Study population

NHANES is a cross-sectional, nationally representative survey of the non-institutionalized civilian population of the United States conducted by the National Center for Health Statistics, Centers for Disease Control and Prevention (NCHS, CDC). Beginning in 1999, the survey has been conducted continuously and released in 2-year cycles. For our study we merged the publicly available files for NHANES cycles 1999–2000 and 2001–2002 using the NCHS recommendations (Johnson et al. 2013). The survey employs a multistage stratified probability sample based on selected counties, blocks, households, and persons within households.

NCHS-trained professionals conducted interviews in participants’ homes, and extensive physical examinations, including blood and urine collection, were conducted at mobile exam centers (MECs). All procedures were approved by the NCHS Research Ethics Review Board (Protocol #98–12 http://www.cdc.gov/nchs/nhanes/irba98.htm), and all participants provided written informed consent. The response rates for all of the examined person were 76.3% for NHANES 1999–2000 and 79.6% for NHANES 2001–2002 (http://www.cdc.gov/nchs/data/nhanes/analytic_guidelines_11_12.pdf).