Prevalence and change of central obesity among US Asian adults: NHANES 2011–2014

All participants in the present study were non-Hispanic Asians from NHANES 2011–2014. This study focused on central obesity among US Asians and NCHS only began to collect data on Asians in 2011 [19]. Non-Hispanic Asians included all persons having origins in the Far East, Southeast Asia, or the Indian subcontinent (including, for example, China, Japan, Korea, Malaysia, the Philippine Islands, Vietnam, Thailand, Cambodia, India, Pakistan,). Our sample did not include Asians with multi-racial/ethnic background. In 2011–2014, primary sample design changes were implemented in NHANES to oversample non-Hispanic Asians and increase the reliability and precision of health estimates for this subgroup [19]. Participants who were less than 20 years were excluded from the study. Pregnant women and those who did not have a WC recorded were also excluded. The final study sample consisted of 1288 non-Hispanic Asian adults.

WC was measured by trained examiners following body measures examination protocol [20]. The measurement room was equipped with wall mirrors designed to facilitate accurate and efficient measurements. A health technologist and recorder worked as a team to assist the examiners. WC data were saved to the study database using the Integrated Survey Information System (ISIS) anthropometry computer application. Each eligible participant was measured without clothing and stood in the pose of crossing arms with hands on opposite shoulders. The examiner made a mark just above the uppermost lateral border of the right ilium of the participant, and then extended the measuring tape around the waist and positioned it in a horizontal plane at the level of the measurement mark. The measuring tape was placed parallel to the floor, and fit snug across but did not compress the skin. WC was taken to the nearest 0.1 cm at the end of the participant’s normal expiration. Central obesity was defined as WC ≥90 cm for men and ≥80 cm for women according to the guidelines of the International Diabetes Federation (IDF) for Asian populations [21].

All the characteristics considered in this study were self-reported through interview questionnaires. Age and gender were demographic factors; education, poverty status, and marital status were socioeconomic factors. Length of stay in the US was used to reflect the acculturation including cultural, psychological, and lifestyle changes. We considered these characteristics to examine how the prevalence of central obesity differed across the groups defined by these characteristics. The selected participants were categorized into three groups: young adults (aged 20–39 years), middle-aged adults (aged 40–59 years), and old adults (aged 60 years or above). In general, 20–39 year olds are more similar phenotypically to each other than they are to those who are middle-aged; equally, 40–59 year olds (especially as women in their 40’s have markedly limited reproductive potential, they resemble more women in their 50’s who are post menopause, rather than women in their 20’s). Also, with aging. Older adults > 60 years realize weight redistribution, greater fat mass per BMI, and therefore are more alike than those in their 40’s. Educational attainments were classified as high school or below and college or above (some college, college graduate or above). Poverty index ratio (PIR) was the ratio of the family’s total income to the family’s appropriate poverty threshold calculated by the family’s mean income and number of individuals in their household. It was used to measure poverty status. A participant was considered poor if PIR < 1.0, and non-poor if PIR ≥ 1.0. Marital status was combined into three categories: never married, married or living with partner, and/or widowed, divorced or separated. Length of stay in the US was defined as follows. Participants were first asked whether they were born in the US; if they answered ‘no’, the length of time living in the US was recorded in years. Thus length of stay in the US was classified, in terms of years in the US, as 1) born in the US, 2) less than 10 years in the US, 3) 10 to 19 years in the US, and 4) 20 years or over.

Data analyses were guided by the NHANES analytic and reporting guidance document [22]. Sampling weights and techniques were considered to account for oversampling and survey nonresponse. Student’s t tests with survey application were used to examine the significance of differences in the means of continuous variables, and chi-square tests were used for the significance of the proportions of categorical variables across central obesity status.