Participants and measures
A cross-sectional, nationally representative sample from the National Health and Nutrition Examination Survey (NHANES) 2005-2006 was obtained using a multistage stratified sampling design. Survey participants were initially interviewed at home and were then invited to a mobile examination center, where they received various examinations and provided blood samples for laboratory tests. All procedures involving human subjects were approved by the Research Ethics Review Board of the National Center for Health Statistics, Centers for Disease Control and Prevention. Written informed consent was obtained from all participants. Details about the NHANES survey design and operation are available elsewhere .
We examined interview and laboratory data from participants aged ≥ 20 years who were noninstitutionalized U.S. civilian. Data on anthropometric measurements were collected by trained health technicians . BMI was calculated from measured weight and height following a standardized protocol. Participants with a BMI of ≥ 25.0 kg/m2 (either overweight or obese) were included in this study. Waist circumference was measured at a point immediately above the iliac crest on the midaxillary line at minimal respiration to the nearest 0.1 cm [31, 32]. Abdominal obesity was defined as waist circumference of > 102 cm for men and > 88 cm for women .
Participants' depressive symptoms were assessed by using the Patient Health Questionnaire-9 (PHQ-9) diagnostic algorithm, which has been described in detail elsewhere . Specifically, participants were asked about how often over the last 2 weeks they had experienced each of the following symptoms: 1) little interest or pleasure in doing things; 2) feeling down, depressed, or hopeless; 3) trouble falling or staying asleep or sleeping too much; 4) feeling tired or having little energy; 5) having a poor appetite or overeating; 6) feeling bad as a failure or having let themselves or their family down; 7) having trouble concentrating on things such as reading the newspaper or watching TV; 8) moving or speaking so slowly that other people could have noticed, or being so fidgety or restless that they had been moving around a lot more than usual; and 9) having thoughts of suicidality or self-injury in some way. Participants were defined as having major depressive symptoms if they had at least five of the nine PHQ-9 criteria for ≥ 7 days (or ≥ several days for "having thoughts of suicidality or self-injury") in the past 2 weeks, one of which must be "loss of interest or pleasure in doing things" or "feel down, depressed, or hopeless" for ≥ 7 days in the past 2 weeks . Alternatively, participants' responses to each item were scored as 0 point for "not at all", 1 point for "having the symptoms for several days", 2 points for "having the symptoms for more than half the days", and 3 points for "having the symptoms for nearly every day". Their scores for each item were then added to produce a total depression severity score, and the cutoff point of ≥ 10 was used to identify participants as having moderate-to-severe depressive symptoms [34, 35]. The PHQ-9 has been shown to provide valid measurements of depression in the general population as well as in patients with diabetes, coronary artery disease, and heart failure. Using a structured mental health professional interview as the criterion standard, a PHQ-9 score of ≥ 10 had a sensitivity of 88% and a specificity of 88% for major depression, and, regardless of diagnostic status, typically represents clinically significant depression [34–36].
Socio-demographic variables used in the analyses included age, sex, race/ethnicity (non-Hispanic white, non-Hispanic black, and other including Mexican American, non-Mexican American, and any other races), educational status (< high school diploma, high school graduate, and > high school diploma), and family poverty-income ratio (calculated as a ratio of family income to poverty threshold). Smoking status was reflected by serum concentrations of cotinine which were measured by an isotope dilution-high performance liquid chromatography/atmospheric pressure chemical ionization tandem mass spectrometry (Perkin-Elmer Sciex Co, Norwalk, CT). Physical activity was calculated as an average daily metabolic equivalent (MET)-hour index that summed transportation, household, and leisure-time physical activity. Alcohol consumption was calculated as the average number of daily drinks for each participant. Heavy alcohol drinking was defined as having > 2 drinks per day in men and having > 1 drink per day in women. The number of chronic conditions including hypertension, diabetes, coronary heart disease, stroke, arthritis, asthma, chronic bronchitis, chronic renal disease, and cancer was also included as a covariate. Most of these conditions were assessed by asking participants whether they had ever been told by a healthcare professional that they had diabetes, coronary heart disease, stroke, arthritis, or cancer, or whether they still had asthma and chronic bronchitis. For blood pressure, up to four readings of systolic and diastolic blood pressure were obtained from participants in the mobile examination centers. The average of the last two measurements of systolic or diastolic blood pressure for participants who had three or four measurements, the last measurement for participants with only two measurements, and the only measurement for participants who had one measurement were used to establish high blood pressure status. According to the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure reports , participants who were on antihypertension medications or had systolic blood pressure ≥ 140 mmHg or diastolic blood pressure ≥ 90 mmHg were defined as having hypertension. For kidney disease, we estimated glomerular filtration rate using the CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) equation , and participants with a glomerular filtration rate of < 60 mL/min/1.73 m2 were defined as having chronic renal disease.
From a total of 3,250 adult participants who were overweight or obese, 231 women were excluded because of pregnancy. After further excluding those who had missing values for any of the study variables, 2,439 participants (1,325 men and 1,114 nonpregnant women) remained in our analyses. The prevalence of having major depressive symptoms or moderate-to-severe depressive symptoms (PHQ-9 score ≥ 10) was age-standardized to the 2000 projected U.S. population. The odds ratios (ORs) with 95% confidence intervals (CIs) for major depressive symptoms or moderate-to-severe depressive symptoms were estimated by conducting logistic regression analyses to test associations between depressive symptoms and waist circumference (used as a continuous variable) or abdominal obesity (used as a categorical variable) while controlling for covariates which included demographic characteristics (age, sex, race/ethnicity, education, and family poverty-income ratio), lifestyle factors (serum concentrations of cotinine, physical activity, and heavy alcohol drinking), and coexistence of multiple chronic conditions (hypertension, diabetes, coronary heart disease, stroke, arthritis, asthma, chronic bronchitis, chronic kidney disease and cancer). SUDAAN (Software for the Statistical Analysis of Correlated Data, Release 9.0, Research Triangle Institute, Research Triangle Park, NC) was used to account for the complex sampling design.