Abstract
This study uses nationally representative data from the National Health and Nutrition Examination Survey to examine changes in obesity prevalence in the US over the last 4 decades among adults aged 18 through 25 years.
Emerging adulthood, from ages 18 through 25 years, is a distinct developmental period characterized by exploration and frequent change (eg, from school to work), ultimately leading to the formation of lifelong habits and adult identity.1,2 Few studies describe obesity in emerging adults; analyses often group these individuals with adolescents (aged 12-19 years) or young adults (aged 20-39 years), limiting opportunities for developmentally informed intervention and treatment.3,4 We examined the changes in obesity prevalence nationally among emerging adults in the US over the last 4 decades.
Methods
We used nationally representative data from the National Health and Nutrition Examination Survey (NHANES), a series of cross-sectional surveys including interviews and physical examinations with a stratified, multistage sampling design. We drew from NHANES II (1976-1980), NHANES III (1988-1994), and the continuous NHANES cycles from 1999 through 2018 (response rate range, 48.8%-80%). We limited our study population to nonpregnant emerging adults (aged 18-25 years) of non-Hispanic Black or non-Hispanic White race with complete data for variables of interest (89.8% of defined population had complete data). Given the changes in how the NHANES assessed race and ethnicity over time, we were limited to the aforementioned groups.
Our outcome of interest was body mass index (BMI; calculated as weight in kilograms divided by height in meters squared). We categorized BMI into standard groups of underweight (<18.5), normal weight (18.5-24.9), overweight (25-29.9), and obesity (≥30).5 Covariates included sex (male or female), race and ethnicity (non-Hispanic Black or non-Hispanic White), and family poverty status (yes or no). We identified family poverty if the NHANES poverty index (the ratio of family income to the US poverty threshold in NHANES II/III and the ratio of family income to the US poverty guidelines in the continuous NHANES) was at or below the poverty threshold or guidelines for that year. We did not include age, given the narrow range of interest.
For each wave, we estimated mean BMI and prevalence of each BMI group along with 95% CIs using NHANES examination weights to account for the complex sampling design and using poststratification to standardize to the internal 1999-2000 study population distribution based on sex, race and ethnicity, and poverty status. To test for trend across all years, we used a nonparametric, Wilcoxon-type test given the time differences between surveys.6 In sensitivity analyses, we combined the continuous NHANES cycles (1999-2018) and conducted a linear or logistic regression, as appropriate, to test for trend over time. Statistical significance was determined by a 2-sided P < .05. All analyses were performed using Stata version 15 (StataCorp). The Johns Hopkins institutional review board reviewed this study and determined that it was not human subjects research.
Results
Across all years, 8015 emerging adults were included. Of these, 3965 were female, 3037 were non-Hispanic Black, and 2386 met criteria for family poverty.
Between 1976 and 2018, mean BMI increased from 23.0 (95% CI, 22.8-23.2) in 1976-1980 to 27.5 (95% CI, 25.5-29.4) in 2017-2018 (P = .005 for trend using a nonparametric test) (Table). The Figure shows the standardized prevalence of BMI groups over time. Between 1976-1980 and 2017-2018, the prevalence of obesity increased from 5.5% (95% CI, 4.3%-7.0%) to 32.6% (95% CI, 22.1%-45.2%; P = .007 for trend using a nonparametric test), whereas normal weight decreased from 70.5% (95% CI, 67.8%-73.0%) to 34.0% (95% CI, 26.1%-42.9%; P = .02 for trend using a nonparametric test). The sensitivity analyses limited to the continuous NHANES cycles had similar results (Table).
Table. Standardized Mean Body Mass Index (BMI) and Prevalence of BMI Groups Among Non-Hispanic Black and Non-Hispanic White Emerging Adults (Aged 18-25 Years), 1976-2018a.
| Survey year | BMIb | BMI groupsb | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| No.c | Mean (95% CI) | Underweight (<18.5) | Normal weight (18.5-24.9) | Overweight (25.0-29.9) | Obesity (≥30.0) | |||||
| No.c | Weighted proportion, % (95% CI)d | No.c | Weighted proportion, % (95% CI)d | No.c | Weighted proportion, % (95% CI)d | No.c | Weighted proportion, % (95% CI)d | |||
| 1976-1980 | 1974 | 23.0 (22.8-23.2) | 127 | 5.9 (4.8-7.3) | 1361 | 70.5 (67.8-73.0) | 351 | 18.2 (16.2-20.3) | 135 | 5.5 (4.3-7.0) |
| 1988-1994 | 1396 | 24.3 (23.7-24.8) | 74 | 5.8 (3.8-8.9) | 811 | 59.4 (54.5-64.2) | 281 | 20.0 (16.3-24.4) | 230 | 14.8 (11.4-18.9) |
| 1999-2000 | 359 | 25.9 (24.8-26.9) | 14 | 4.5 (1.6-12.0) | 183 | 48.1 (39.4-57.0) | 83 | 27.5 (21.1-34.9) | 79 | 19.9 (14.1-27.3) |
| 2001-2002 | 550 | 25.8 (25.1-26.6) | 33 | 4.2 (2.4-7.2) | 293 | 52.6 (48.0-57.3) | 118 | 23.8 (18.6-30.0) | 106 | 19.3 (13.9-26.2) |
| 2003-2004 | 608 | 26.4 (25.2-27.5) | 31 | 4.2 (2.5-6.8) | 304 | 49.0 (41.9-56.2) | 130 | 23.1 (19.7-26.9) | 143 | 23.8 (16.7-32.7) |
| 2005-2006 | 616 | 26.5 (25.5-27.6) | 23 | 5.8 (3.0-11.1) | 296 | 47.1 (39.9-54.4) | 135 | 22.6 (17.2-29.0) | 162 | 24.5 (17.2-33.6) |
| 2007-2008 | 415 | 26.3 (24.5-28.0) | 18 | 5.1 (2.7-9.1) | 200 | 50.2 (40.6-59.9) | 100 | 23.1 (17.8-29.4) | 97 | 21.6 (15.1-29.9) |
| 2009-2010 | 484 | 26.8 (25.6-28.0) | 20 | 2.7 (1.4-5.3) | 242 | 48.7 (37.5-60.1) | 98 | 22.6 (15.8-31.4) | 124 | 26.0 (19.9-33.1) |
| 2011-2012 | 481 | 25.9 (24.9-26.9) | 25 | 5.7 (3.8-8.4) | 213 | 51.7 (42.3-60.9) | 111 | 21.6 (16.6-27.5) | 132 | 21.1 (15.7-27.7) |
| 2013-2014 | 477 | 26.7 (25.1-28.3) | 22 | 4.2 (1.8-9.5) | 223 | 48.3 (42.5-54.2) | 112 | 24.5 (20.2-29.4) | 120 | 22.9 (17.3-29.8) |
| 2015-2016 | 337 | 27.5 (26.3-28.7) | 14 | 2.9 (2.2-6.8) | 153 | 42.6 (37.0-48.4) | 85 | 26.2 (20.2-33.3) | 85 | 27.3 (21.0-34.7) |
| 2017-2018 | 318 | 27.5 (25.5-29.4) | 22 | 8.7 (4.4-16.4) | 127 | 34.0 (26.1-42.9) | 64 | 24.8 (18.2-32.8) | 105 | 32.6 (22.1-45.2) |
| P valuee | .005 | .26 | .02 | .06 | .007 | |||||
| P value for sensitivity analysisf | .04 | .58 | .02 | .72 | .03 | |||||
Data are from the National Health and Nutrition Examination Survey (NHANES). All estimates were standardized to the internal 1999-2000 study population based on sex, race/ethnicity (non-Hispanic Black vs non-Hispanic White), and poverty status. Race and ethnicity options in NHANES III and the continuous NHANES cycles were defined by the NHANES survey developers and chosen by participants. For NHANES II, the race options were defined by the NHANES survey developers and individuals were classified by observation except in cases when the interviewer was unable to do so, in which case the participant was asked. Ethnicity was not directly assessed. A Hispanic ethnicity variable was constructed based on participant-reported natural origin or ancestry.
Calculated as weight in kilograms divided by height in meters squared.
Counts are unweighted.
Examination weights were used to calculate weighted prevalence estimates and 95% CIs. The rare strata with single sampling units in the data were treated as certainty units to perform variance estimation.
Estimated using a nonparametric, Wilcoxon-type test for trend.
Estimated using survey-weighted, adjusted regressions for pooled data from continuous NHANES, 1999-2018.
Figure. Standardized Prevalence of Body Mass Index Groups Among Emerging Adults (Aged 18-25 Years) in the National Health and Nutrition Examination Survey, 1976-2018.
Prevalence estimates calculated using survey weights and standardized to the internal 1999-2000 study population based on sex, race and ethnicity, and poverty status. Groups were defined using body mass index (calculated as weight in kilograms divided by height in meters squared) as underweight (<18.5), normal weight (18.5-24.9), overweight (25.0-29.9), and obesity (≥30.0).
Discussion
This analysis found that from 1976 to 2018, the prevalence of obesity among emerging adults in the US increased significantly. Limitations of this study include the population being limited to non-Hispanic Black and non-Hispanic White individuals, patterns among whom may not be generalizable to other races or ethnicities, as well as a decline in the NHANES response rate over time. Additionally, survey weights cannot be used in the poststratification analysis, as the initial results rather than the study design determine the relative number in each stratum at this stage.
Emerging adulthood may be a key period for preventing and treating obesity given that habits formed during this period often persist through the remainder of the life course. There is an urgent need for research on risk factors contributing to obesity during this developmental stage to inform the design of interventions as well as policies aimed at prevention.
Section Editors: Jody W. Zylke, MD, Deputy Editor; Kristin Walter, MD, Senior Editor.
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