2 during 1960--1962 (9), whereas the comparable prevalences were 46% (white women) and 66% (black women) during 2005--2008.

Recent increases in obesity likely result from the interaction of biologic, social, and cultural factors with an environment characterized by limited opportunities for physical activity and an abundance of high-calorie foods (10,11). For example, during the 1980s and 1990s, substantial increases occurred in the availability of processed foods and in the number of meals eaten away from home (12). Neighborhoods with large minority populations have fewer chain supermarkets and produce stores, increasing the difficulty and expense in obtaining healthy foods (13). In addition, breastfeeding, which is inversely associated with childhood obesity, is more prevalent among white women than among black women (14). Furthermore, compared with whites, blacks and Mexican Americans are less likely to engage in regular (nonoccupational) physical activity (15). Differences also exist in attitudes and cultural norms concerning body weight. For example, black and Hispanic women have been reported to be more satisfied with their body size and therefore less likely to try to lose weight than white women (16). Additional cultural factors, such as equating overweight with healthiness in children or the use of food treats by parents as tokens of love and caring, might influence childhood obesity among Mexican Americans (17).

The results of this study are subject to at least two limitations. First, NHANES data do not include an adequate number of persons who are minorities (other than black and Mexican American) to estimate obesity prevalence; other studies (18) have reported high prevalences among American Indians/Alaska Natives. Second, the NHANES racial/ethnic categories differed somewhat between surveys conducted during 1988--1994 and 1999--2008, with the latter surveys including a multiracial category. On the basis of data from the three NHANES (1999--2000, 2001--2002, and 2003--2004) that included both racial/ethnic classifications, approximately 1% of white persons and 2% of black persons in 1988--1994 would have been classified into the category of other race, which included multirace. However, this limited amount of reclassification likely did not influence substantially the results in this report.

Substantial differences exist in obesity prevalence across racial/ethnic groups. An increased emphasis on policy and environmental strategies that support healthy eating and active living, in addition to education campaigns, might reduce these disparities. Environmental approaches supported by CDC through funded programs include ways to improve access to healthy foods in underserved communities (19,20), such as increased accessibility of supermarkets; expanding programs that promote the delivery of regionally grown farm produce to community institutions, farmers' markets, and individuals (Farm-to-Where-You-Are); and promotion of food policy councils to improve the food environment at the state and local levels. Work sites can follow recommendations of the Task Force on Community Preventive Services to implement programs intended to improve the diet and increase the physical activity of employees (21). Strategies to increase low- or no-cost physical activity opportunities in communities, including trails and parks, along with improvements to sidewalks, might also help to reduce disparities in obesity.

References

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