NHANES III was conducted in 1988-1994 on a nationwide multistage probability sample of approximately 40,000 persons from the civilian, noninstitu-tionalized population of the United States aged two months and over excluding reservation lands of American Indians. Of these, 31,311 were examined. The analysis was restricted to men to eliminate confounding by gender, pregnancy, menopause, parity, or female hormone use. IgG antibodies to H. pylori were measured only in the first half of the survey (Phase I), itself a representative sample of the U.S. population. The analyses in this report are limited to examined men aged 40-74 years for whom a valid serum H. pylori antibody measure and data on history of doctor diagnosed diabetes and glycated hemoglobin measured in the survey were available. The study was restricted to men older than 40 and younger than 75 years because that was the age range of the subsample, which received an oral glucose tolerance test in NHANES III. The analyses of serum antibody and fasting serum glucose, insulin, and triglycerides are restricted to men examined in the morning after fasting nine-to-24 hours with valid serum antibody and insulin data, no history of diabetes, and not taking insulin or oral hypoglycemic agents. Numbers of persons in various regression analyses that follow may vary slightly due to differing numbers with missing values on selected other variables. Details of the plan, sampling, operation, and response have been published as have procedures used to obtain informed consent and to maintain confidentiality of information obtained.
Demographic information; medical history, including doctor-diagnosed and coronary heart disease; and behavioral information were collected by household interview prior to the examination. Race and Mexican-American ethnicity were determined by self-report. Examinations were carried out in a mobile examination center. Blood samples were obtained at the examination center. Blood in a redtop Vacutainer tube was allowed to stand for 45 min. at room temperature to allow complete clotting and clot retraction. Samples were centrifuged at 1,500 x g for 30 min at 4°C and were frozen at -20°C. H. pylori serologic testing was done using a commercial IgG ELISA (Wampole Laboratories, Cranbury, NJ). Immune status ratio (ISR) was calculated as the quotient of specimen optical density and the mean optical density of three cutoff controls (negative, high positive, and low positive). For this analysis, negative specimens had ISR 01.09, and positive >1.09. Retesting in a sample of 900 specimens indicated a 97% reproducibility. Previous validation studies indicated a test sensitivity of 91% and specificity of 96%. Overall sero-prevalence of H. pylori in this survey was 32.5% in adults aged 20 and over. Quality-control methods are described elsewhere.
Frozen serum was sent to the Missouri Diabetes Diagnostic Laboratory and stored at -70°C until analysis for serum insulin concentration. Insulin radioimmunoassay (RIA) was performed using the Pharmacia Insulin RIA kit (Pharmacia Diagnostics AB, Uppsala, Sweden) for the majority of samples. (Prior to November 1990, RIA kits purchased from Cambridge Laboratories, Cambridge, MA and its successor, Ventrex, Inc., Cambridge, MA were used. Based on simultaneous analyses using all three assays, results from these kits were converted to Pharmacia equivalence.) Quality control procedures included the reanalysis of 5% of specimens randomly selected either within-assay or between-assay, and the analysis of batch specimens consisting of four levels of control pools before and after all survey specimens. The internal reference range for fasting serum insulin in nonobese, nondiabetic adults (mean age 28.1 years) was 3.08-11.92 uIU/mL. The cross-reactivity of Pharmacia insulin antibody with proin-sulin is approximately 40%. Concentration of C-peptide in serum was determined by RIA in a three-day, batch, sequential-saturation method with two incubations. The internal reference range for fasting serum C-peptide was 0.266-1.079 pmol/mL. Frozen plasma was sent to the Missouri Diabetes Diagnostic Laboratory for determination of plasma glucose using a modified hexokinase enzymatic method on the Cobas Mira Chemistry System (Roche Diagnostic Systems, Inc., Montclair, NJ). Within- and between-assay quality-control procedures were used. During the six years of the survey, the coefficient of variation of the method was 1.6-3.7%. Glycated hemoglobin (HbAIC) in whole blood was determined using a high-performance liquid chromatographic assay on the Diamat automated HPLC system, model 723 (Bio-Rad Laboratories, Hercules, CA). The upper limit of normal for HbAiC in this system has been defined as 6.1%.
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Technicians measured height to the nearest 0.1 cm; weight to the nearest 0.01 kg; triceps, subscapular, suprailiac, and mid-thigh skinfold thickness to the nearest 0.1 mm; and waist and buttocks circumference to the nearest 0.1 cm, as described in detail elsewhere. With the sample person standing at minimal respiration, waist circumference was measured in a horizontal plane at the level of the high point of the iliac crest to the nearest 0.1 cm. Hip circumference was measured in a horizontal plane at the maximum extension of the buttocks. The following were computed: waist-to-hip circumference ratio (WHR) and body mass index (BMI=weight /height2, kg/m2). Extensive descriptive data on prevalence of H. pylori infection, diabetes, glucose tolerance, height, weight, BMI, and obesity prevalence in the NHANES III population have been published elsewhere and will not be duplicated here.
The plan of the present analyses was as follows. H. pylori infection was considered the exposure variable for all analyses. Detailed descriptive statistics and measures of association were computed using the Statistical Analysis System (SAS). Analysis of covari-ance was used to assess the association of the mean levels of continuous risk variables in persons with and without infection controlling for age. Multivariate logistic regression analysis was used to develop models for controlling for confounding of the association of infection status with history of doctor-diagnosed acute myocardial infarction, stroke, or diabetes mellitus. Linear multivariate regression analysis was used to develop models for controlling for confounding of the association of infection status with concentrations of glycated hemoglobin, fasting serum insulin, and other continuous variables. Only variables with prespeci-fied hypotheses were entered into the regression models. Population estimates for means and percentiles of variables were produced using weighted SAS or SUDAAN (statistical software packages) procedures. Age-adjusted means were performed using SAS weighted analysis, and all statistical testing and variance estimation were performed using the PROC LOGISTIC and PROC REGRESS procedure for regression models in the SUDAAN system. These weighted analyses used techniques that incorporated sampling weights and design features of the survey.
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