This longitude cohort consisted of 11,357 participants residing in Shijiazhuang city which is the provincial capital city of Hebei province in China. All participants have attended the health examinations conducted in 21 public medical institutions during 2000–2019. The survey was initially conducted during March to April 2000 to March to April 2001 for participants aged over 18 years old. But participants with missing data including age, WC, FVE1 and FVC (n = 2,589), as well as pregnant and breastfeeding participants (n = 157) were excluded. Additionally, participants were excluded with underlying lung disease: asthma and asthma-like symptoms including wheeze, nocturnal chest tightness, attacks of breathlessness after activity or at rest or at night time, use of inhaled or oral medicine for breathing problems (n = 392). Considering possible confounding by disease- and metabolic WC change, we also excluded participants who had diabetes (n = 52) and cancer (n = 3). Finally, our study included 8,164 participants (3,929 men and 4,235 women) to estimate the risk of incident COPD with change in WC during the 18-year follow-up.
Ethical approval was obtained from the ethics committees of all participating institutions and all participants provided informed written consent.
Before the health examination, all participants need to finish an overnight fast. At baseline, the questionnaires were used to collect the information of social and demographic factors including age, gender, educational level, smoking habits, alcohol consumption and physical activities. The same questionnaires were applied for the information collection in each year. Educational level was categorized as low level (lower than senior high school) and high level (college or university degree and master’s degree or higher). Smoking status was categorized as currently smoking, ever smoking or never smoking. Alcohol drinking was categorized as drinker (alcohol consumption 12 or more timed in the last year) and non-drinker (alcohol consumption less than 12 times in the last year). Urbanization was categorized as urban and rural living area. The metabolic equivalent index (MET) was calculated by the codes and the detailed questions of the physical activity survey have been published elsewhere. [16–18] The same measurements of the health examination at baseline were taken after 18-year follow-up.
All measurements of WC and BMI were collected according to the World Health Organization standard.[19–20] WC was measured with gentle breathing at the midpoint between the lowest rib and the iliac crest to the nearest 0.1 cm. The difference value (D-value) in WC was calculated by that the WC measured at baseline subtracted that measured at follow-up. Percent of WC gain ༞0% indicated that WC increased from baseline to follow-up, the greater the percent WC gain, the greater the increase in WC. Weight and height were measured with participants in light clothing and without shoes. The measurements of weight and height corrected to the nearest 0.5 kg and 0.1 cm, respectively. Body mass index (BMI) was calculated as the ratio of weight (kg) to the square of height (m2).
Lung function was measured by respiratory physician using the same spirometer (MasterScreen Pneumo, Jaeger, Germany) and all examinations were in accordance with American Thoracic Society recommendation. Firstly, all participants were led through practice the exhalations before the examination of lung function. Then, pre- and post-bronchodilator forced expiratory volume in one second (FEV1) and forced vital capacity (FVC) was measured as the primary outcomes and the ratio of these two measurements (FEV1: FVC) was calculated for each subject to assess the risk of COPD. Participants were asked to return on another day for an additional spirometry test, when the result of examination was regarded as low quality.
Overnight fasting blood was collected into vacuum tubes for assessing fasting plasma glucose (FPG), total cholesterol (TC), triglyceride (TG) and high-density lipoprotein cholesterol (HDL-C). Details about the storage and measurements methods were in accordance with the Clinical Practices standards.
Abdominal obesity for Chinese adults was defined by WC ≥ 90 cm for men and WC ≥ 80 cm for women according to the criteria of the Working Group on Obesity in China.
According to the Global Initiative for Chronic Obstructive Lung Disease (GOLD) criteria, COPD was defined as a post-bronchodilator FEV1:FVC < 70%. Combined with the diagnose criteria for COPD in China, we also considered patients’ history of illness, results of X-ray and biochemical examination when participants were diagnosed as COPD. Patients of COPD were defined as a participant who reported a previous diagnosis of COPD through spirometry testing by physician. Participants were also defined as COPD patients by self-reporting that they received any inhaled short-acting or long-acting bronchodilator or corticosteroid therapy before.
All continuous variables were presented as mean (Standard Deviation) and categorical data as number (percentage). Kruskal-Wallis test or chi-square test was used to estimate the differences of variables in each group at baseline. To examine the association of WC dynamic change and incident COPD risk in detail, the range of percent WC change was divided into small categories: ≤-2.5%, -2.5–2.5%, 2.5–5% and >5%.
The three Poisson regression models were used to assess the association of the risk of COPD and WC gain by relative risk (RR) and 95% confidence interval(CI).[26–27] For the first part analysis, WC gain − 2.5–2.5% was regarded as the reference group. Participants with normal WC at both baseline and follow-up were regarded as the reference group in the second part analysis. All potential confounding factors in the regression models were adjusted based on the baseline variables. Model 1 adjusted for age firstly. Then, model 2 adjusted for age as well as education level, smoking status, alcohol drinking, and physical activity. Finally, model 3 adjusted for all variables in model 2 as well as body mass index, systolic blood pressure (SBP), diastolic blood pressure (DBP), fasting plasma glucose (FPG), total cholesterol (TC), triglycerides (TG), high density lipoprotein cholesterol (HDL-C), forced expiratory volume in 1 second (FEV1) and forced vital capacity (FVC).
All analyses were performed by Stata, 12.0 version (Stata Corporation, College Station, TX, USA). A two-tailed p value less than 0.05 was regarded significantly.