Trends in the Association between Obesity and Socioeconomic Status in U.S. Adults: 1971 to 2000 Qi Zhang* and Youfa Wang† Abstract Discussion: The association between SES and obesity has ZHANG, QI AND YOUFA WANG. Trends in the been weakened over the past 3 decades, when the preva- association between obesity and socioeconomic status in lence of obesity increased dramatically. There are consid- U.S. adults: 1971 to 2000. Obes Res. 2004;12:1622–1632. erable variations in the changes in the associations across Objective: To study the secular trends in the disparity of gender and ethnic groups. Our findings suggest that indi- obesity across socioeconomic status (SES) groups among vidual characteristics are not likely the main cause of the U.S. adults. current obesity epidemic in the U.S., whereas social-envi- Research Methods and Procedures: We used national rep- ronmental factors play an important role. Strategies for resentative data collected in the National Health and Nutri- obesity prevention and management should target all SES tion Examination Surveys conducted in 1971 to 1974, 1976 groups from a societal perspective. to 1980, 1988 to 1994, and 1999 to 2000 from 28,543 adults 20 to 60 years old. Obesity was defined based on BMI Key words: BMI, education, NHANES, socioeconomic calculated using measured weight and height. Trends in the status, trends relationship between obesity and education levels were an- alyzed controlling for age, gender, and ethnicity. Introduction Results: The disparity in obesity across SES (less than high National survey data show that the prevalence of obesity school, high school, and college or above to indicate low, has doubled over the past 3 decades, and it continues to medium, and high SES, respectively) has decreased over the increase in the United States (1–3). Currently, approxi- past 3 decades. In National Health and Nutrition Examina- mately two-thirds of American adults are overweight or tion Surveys I (1971 to 1974), there was as much as a 50% obese (3). Obesity increases the risks of a number of dis- relative difference in the obesity prevalence across the three eases and health conditions, such as cardiovascular disease, groups, but by 1999 to 2000, it decreased to 14%. This trend hypertension, type 2 diabetes, and certain types of cancer was more pronounced in women. The trends of diminishing (4,5). A recent study suggests that an estimated 300,000 disparities in obesity were also revealed by our logistic and deaths in the U.S. are related to the condition (6). The total linear regression analyses. The odds ratio converged to 1 direct and indirect costs associated with obesity have been from the 1970s to 2000. In most sociodemographic groups, estimated at $117 billion in 2000 (7). the relationship between BMI and SES (coefficients) has Socioeconomic status (SES)1 has, in the past, been an been weakened over time. important factor associated with obesity, particularly in women, because SES influences individuals’ energy intake and energy expenditure and, as a result, affects their body Received for review March 2, 2004. fat storage (8). Previous studies have shown that the asso- Accepted in final form July 23, 2004. The costs of publication of this article were defrayed, in part, by the payment of page ciation between SES and obesity may vary by populations, charges. This article must, therefore, be hereby marked “advertisement” in accordance with by gender, and by age (8–10). In general, in industrialized 18 U.S.C. Section 1734 solely to indicate this fact. *Section of General Internal Medicine, University of Chicago, Chicago, Illinois; and countries, low-SES groups are more likely to be obese than †Department of Human Nutrition and Division of Epidemiology and Biostatistics, Univer- their high-SES counterparts, whereas in developing coun- sity of Illinois, Chicago, Illinois. Address correspondence to Qi Zhang, Section of General Internal Medicine, University of Chicago, 5841 South Maryland Avenue, MC 2007, Chicago, IL 60637. E-mail: [email protected] 1 Nonstandard abbreviations: SES, socioeconomic status; NHANES, National Health and Copyright © 2004 NAASO Nutrition Examination Surveys; OR, odds ratio; CI, confidence interval. 1622 OBESITY RESEARCH Vol. 12 No. 10 October 2004 Association between Obesity and Education, Zhang and Wang tries, high-SES groups are more likely to be obese (11–13). reported race and ethnicity. In NHANES 1999 to 2000, Numerous studies conducted in the U.S. have shown that subjects were classified into non-Hispanic white, non-His- low-SES and minority groups have a higher prevalence of panic black, Mexican American, other race (including mul- obesity (14–16). However, to our knowledge, few studies tiracial), and other Hispanic. have systematically examined whether the relationship be- tween obesity and SES has changed over time; in other Measures words, whether the disparity in obesity across SES groups Definitions of Overweight and Obesity. has changed. If the relationship has changed, are there BMI [weight differences across sociodemographic (e.g., gender and eth- (kilograms)/height (meters squared)] was calculated for nic) groups? The present study attempts to answer these each individual based on measured weight and height. Ac- questions. cording to the World Health Organization’s recommenda- Ն Research on the secular trend in the disparity of obesity tion, individuals with a BMI 30 were considered obese across different SES groups is of special importance for (5). Pregnant women were excluded from our analysis. several reasons. First, it can help increase our understanding SES. Most commonly used variables to measure SES of the underlying causes of the rising obesity epidemic in were education, income, and occupational status. Each SES the U.S. Second, it can help predict future trends in obesity measure had its own strengths and limitations when the prevalence across SES groups. Finally, such research find- relationship between SES and health outcomes was studied. ings can guide the development of effective programs and Many researchers suggested that education is the most sta- policies for the prevention and management of obesity in ble and robust indicator of SES (21–23). different SES groups. It can help set priorities for future We selected education level of the subjects as the primary research and interventions, and for government policy to measure of SES because: education is the only available reduce obesity. In the present study, using successive na- SES variable in NHANES 1999 to 2000; education is less tional representative survey data that were collected over likely to be affected by a subject’s body weight status, the past 4 decades in the National Health and Nutrition whereas people’s income might be affected; and education Examination Surveys (NHANES), we examined the secular is more comparable across time than income or occupation. changes in the relationship between obesity and SES in the In addition, education had little missing data, compared U.S. We also examined whether the changes in the associ- with income. We coded the education level as follows: low ations varied across gender and ethnic groups. education (less than high school, meaning ninth grade or less), medium education (high school, meaning 10th to 12th grades), and high education (college or higher) to indicate Research Methods and Procedures low, medium, and high SES. Note that our findings regard- ing the trends in the association between SES and obesity Data were similar if income was used as the indicator of SES in The NHANES include a series of cross-sectional surveys analysis using the NHANES I to III data (data not shown). that have provided nationally representative information on Confounding Variables. the nutrition and health status of the U.S. civilian popula- Previous studies suggest that tion. The National Center for Health Statistics conducted obesity is associated with demographic characteristics such the first and second NHANES (I and II) in 1971 to 1975 and as gender, age, and race/ethnicity (24,25). Meanwhile, SES 1976 to 1980, respectively; NHANES III was conducted may also be correlated with gender, age, and ethnicity. during 1988 to 1994. Since 1999, NHANES has been a Thus, these variables are potential confounders, and it is continuous survey, and the data are now available for the necessary to control for them when studying the relationship first 2 years of that period (1999 to 2000). All of the four between SES and obesity. When studying the difference rounds of NHANES used a stratified, multistage probability across ethnic groups, because of small sample sizes and cluster sampling design. Detailed descriptions of the sample limited information about subjects with Hispanic origin design, interviewing procedures, and physical examinations available in NHANES I and II, we recoded race/ethnicity conducted have been published elsewhere (17–20). into three groups (white, black, and other) to make the In each survey, standardized protocols were used for all analysis comparable across surveys. In this report, we fo- interviews and examinations. Data on weight and height cused on the comparison between white and black. were collected for each individual through direct physical Because there were two gender groups, three ethnic examination in a mobile examination center. In NHANES I groups, and three SES levels, 18 gender-race-education and II, race-ethnic group was classified as white, black, and groups were created in our analysis. If we would have “other” based on observation. In NHANES III, subjects stratified the subjects further into four age groups, the were classified as non-Hispanic white, non-Hispanic black, sample size for each gender-race-age-SES group would Mexican American, and other ethnic groups, based on self- have been too small. Therefore, we reported prevalence OBESITY RESEARCH Vol. 12 No. 10 October 2004 1623 Association between Obesity and Education, Zhang and Wang Table 1. Sociodemographic characteristics and anthropometric measures of U.S.
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