Obesity prevalence has been increasing since the 1980s among adults, but among youth, prevalence plateaued between 2005-2006 and 2013-2014.1,2 We analyzed trends in obesity prevalence among US youth and adults between 2007-2008 and 2015-2016 in order to determine recent changes.
The National Health and Nutrition Examination Survey (NHANES) is a cross-sectional survey with a complex, multistage probability design that represents the civilian, noninstitutionalized population with a response rate of 75.4% in 2007-2008 and 58.7% in 2015-2016.3 Participants 18 years or older provided written consent, youth aged 7 to 17 years provided written assent, and parental permission was obtained in writing for youth younger than 18 years. NHANES was approved by the National Center for Health Statistics research ethics review board. Standardized measurements of weight and height were obtained.3
Among adults aged 20 years and older, obesity was defined as a body mass index (BMI; calculated as weight in kilograms divided by height in meters squared) of 30 or more and severe obesity was defined as a BMI of 40 or more.4 Among youth aged 2 to 19 years, obesity was defined as a BMI at or above the 95th percentile of sex-specific BMI-for-age and severe obesity was defined as a BMI at or above 120% of the 95th percentile.1 Pregnant females were excluded.
Prevalence and 95% CIs of obesity and severe obesity were estimated overall5 and stratified by sex and age (2-5, 6-11, 12-19, 20-39, 40-59, and ≥60 years). Linear and quadratic trends overall and stratified by sex and age were examined in regression models with 2-year survey cycles modeled as an orthogonal polynomial and in adjusted models (including survey cycle, sex, age, race/Hispanic origin [non-Hispanic white, non-Hispanic black, Hispanic, or other], education [high school graduate or less, some college, and college graduate; education of household head for youth], and, among adults, smoking status [never, former, or current smoker]) to determine if trends could be explained by these factors. Interactions between survey cycle with sex and age were tested among youth and adults separately to supplement stratified analyses and were not significant. A 2-sided P value of .05 was used to assess statistical significance.
Statistical analyses accounted for the complex survey design, including examination sample weights, which adjusted for nonresponse and took into account loss between the screener and interview and between the interview and the examination. Analyses were conducted using R (R statistics), version 3.4.16; SAS (SAS Institute), version 9.4; and SUDAAN (RTI International), version 11.0.
Data from 16 875 youth (Table 1) and 27 449 adults (Table 2) were analyzed. Among youth, obesity prevalence was 16.8% (95% CI, 14.2%-19.8%) in 2007-2008 and 18.5% (95% CI, 15.8%-21.3%) in 2015-2016. Based on the unadjusted model, there were no significant linear trends in the prevalence of obesity or severe obesity overall, by sex or age group (P range = .17 to .78) (Table 1). Obesity prevalence among children aged 2 to 5 years showed a quadratic trend (P = .04), decreasing from 10.1% in 2007-2008 to 8.4% in 2011-2012 and then increasing to 13.9% in 2015-2016. Adjusted overall linear and quadratic trends for obesity and severe obesity among youth aged 2 to 19 years remained nonsignificant.
Age-standardized prevalence of obesity among adults increased from 33.7% (95% CI, 31.5%-36.1%) in 2007-2008 to 39.6% (95% CI, 36.1%-43.1%) in 2015-2016 (P = .001) (Table 2). Prevalence increased among women, and in adults aged 40 to 59 years and 60 years or older. The observed increases in men and adults aged 20 to 39 years did not reach statistical significance. There were no significant quadratic trends. The adjusted model also showed a significant overall linear trend for obesity among adults (P < .001; data not shown).
Age-standardized prevalence of severe obesity in adults increased from 5.7% (95% CI, 4.9%-6.7%) in 2007-2008 to 7.7% (95% CI, 6.6%-8.9%) in 2015-2016 (P = .001). Prevalence increased in men, women, adults aged 20 to 39 years and 40 to 59 years. There was no significant linear trend among adults 60 years and older. There were no significant quadratic trends. The adjusted model also showed a significant overall linear trend for severe obesity (P < .001; data not shown).