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More Overweight Adolescents Think They Are Just Fine
More Overweight Adolescents Think They Are Just Fine Generational Shift in Body Weight Perceptions Among Adolescents in the U.S. Hui Lu, PhD,1,2 Yelena N. Tarasenko, DrPH,3,4 Farrah C. Asgari-Majd, MPH,4 Cherell Cottrell-Daniels, MPH,4 Fei Yan, MD, PhD,2 Jian Zhang, MD, DrPH4 Introduction: As obesity prevalence has increased, adolescents’ self-perceived body weight might have shifted accordingly. This study aims to investigate the generational shift in adolescents’ perception of their body weight. Methods: We used data from adolescents aged 12–16 years who participated in the National Health and Nutrition Examination Survey in 1988–1994 (early, n¼1,720) or 2007–2012 (recent, n¼2,518). Self-perceived weight status was assessed by face-to-face interviews, and BMI z-scores were calculated using directly measured weight and height with the 2000 CDC Growth Charts as the reference. The analysis was conducted in 2013. Results: The median BMI z-score of self-perceived overweight adolescents increased from 1.32 (95% CI¼1.18, 1.46) among adolescents interviewed early to 1.82 (1.74, 1.90) among adolescents interviewed recently. After adjusting for age, race/ethnicity, sex, and family income, the probability of self-perceiving as “overweight” declined by 29% for overweight/obese adolescents interviewed recently (probability ratio [PR]¼0.71 [0.62, 0.82]) compared with adolescents with the same z-scores but interviewed early. The declining tendency of accurately self-perceiving as overweight was most pronounced among whites (PR¼0.64 [0.48, 0.85]), and least among blacks (PR¼0.76 [0.58, 0.99]). Both boys and girls interviewed recently were significantly less likely to accurately self-perceive as overweight (PR¼0.70 [0.56, 0.89] and 0.73 [0.61, 0.87], respectively) compared with their counterparts who were interviewed early.
Conclusions: Fewer overweight/obese adolescents self-perceived as such in the 2007–2012 survey compared with the 1988–1994 survey. The declining tendency among overweight or obese adolescents may be indicative of a generational shift in body weight perceptions. (Am J Prev Med 2015;](]):]]]–]]]) & 2015 American Journal of Preventive Medicine
Introduction
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arious biological, behavioral, and social factors along with the complex interactions between them contribute to obesity in adolescence. These factors
From the 1Department of Social Medicine and Health Education, School of Public Health, Nanjing Medical University, Nanjing, Jiangsu, China; 2 Department of Social Medicine, School of Public Health, Fudan University, Shanghai, China; 3Department of Health Policy and Management, Jiann-Ping Hsu College of Public Health, Georgia Southern University, Statesboro, Georgia, and 4Department of Epidemiology, Jiann-Ping Hsu College of Public Health, Georgia Southern University, Statesboro, Georgia Address correspondence to: Jian Zhang, MD, DrPH, Jiann-Ping Hsu College of Public Health, Georgia Southern University, PO Box 8015, Statesboro GA 30460. E-mail: [email protected]. 0749-3797/$36.00 http://dx.doi.org/10.1016/j.amepre.2015.03.024
& 2015 American Journal of Preventive Medicine
have been targeted with obesity prevention and control programs, many of which have yielded modest results.1,2 Failure to accurately recognize one’s weight status as overweight or obese may be a barrier to engagement in effective weight-loss strategies and behaviors.3–5 According to the cognitive–behavioral theory, becoming conscious of one’s excess weight and gaining an understanding of the health risk posed by excess weight are the precursors to adopting behavioral changes necessary for appropriate weight control.6,7 Adolescents with accurate self-perceptions of their body weight have greater readiness to make weight-related behavioral changes and are more effective in making the changes.8–13 By contrast, overweight adolescents who do not perceive their weight status properly are less likely to desire weight loss,9 and are more likely to have a poor diet.11
Published by Elsevier Inc.
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Correlates of adolescents’ self-perceptions of their weight status include, but are not limited to, actual weight status, race, sex, and socioeconomic characteristics.14–19 In addition to these individual-level factors, sociocultural factors have been suggested as potential correlates of perceptions among adolescents.20 As obesity prevalence has more than doubled in adolescents during the past 20 years,21 socially accepted “normal” weight may also be shifting accordingly. Large national surveys show that desired weight among adults has been shifting upward in a similar pattern to the increase in obesity,22 and social norms related to body weight are shifting toward heavier among the adult population.23 However, to our knowledge, shifting of socially accepted norm weight has not been examined among adolescents—a subpopulation experiencing pronounced emotional, social, physical changes, and a significant increase of obesity prevalence. Thoroughly examining the evolution of adolescents’ self-perceptions may yield instructive information for fine-tuning obesity prevention strategies. Hence, the objective of our study is to examine the generational shift in adolescents’ perceptions of their weight status.
Methods Study Population We used data from the third National Health and Nutrition Examination Survey (NHANES III, 1988–1994, hereafter referred to as “early survey”) and three cycles of the Continuous NHANES (2007–2008, 2009–2010, and 2011–2012, referred to as “recent survey”). The NHANES is conducted by the CDC National Center for Health Statistics to assess the health and nutritional status of the U.S. civilian, non-institutionalized population. It uses a complex multistage probability sampling design, with some subgroups oversampled. The unweighted response rate for the total examined sample of the early survey was 78%, and ranged from 75% to 77% across three recent survey cycles.24 Data used in the current report were collected through in-person interviews and standardized physical examinations during which body weight and height were measured in mobile examination centers.24,25 The early survey included 2,028, and the recent one included 2,984 adolescents aged 12–16 years. Among these, 145 underweight (i.e., BMI percentile less than fifth) adolescents were excluded because the small number of underweight adolescents limited our ability to produce stable estimates. Among the remaining, 270 did not answer the question about self-perceived body weight, 326 had no data on family income, and 33 were interviewed on body perception via proxies. After excluding the underweight and those with missing data, the final samples for the current report consisted of 1,720 and 2,518 adolescents from the early and recent surveys, respectively. The NHANES protocol was reviewed and approved by the National Center for Health Statistic’s IRB, and the current study was exempt from ethics review by Georgia Southern University’s IRB committee.
Measures NHANES body measurement data were collected by trained technicians following a standard protocol. Body weight was taken using a digital floor scale and determined to the nearest 0.05 kg (Toledo 2181 Scale). Standing height was measured to the nearest 0.1 cm with standardized measuring equipment (Holtain Height Stadiometer). A more detailed description of the anthropometry examination can be found on the NHANES website.26 Using a computerized formula derived from the 2000 CDC Growth Charts, we translated directly measured weight and height into to sex- and agespecific BMI percentiles and z-scores.27,28 Based on IOM recommendations,29 we categorized adolescents as obese if they had a sex- and age-specific BMI at or above the 95th percentile, as overweight if at or above the 85th but less than the 95th percentile, and as normal weight if at or above the fifth but less than the 85th percentile. The self-perception of the participant’s weight (hereafter referred to as “self-perception”) was obtained from the Youth Questionnaires in the early survey and the Weight History Module in the recent survey. In both surveys, the respondents were asked, Do you consider yourself to be overweight, underweight, or just about the right weight? Adolescents’ responses to this question were used to operationalize their self-perceived body status as “overweight,” “underweight,” and “about the right weight.” We explored the shift in self-perceptions by major demographic variables, including sex; race/ethnicity (non-Hispanic white, nonHispanic black, Hispanic, and other); and family income level. The demographics file of NHANES included information that was collected using the Sample Person and Family Demographics questionnaires, and provided family- and individual-level information. People aged Z16 years and emancipated minors were interviewed directly. A proxy provided information on family income for the adolescents included in the current study who were aged o16 years.24,25 The family income was measured by the poverty–income ratio (PIR), the ratio of income to the family’s poverty threshold defined by the U.S. Census Bureau. Families with PIR o1.0 were defined as “poor,” 1r PIR o2 as “near poor,” 2r PIR o4 as “middle-income,” and PIR Z4 as “high income.”30
Statistical Analysis To obtain national estimates adjusted for the oversampling of specific populations and nonresponse bias, we used the SAS procedures specifically developed for survey data (SAS, version 9.3) with appropriate weighting and nesting variables. The means and medians of the age- and sex-specific BMI z-scores were compared among self-perceived overweight adolescents to assess the shift in the distribution of BMI z-scores from the early to the recent survey. The differences of median score among selfperceived overweight adolescents were further examined by the sociodemographic stratum (i.e., age group; boys and girls; whites, blacks, Hispanic Americans, and others; poor, near poor, middle, and high income) to identify the subpopulation experiencing the most significant shift. The probability ratios (PRs; recent survey over early survey) were calculated to measure the change in the probability of self-perceiving as “overweight” among actually overweight or obese adolescents. Multivariable adjusted PRs were obtained using modified Poisson regression analyses.31 We combined the data from the two survey periods and created a dummy variable to differentiate the adolescents interviewed recently from www.ajpmonline.org
Lu et al / Am J Prev Med 2015;](]):]]]–]]] those interviewed early. The covariates adjusted for in the multivariable analyses included BMI z-scores, age, sex, race/ethnicity, and family income level. The critical p-value to judge the statistical significance or calculate confidence intervals was 0.05 (two-sided). The analyses were performed in 2013.
Results Figure 1 illustrates the mismatch between perceived and measured body weight by sex and the survey period. Across all three panels, for normal weight (left panel); overweight (middle panel); obese (right panel); boys (top section); and girls (bottom section), the percentages of self-perceived overweight declined between the
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two survey periods. Less than a quarter (21%) of overweight boys correctly perceived themselves as overweight in recent survey in contrast to 28% of their counterparts in the earlier survey (po0.05, top section of middle panel). The percentage of the overweight girls who appropriately perceived themselves as overweight declined from 79% in the early survey to 36% in the recent survey (po0.05, bottom section of middle panel), declining by more than 50%. The percentage of obese boys who accurately perceived their weight status declined from 80% to 58% (po0.05). Overall, girls’ self-perception was more accurate than boys’ in both early and recent surveys.
Figure 1. Self-perceived body weight by sex and BMI category. Adolescents aged 12–16, NHANES 1988–1994 (n¼1,720) and 2007–2012 (n¼2,518), based on directly measured height and weight and the CDC 2000 growth chart. The extreme values, i.e., BMI z-score 44, were excluded. Overweight was defined as age- and sex-specific BMI percentiles between 85 and 94.99, and obese as BMI percentiles Z95.29 NHANES, the National Health Examination and Nutrition Survey.
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To graphically illustrate the shifting of self-perceptions, we present the distributions of the BMI z-scores among self-perceived overweight adolescents in Figure 2, with the top panel representing the early (unweighted n¼528) and the bottom representing the recent survey (n¼570). The bars indicate the number of children in each BMI z-score group categorized with 0.2 as the interval. Compared with that of the early survey, the distribution of the recent survey not only skewed to the left (skewness, –0.49 for the early survey and –0.87 for the recent survey) but also shifted to the right (median¼1.32, 95% CI¼1.18, 1.46, for the early survey and 1.82, 95% CI¼1.74, 1.90, for the recent survey). With statistical significance, the means shifted from 1.23 (95% CI¼1.11, 1.35) to 1.74 (95% CI¼1.66, 1.82) accordingly. We also examined the differences of the BMI z-scores between the two survey periods among self-perceived overweight adolescents by the level of sociodemographic stratum (Appendix Table 1). Compared with the early survey, the median BMI z-scores increased unevenly across the strata of demographic variables. The largest
increase occurred among those aged 12 years (Δ z-score¼0.73, po0.05) and the smallest with those aged 16 years (Δ z-score¼0.05, p40.05). Although the median scores increased for both boys and girls (Δ z-score¼0.40 for boys and 0.63 for girls, po0.05 for both), self-perceived overweight boys showed a higher median score compared with their female peers. The median score was 1.92 (95% CI¼1.79, 2.06) for boys and 1.78 (95% CI¼1.68, 1.88) for girls (p40.05) in the recent survey. Among black adolescents, a significant increase of median BMI z-score occurred between the two survey periods (Δ z-score¼0.60, po0.05), and the median score among self-perceived overweight blacks was the highest in all sociodemographic strata (2.10, 95% CI¼1.93, 2.27 in the recent survey). White adolescents who self-perceived as overweight, however, experienced a larger increase in the median BMI zscore (Δ z-score¼0.62) compared with their counterparts from all other racial/ethnic groups. The forest plot (Figure 3) displays the adjusted PRs of accurately self-perceiving as overweight between the two survey periods among actually overweight/obese
Figure 2. Shifting distribution of BMI z-scores between two survey periods. Self-perceived overweight adolescents, NHANES 1988–1994 and 2007–2012. Age and sex-specific BMI percentiles were calculated based on directly measured height and weight and the CDC 2000 growth chart. Extreme values, i.e., BMI z-score 44 were excluded.28 The statistics were calculated using SAS survey procedures with appropriate weighting and nesting variables; however, the histograms were generated without the weighting and nesting variable using PROC UNIVARIATE of SAS. *Statistically different between two survey periods (po0.05). NHANES, the National Health Examination and Nutrition Survey.
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Figure 3. The adjusted probability ratios of self-perceiving as “overweight” among actually overweight/obese adolescents, NHANES 1988–1994 and 2007–2012. The NHANES III (1988-1994) was used as the reference survey period. Modified Poisson regression was used to adjust for age, sex, race/ethnicity, family income, and the BMI z-scores.31 %, percentage of actually overweight/obese children who were also self-perceived as overweight; N, un-weighted numbers of actually overweight/ obese children; NHANES, the National Health and Nutrition Examination Survey; PR, probability ratio.
adolescents (unweighted n¼533 for the early survey, the reference period; n¼988 for the recent survey). After adjusting for the sociodemographic characteristics and BMI z-scores, the point estimates of PRs for all sociodemographic strata were lower than 1 (to left side of the reference line). Significantly fewer overweight/obese adolescents perceived themselves as such in the recent survey compared with those in the early survey (PR¼0.63, 95% CI¼0.48, 0.84, for the 12-year-old group; PR¼0.72, 95% CI¼0.53, 0.96, for the 13-yearold group). Across racial/ethnic strata, the declining tendency of accurately self-perceiving as overweight among overweight/obese adolescents was most noticeable among white adolescents (PR¼0.64, 95% CI¼0.48, 0.85), and least among black adolescents (PR¼0.76, 95% CI¼0.58, 0.99). Both boys and girls interviewed recently were less likely to correctly self-perceive as overweight (PR¼0.70, 95% CI¼0.56, 0.89, for boys and PR¼0.73, 95% CI¼0.61, 0.87, for girls). Overall, after ] 2015
controlling for age, race/ethnicity, sex, and family income level, overweight/obese adolescents interviewed recently were 29% less likely to accurately self-perceive as overweight (PR¼0.71, 95% CI¼0.62, 0.82) when compared with their counterparts with the same body weight but interviewed earlier.
Discussion Using nationally representative data, we show an increase in the mean and median BMI z-scores among adolescents who self-perceived themselves as overweight. A declining tendency to accurately self-perceive as overweight among actually overweight or obese adolescents was observed before and after adjusting for sociodemographic characteristics as well as age- and sex-specific BMI z-scores. The gap between the reality and perceptions about body weight status has been expanding among adolescents.
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There are several potential explanations for the declining tendency to correctly self-perceive as “overweight” among overweight/obese adolescents. In the wake of the obesity pandemic, the media, weight-loss industries, and medical communities have encouraged adolescents to maintain slender frames.32 Facing harsher messages, more and more overweight/obese adolescents may be increasingly reluctant to admit that they are overweight.33 The social comparison theory provides an additional explanation. The theory postulates that individuals assess themselves in relation to others rather than against an absolute scale.34,35 Paralleled with the increasing prevalence of obesity, increasingly more adolescents compare themselves to their peers with heavier body weight, to maintain a positive image of their own weight status. The numbers and the growth charts recommended by professional organizations are more remote than the peers surrounding them daily and are less likely to be used by adolescents as references. In addition to the current study, the Quebec Child and Adolescent Health and Social Survey also obtained evidence to support the social comparison theory. The Quebec study observed that overweight/obese adolescents whose schoolmates had a higher BMI score were more likely to perceive themselves to be of normal weight.36 Additionally, the confusing nature of the definition of “being overweight or obese” may underlie the increasing number of overweight/obese adolescents who failed to recognize their overweight status. Unlike the assessment of body weight for adults, defining obesity or overweight in adolescents is more formidable because this group continuously experiences changes in body composition affecting or being affected by puberty.37,38 Instead of setting thresholds for categorization, the BMI-for-age percentile from a reference population is used to allow comparison among adolescents of the same sex and age. The BMI-for-age percentile, however, is poorly correlated with the degree of adiposity among adolescents, and a study found that less than one half of children and adolescents whose BMIs-for-age are in the overweight range (85th–94.9th percentile) have high adiposity.39 The confusing nature is further compounded by the change of definition of overweight. In 1994, the Expert Committee on Clinical Guidelines for Overweight in Adolescent Preventive Services recommended that the children whose BMI was at or above the 95th percentile of the 2000 CDC Growth Charts should be considered “overweight,” and those at or above the 85th percentile as “at the risk of overweight.”40 However, to convey the urgency of childhood obesity, IOM redefined pediatric overweight and obese in 2005. Children previously defined as “overweight” were redefined as “obese,” whereas children “at the risk of overweight” were
redefined as “overweight.”41 The fundamental changes of the definitions took place between the two survey periods covered by this study. Becoming conscious of one’s excess weight is the precursor to adopting behavioral changes necessary for appropriate weight control.6,42 The declining tendency of correctly perceiving overweight status presents a vast challenge to obesity prevention among adolescents, making the overweight and obese adolescents less motivated to actively engage in effective weight-loss behaviors.3–5 On the other hand, overweight and obese adolescents experience psychological distress, including depressive mood.43,44 In certain extreme cases, body dissatisfaction has a devastating impact on adolescents’ psycho-emotional well-being and may cause adolescents to become overly concerned or impose detrimental levels of dietary restrictions.45,46 Increasing percentages of overweight/obese adolescents self-perceiving their body weight as “about the right weight” may suggest a reduction in social pressure on adolescents to maintain lower body weight. The generational shift of body weight perception declares urgency to explore the novel strategies to delicately protect adolescents’ attitude toward body image while correcting the body misperception to embark on lifestyle change for a healthy weight. In addition to family-based prevention strategies, officebased physician engagement should be increased. Discussion of body weight concerns between a physician and adolescents might be an effective tool to empower adolescents to actively engage in preventive efforts.45
Limitations This study is subject to limitations. A large number of adolescents were excluded from the current analyses for various reasons, and significant differences in SES were identified between the included and excluded NHANES participants (Appendix Table 2). Therefore, the generalizability of the current analyses may be compromised. More than two survey periods may be needed to assess the generational shift over a long time scale. The current study has its unique strengths. Although the NHANES is a cross-sectional survey, observations from different survey periods allowed for exploration of the overall trends. To our knowledge, no prior studies have used nationally representative samples to investigate a generational shift in body weight perceptions among adolescents in the U.S. The spectrum of participants, including three major races/ethnicities and other sociodemographic groups, allowed us to note the common thread among adolescents from various subpopulations. www.ajpmonline.org
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Conclusions In spite of an increase in the body weight of adolescents, fewer adolescents aged 12–16 years accurately perceived their weight status in the early 21st century compared with the late 20th century. The declining tendency may be indicative of a generational shift in body weight perceptions, presenting a vast challenge to obesity prevention among adolescents. Further studies are warranted on how to correct the misperceptions, promote adolescents’ engagement in appropriate weight control behaviors, and reduce the weight-related social pressure on adolescents while maintaining appropriate motivations for a healthier lifestyle. Methodologic studies examining the correlation between body perceptions and the degree of adiposity among adolescents are also needed. A draft of this manuscript was initially prepared as a group project in a summer workshop on advanced research methods sponsored by the National Key Laboratory of Public Health Security, School of Public Health, Fudan University, Shanghai, China. The workshop was led by Dr. Zhang from Jiann-Ping Hsu College of Public Health, Georgia Southern University, and Dr. Yan from Fudan University, China. We are indebted to the graduate students who have contributed substantially to the early drafts with care. The project was conducted with no specific funding. However, the National Key Laboratory of Public Health Security, School of Public Health, Fudan University, China, sponsored a workshop, allowing this project to be completed in a timely manner. No financial disclosures were reported by the authors of this paper.
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24. National Center for Health Statistics. NHANES Response Rates and Population Totals. 2013. National Health and Nutrition Examination Survey. Atlanta, GA: CDC, National Center for Health Statistics, November 20, 2013. 25. CDC. National Health and Nutrition Examination Survey—questionnaires, datasets, and related documentation. www.cdc.gov/nchs/ nhanes/about_nhanes.htm. Published November 3, 2009. 26. Westat I. National Health and Nutrition Examination Survey—body measurements (anthropometry). www.cdc.gov/nchs/nhanes/about_ nhanes.htm. Published October 3, 1988. 27. Kuczmarski RJ, Ogden CL, Grummer-Strawn LM, et al. CDC growth charts: United States. Adv Data. 2000;314:1–27. 28. Kuczmarski RJ, Ogden CL, Guo SS, et al. CDC growth charts for the United States: methods and development. Vital Health Stat 11. 2000;2002(246):1–190. 29. Krebs NF, Himes JH, Jacobson D, et al. Assessment of child and adolescent overweight and obesity. Pediatrics. 2007;120(suppl 4): S193–S228. http://dx.doi.org/10.1542/peds.2007-2329D. 30. Housing and Household Economic Statistics Division, U.S. Census Bureau. How the Census Bureau measures poverty (official measure). www.census.gov/hhes/www/poverty/about/overview/measure.html. Published September 29, 2009. Updated February 11, 2010. 31. Spiegelman D, Hertzmark E, Easy SAS. calculations for risk or prevalence ratios and differences. Am J Epidemiol. 2005;162(3):199– 200. http://dx.doi.org/10.1093/aje/kwi188. 32. Frisco ML, Houle JN, Martin MA. The image in the mirror and the number on the scale: weight, weight perceptions, and adolescent depressive symptoms. J Health Soc Behav. 2010;51(2):215–228. http: //dx.doi.org/10.1177/0022146510372353. 33. Latner JD, Stunkard AJ. Getting worse: the stigmatization of obese children. Obes Res. 2003;11(3):452–456. http://dx.doi.org/10.1038/ oby.2003.61. 34. Suls J, Martin R, Wheeler L. Social comparison: why, with whom, and with what effect? Curr Dir Psychol Sci. 2002;11(5):159–163. http://dx. doi.org/10.1111/1467-8721.00191. 35. Festinger L. Social psychology and group processes. Annu Rev Psychol. 1955;6:187–216. http://dx.doi.org/10.1146/annurev.ps.06.020155. 001155. 36. Maximova K, McGrath JJ, Barnett T, et al. Do you see what I see? Weight status misperception and exposure to obesity among children and adolescents. Int J Obes (Lond). 2008;32(6):1008–1015. http://dx. doi.org/10.1038/ijo.2008.15.
37. Adair LS. Child and adolescent obesity: epidemiology and developmental perspectives. Physiol Behav. 2008;94(1):8–16. http://dx.doi.org/ 10.1016/j.physbeh.2007.11.016. 38. Wang Y. Epidemiology of childhood obesity—methodological aspects and guidelines: what is new? Int J Obes Relat Metab Disord. 2004;28 (suppl 3):S21–S28. http://dx.doi.org/10.1038/sj.ijo.0802801. 39. Flegal KM, Ogden CL, Yanovski JA, et al. High adiposity and high body mass index-for-age in U.S. children and adolescents overall and by race-ethnic group. Am J Clin Nutr. 2010;91(4):1020–1026. http://dx. doi.org/10.3945/ajcn.2009.28589. 40. Himes JH, Dietz WH. Guidelines for overweight in adolescent preventive services: recommendations from an expert committee. The Expert Committee on Clinical Guidelines for Overweight in Adolescent Preventive Services. Am J Clin Nutr. 1994;59(2):307–316. 41. Koplan JP, Liverman CT, Kraak VI. Preventing childhood obesity: health in the balance: executive summary. J Am Diet Assoc. 2005;105 (1):131–138. http://dx.doi.org/10.1016/j.jada.2004.11.023. 42. Brener ND, Eaton DK, Lowry R, McManus T. The association between weight perception and BMI among high school students. Obes Res. 2004;12(11):1866–1874. http://dx.doi.org/10.1038/oby.2004.232. 43. Neumark-Sztainer D, Wall M, Guo J, et al. Obesity, disordered eating, and eating disorders in a longitudinal study of adolescents: how do dieters fare 5 years later? J Am Diet Assoc. 2006;106(4):559–568 http://dx.doi.org/10.1016/j.jada.2006.01.003. 44. Goodman E, Whitaker RC. A prospective study of the role of depression in the development and persistence of adolescent obesity. Pediatrics. 2002;110(3):497–504. http://dx.doi.org/10.1542/peds.110.3.497. 45. Hayward J, Millar L, Petersen S, Swinburn B, Lewis AJ. When ignorance is bliss: weight perception, body mass index and quality of life in adolescents. Int J Obes (Lond). 2014;38(10):1328–1334. http://dx. doi.org/10.1038/ijo.2014.78. 46. Rhee KE, De Lago CW, Arscott-Mills T, Mehta SD, Davis RK. Factors associated with parental readiness to make changes for overweight children. Pediatrics. 2005;116(1):e94–e101. http://dx.doi.org/10.1542/ peds.2004-2479.
Appendix Supplementary data Supplementary data associated with this article can be found at http://dx.doi.org/10.1016/j.amepre.2015.03.024.
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Conclusions: Fewer overweight/obese adolescents self-perceived as such in the 2007–2012 survey compared with the 1988–1994 survey. The declining tendency among overweight or obese adolescents may be indicative of a generational shift in body weight perceptions. (Am J Prev Med 2015;](]):]]]–]]]) & 2015 American Journal of Preventive Medicine
Introduction
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arious biological, behavioral, and social factors along with the complex interactions between them contribute to obesity in adolescence. These factors
From the 1Department of Social Medicine and Health Education, School of Public Health, Nanjing Medical University, Nanjing, Jiangsu, China; 2 Department of Social Medicine, School of Public Health, Fudan University, Shanghai, China; 3Department of Health Policy and Management, Jiann-Ping Hsu College of Public Health, Georgia Southern University, Statesboro, Georgia, and 4Department of Epidemiology, Jiann-Ping Hsu College of Public Health, Georgia Southern University, Statesboro, Georgia Address correspondence to: Jian Zhang, MD, DrPH, Jiann-Ping Hsu College of Public Health, Georgia Southern University, PO Box 8015, Statesboro GA 30460. E-mail: [email protected]. 0749-3797/$36.00 http://dx.doi.org/10.1016/j.amepre.2015.03.024
& 2015 American Journal of Preventive Medicine
have been targeted with obesity prevention and control programs, many of which have yielded modest results.1,2 Failure to accurately recognize one’s weight status as overweight or obese may be a barrier to engagement in effective weight-loss strategies and behaviors.3–5 According to the cognitive–behavioral theory, becoming conscious of one’s excess weight and gaining an understanding of the health risk posed by excess weight are the precursors to adopting behavioral changes necessary for appropriate weight control.6,7 Adolescents with accurate self-perceptions of their body weight have greater readiness to make weight-related behavioral changes and are more effective in making the changes.8–13 By contrast, overweight adolescents who do not perceive their weight status properly are less likely to desire weight loss,9 and are more likely to have a poor diet.11
Published by Elsevier Inc.
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Correlates of adolescents’ self-perceptions of their weight status include, but are not limited to, actual weight status, race, sex, and socioeconomic characteristics.14–19 In addition to these individual-level factors, sociocultural factors have been suggested as potential correlates of perceptions among adolescents.20 As obesity prevalence has more than doubled in adolescents during the past 20 years,21 socially accepted “normal” weight may also be shifting accordingly. Large national surveys show that desired weight among adults has been shifting upward in a similar pattern to the increase in obesity,22 and social norms related to body weight are shifting toward heavier among the adult population.23 However, to our knowledge, shifting of socially accepted norm weight has not been examined among adolescents—a subpopulation experiencing pronounced emotional, social, physical changes, and a significant increase of obesity prevalence. Thoroughly examining the evolution of adolescents’ self-perceptions may yield instructive information for fine-tuning obesity prevention strategies. Hence, the objective of our study is to examine the generational shift in adolescents’ perceptions of their weight status.
Methods Study Population We used data from the third National Health and Nutrition Examination Survey (NHANES III, 1988–1994, hereafter referred to as “early survey”) and three cycles of the Continuous NHANES (2007–2008, 2009–2010, and 2011–2012, referred to as “recent survey”). The NHANES is conducted by the CDC National Center for Health Statistics to assess the health and nutritional status of the U.S. civilian, non-institutionalized population. It uses a complex multistage probability sampling design, with some subgroups oversampled. The unweighted response rate for the total examined sample of the early survey was 78%, and ranged from 75% to 77% across three recent survey cycles.24 Data used in the current report were collected through in-person interviews and standardized physical examinations during which body weight and height were measured in mobile examination centers.24,25 The early survey included 2,028, and the recent one included 2,984 adolescents aged 12–16 years. Among these, 145 underweight (i.e., BMI percentile less than fifth) adolescents were excluded because the small number of underweight adolescents limited our ability to produce stable estimates. Among the remaining, 270 did not answer the question about self-perceived body weight, 326 had no data on family income, and 33 were interviewed on body perception via proxies. After excluding the underweight and those with missing data, the final samples for the current report consisted of 1,720 and 2,518 adolescents from the early and recent surveys, respectively. The NHANES protocol was reviewed and approved by the National Center for Health Statistic’s IRB, and the current study was exempt from ethics review by Georgia Southern University’s IRB committee.
Measures NHANES body measurement data were collected by trained technicians following a standard protocol. Body weight was taken using a digital floor scale and determined to the nearest 0.05 kg (Toledo 2181 Scale). Standing height was measured to the nearest 0.1 cm with standardized measuring equipment (Holtain Height Stadiometer). A more detailed description of the anthropometry examination can be found on the NHANES website.26 Using a computerized formula derived from the 2000 CDC Growth Charts, we translated directly measured weight and height into to sex- and agespecific BMI percentiles and z-scores.27,28 Based on IOM recommendations,29 we categorized adolescents as obese if they had a sex- and age-specific BMI at or above the 95th percentile, as overweight if at or above the 85th but less than the 95th percentile, and as normal weight if at or above the fifth but less than the 85th percentile. The self-perception of the participant’s weight (hereafter referred to as “self-perception”) was obtained from the Youth Questionnaires in the early survey and the Weight History Module in the recent survey. In both surveys, the respondents were asked, Do you consider yourself to be overweight, underweight, or just about the right weight? Adolescents’ responses to this question were used to operationalize their self-perceived body status as “overweight,” “underweight,” and “about the right weight.” We explored the shift in self-perceptions by major demographic variables, including sex; race/ethnicity (non-Hispanic white, nonHispanic black, Hispanic, and other); and family income level. The demographics file of NHANES included information that was collected using the Sample Person and Family Demographics questionnaires, and provided family- and individual-level information. People aged Z16 years and emancipated minors were interviewed directly. A proxy provided information on family income for the adolescents included in the current study who were aged o16 years.24,25 The family income was measured by the poverty–income ratio (PIR), the ratio of income to the family’s poverty threshold defined by the U.S. Census Bureau. Families with PIR o1.0 were defined as “poor,” 1r PIR o2 as “near poor,” 2r PIR o4 as “middle-income,” and PIR Z4 as “high income.”30
Statistical Analysis To obtain national estimates adjusted for the oversampling of specific populations and nonresponse bias, we used the SAS procedures specifically developed for survey data (SAS, version 9.3) with appropriate weighting and nesting variables. The means and medians of the age- and sex-specific BMI z-scores were compared among self-perceived overweight adolescents to assess the shift in the distribution of BMI z-scores from the early to the recent survey. The differences of median score among selfperceived overweight adolescents were further examined by the sociodemographic stratum (i.e., age group; boys and girls; whites, blacks, Hispanic Americans, and others; poor, near poor, middle, and high income) to identify the subpopulation experiencing the most significant shift. The probability ratios (PRs; recent survey over early survey) were calculated to measure the change in the probability of self-perceiving as “overweight” among actually overweight or obese adolescents. Multivariable adjusted PRs were obtained using modified Poisson regression analyses.31 We combined the data from the two survey periods and created a dummy variable to differentiate the adolescents interviewed recently from www.ajpmonline.org
Lu et al / Am J Prev Med 2015;](]):]]]–]]] those interviewed early. The covariates adjusted for in the multivariable analyses included BMI z-scores, age, sex, race/ethnicity, and family income level. The critical p-value to judge the statistical significance or calculate confidence intervals was 0.05 (two-sided). The analyses were performed in 2013.
Results Figure 1 illustrates the mismatch between perceived and measured body weight by sex and the survey period. Across all three panels, for normal weight (left panel); overweight (middle panel); obese (right panel); boys (top section); and girls (bottom section), the percentages of self-perceived overweight declined between the
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two survey periods. Less than a quarter (21%) of overweight boys correctly perceived themselves as overweight in recent survey in contrast to 28% of their counterparts in the earlier survey (po0.05, top section of middle panel). The percentage of the overweight girls who appropriately perceived themselves as overweight declined from 79% in the early survey to 36% in the recent survey (po0.05, bottom section of middle panel), declining by more than 50%. The percentage of obese boys who accurately perceived their weight status declined from 80% to 58% (po0.05). Overall, girls’ self-perception was more accurate than boys’ in both early and recent surveys.
Figure 1. Self-perceived body weight by sex and BMI category. Adolescents aged 12–16, NHANES 1988–1994 (n¼1,720) and 2007–2012 (n¼2,518), based on directly measured height and weight and the CDC 2000 growth chart. The extreme values, i.e., BMI z-score 44, were excluded. Overweight was defined as age- and sex-specific BMI percentiles between 85 and 94.99, and obese as BMI percentiles Z95.29 NHANES, the National Health Examination and Nutrition Survey.
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To graphically illustrate the shifting of self-perceptions, we present the distributions of the BMI z-scores among self-perceived overweight adolescents in Figure 2, with the top panel representing the early (unweighted n¼528) and the bottom representing the recent survey (n¼570). The bars indicate the number of children in each BMI z-score group categorized with 0.2 as the interval. Compared with that of the early survey, the distribution of the recent survey not only skewed to the left (skewness, –0.49 for the early survey and –0.87 for the recent survey) but also shifted to the right (median¼1.32, 95% CI¼1.18, 1.46, for the early survey and 1.82, 95% CI¼1.74, 1.90, for the recent survey). With statistical significance, the means shifted from 1.23 (95% CI¼1.11, 1.35) to 1.74 (95% CI¼1.66, 1.82) accordingly. We also examined the differences of the BMI z-scores between the two survey periods among self-perceived overweight adolescents by the level of sociodemographic stratum (Appendix Table 1). Compared with the early survey, the median BMI z-scores increased unevenly across the strata of demographic variables. The largest
increase occurred among those aged 12 years (Δ z-score¼0.73, po0.05) and the smallest with those aged 16 years (Δ z-score¼0.05, p40.05). Although the median scores increased for both boys and girls (Δ z-score¼0.40 for boys and 0.63 for girls, po0.05 for both), self-perceived overweight boys showed a higher median score compared with their female peers. The median score was 1.92 (95% CI¼1.79, 2.06) for boys and 1.78 (95% CI¼1.68, 1.88) for girls (p40.05) in the recent survey. Among black adolescents, a significant increase of median BMI z-score occurred between the two survey periods (Δ z-score¼0.60, po0.05), and the median score among self-perceived overweight blacks was the highest in all sociodemographic strata (2.10, 95% CI¼1.93, 2.27 in the recent survey). White adolescents who self-perceived as overweight, however, experienced a larger increase in the median BMI zscore (Δ z-score¼0.62) compared with their counterparts from all other racial/ethnic groups. The forest plot (Figure 3) displays the adjusted PRs of accurately self-perceiving as overweight between the two survey periods among actually overweight/obese
Figure 2. Shifting distribution of BMI z-scores between two survey periods. Self-perceived overweight adolescents, NHANES 1988–1994 and 2007–2012. Age and sex-specific BMI percentiles were calculated based on directly measured height and weight and the CDC 2000 growth chart. Extreme values, i.e., BMI z-score 44 were excluded.28 The statistics were calculated using SAS survey procedures with appropriate weighting and nesting variables; however, the histograms were generated without the weighting and nesting variable using PROC UNIVARIATE of SAS. *Statistically different between two survey periods (po0.05). NHANES, the National Health Examination and Nutrition Survey.
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Figure 3. The adjusted probability ratios of self-perceiving as “overweight” among actually overweight/obese adolescents, NHANES 1988–1994 and 2007–2012. The NHANES III (1988-1994) was used as the reference survey period. Modified Poisson regression was used to adjust for age, sex, race/ethnicity, family income, and the BMI z-scores.31 %, percentage of actually overweight/obese children who were also self-perceived as overweight; N, un-weighted numbers of actually overweight/ obese children; NHANES, the National Health and Nutrition Examination Survey; PR, probability ratio.
adolescents (unweighted n¼533 for the early survey, the reference period; n¼988 for the recent survey). After adjusting for the sociodemographic characteristics and BMI z-scores, the point estimates of PRs for all sociodemographic strata were lower than 1 (to left side of the reference line). Significantly fewer overweight/obese adolescents perceived themselves as such in the recent survey compared with those in the early survey (PR¼0.63, 95% CI¼0.48, 0.84, for the 12-year-old group; PR¼0.72, 95% CI¼0.53, 0.96, for the 13-yearold group). Across racial/ethnic strata, the declining tendency of accurately self-perceiving as overweight among overweight/obese adolescents was most noticeable among white adolescents (PR¼0.64, 95% CI¼0.48, 0.85), and least among black adolescents (PR¼0.76, 95% CI¼0.58, 0.99). Both boys and girls interviewed recently were less likely to correctly self-perceive as overweight (PR¼0.70, 95% CI¼0.56, 0.89, for boys and PR¼0.73, 95% CI¼0.61, 0.87, for girls). Overall, after ] 2015
controlling for age, race/ethnicity, sex, and family income level, overweight/obese adolescents interviewed recently were 29% less likely to accurately self-perceive as overweight (PR¼0.71, 95% CI¼0.62, 0.82) when compared with their counterparts with the same body weight but interviewed earlier.
Discussion Using nationally representative data, we show an increase in the mean and median BMI z-scores among adolescents who self-perceived themselves as overweight. A declining tendency to accurately self-perceive as overweight among actually overweight or obese adolescents was observed before and after adjusting for sociodemographic characteristics as well as age- and sex-specific BMI z-scores. The gap between the reality and perceptions about body weight status has been expanding among adolescents.
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There are several potential explanations for the declining tendency to correctly self-perceive as “overweight” among overweight/obese adolescents. In the wake of the obesity pandemic, the media, weight-loss industries, and medical communities have encouraged adolescents to maintain slender frames.32 Facing harsher messages, more and more overweight/obese adolescents may be increasingly reluctant to admit that they are overweight.33 The social comparison theory provides an additional explanation. The theory postulates that individuals assess themselves in relation to others rather than against an absolute scale.34,35 Paralleled with the increasing prevalence of obesity, increasingly more adolescents compare themselves to their peers with heavier body weight, to maintain a positive image of their own weight status. The numbers and the growth charts recommended by professional organizations are more remote than the peers surrounding them daily and are less likely to be used by adolescents as references. In addition to the current study, the Quebec Child and Adolescent Health and Social Survey also obtained evidence to support the social comparison theory. The Quebec study observed that overweight/obese adolescents whose schoolmates had a higher BMI score were more likely to perceive themselves to be of normal weight.36 Additionally, the confusing nature of the definition of “being overweight or obese” may underlie the increasing number of overweight/obese adolescents who failed to recognize their overweight status. Unlike the assessment of body weight for adults, defining obesity or overweight in adolescents is more formidable because this group continuously experiences changes in body composition affecting or being affected by puberty.37,38 Instead of setting thresholds for categorization, the BMI-for-age percentile from a reference population is used to allow comparison among adolescents of the same sex and age. The BMI-for-age percentile, however, is poorly correlated with the degree of adiposity among adolescents, and a study found that less than one half of children and adolescents whose BMIs-for-age are in the overweight range (85th–94.9th percentile) have high adiposity.39 The confusing nature is further compounded by the change of definition of overweight. In 1994, the Expert Committee on Clinical Guidelines for Overweight in Adolescent Preventive Services recommended that the children whose BMI was at or above the 95th percentile of the 2000 CDC Growth Charts should be considered “overweight,” and those at or above the 85th percentile as “at the risk of overweight.”40 However, to convey the urgency of childhood obesity, IOM redefined pediatric overweight and obese in 2005. Children previously defined as “overweight” were redefined as “obese,” whereas children “at the risk of overweight” were
redefined as “overweight.”41 The fundamental changes of the definitions took place between the two survey periods covered by this study. Becoming conscious of one’s excess weight is the precursor to adopting behavioral changes necessary for appropriate weight control.6,42 The declining tendency of correctly perceiving overweight status presents a vast challenge to obesity prevention among adolescents, making the overweight and obese adolescents less motivated to actively engage in effective weight-loss behaviors.3–5 On the other hand, overweight and obese adolescents experience psychological distress, including depressive mood.43,44 In certain extreme cases, body dissatisfaction has a devastating impact on adolescents’ psycho-emotional well-being and may cause adolescents to become overly concerned or impose detrimental levels of dietary restrictions.45,46 Increasing percentages of overweight/obese adolescents self-perceiving their body weight as “about the right weight” may suggest a reduction in social pressure on adolescents to maintain lower body weight. The generational shift of body weight perception declares urgency to explore the novel strategies to delicately protect adolescents’ attitude toward body image while correcting the body misperception to embark on lifestyle change for a healthy weight. In addition to family-based prevention strategies, officebased physician engagement should be increased. Discussion of body weight concerns between a physician and adolescents might be an effective tool to empower adolescents to actively engage in preventive efforts.45
Limitations This study is subject to limitations. A large number of adolescents were excluded from the current analyses for various reasons, and significant differences in SES were identified between the included and excluded NHANES participants (Appendix Table 2). Therefore, the generalizability of the current analyses may be compromised. More than two survey periods may be needed to assess the generational shift over a long time scale. The current study has its unique strengths. Although the NHANES is a cross-sectional survey, observations from different survey periods allowed for exploration of the overall trends. To our knowledge, no prior studies have used nationally representative samples to investigate a generational shift in body weight perceptions among adolescents in the U.S. The spectrum of participants, including three major races/ethnicities and other sociodemographic groups, allowed us to note the common thread among adolescents from various subpopulations. www.ajpmonline.org
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Conclusions In spite of an increase in the body weight of adolescents, fewer adolescents aged 12–16 years accurately perceived their weight status in the early 21st century compared with the late 20th century. The declining tendency may be indicative of a generational shift in body weight perceptions, presenting a vast challenge to obesity prevention among adolescents. Further studies are warranted on how to correct the misperceptions, promote adolescents’ engagement in appropriate weight control behaviors, and reduce the weight-related social pressure on adolescents while maintaining appropriate motivations for a healthier lifestyle. Methodologic studies examining the correlation between body perceptions and the degree of adiposity among adolescents are also needed. A draft of this manuscript was initially prepared as a group project in a summer workshop on advanced research methods sponsored by the National Key Laboratory of Public Health Security, School of Public Health, Fudan University, Shanghai, China. The workshop was led by Dr. Zhang from Jiann-Ping Hsu College of Public Health, Georgia Southern University, and Dr. Yan from Fudan University, China. We are indebted to the graduate students who have contributed substantially to the early drafts with care. The project was conducted with no specific funding. However, the National Key Laboratory of Public Health Security, School of Public Health, Fudan University, China, sponsored a workshop, allowing this project to be completed in a timely manner. No financial disclosures were reported by the authors of this paper.
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Appendix Supplementary data Supplementary data associated with this article can be found at http://dx.doi.org/10.1016/j.amepre.2015.03.024.
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