Study Cohort
The Kailuan Study is an ongoing cohort study based on functional community population that began in 2006. All the participants underwent questionnaires interviews, clinical examinations and laboratory tests every 2 years at 11 Kailuan Group affiliated hospitals by well-trained physicians or nurses using a standardized protocol, and follow-up data were obtained for events including UC, allowing us to investigate the relationship between transitions in MetS and MO phenotypes and UC. More details about the Kailuan Study can be found elsewhere [25, 26]. Participants who met the following criteria were recruited in this study: (1) included in the baseline survey (2006–2007) population of the Kailuan Study, (2) age ≥ 18 years and (3) signed the informed consent form. The exclusion criterion was history of malignancy. The study was conducted in accordance with the Declaration of Helsinki and was approved by the Ethics Committee of Kailuan General Hospital.
A total of 101,510 participants (81,110 males and 20,400 females; aged 18 − 98 years) met the criteria and were recruited in this study, and 379 participants with a history of cancer at baseline (2006 − 2007), 1,468 participants with missing data on BMI and 1,766 participants with missing data on MetS status were excluded. Finally, 97,897 participants were included in the analysis of the association between MetS and MO status at baseline and the risk of UC. To explore the effects of transitions in MetS and MO status over time (from 2006–2007 to 2008 − 2009) on the risk of UC, 35,178 participants, amongst whom 24,162 did not participate in the 2008 − 2009 survey, 34 with incident UC between the 2006 − 2007 and 2008 − 2009 surveys, 3,186 participants with missing data on BMI and 7,796 participants with missing data on MetS components in the 2008–2009 survey were further excluded, leaving 62,719 participants for assessing the association of transitions in MetS and MO status and risk of UC (Fig. 1).
Collection of Exposure Information
The baseline data included sociodemographic characteristics (age, gender, occupation, education level, economic income and marital status), lifestyle characteristics (smoking status, alcohol consumption, salt intake and sitting time), history of previous diseases, physical examination data [body weight, height, waist circumference (WC) and blood pressure (BP)] and blood indices (fasting glucose and lipids).
Participants wore light clothing, removed their shoes and hats, and had their body weight and height measured. BMI was calculated by weight in kilograms divided by height in square meters. WC was measured at the level of the midpoint between the anterior superior iliac crest and the lower rib cage. Systolic and diastolic blood pressure were measured using a mercury sphygmomanometer with a suitable cuff on the left arm of the subject after 5 min of rest and then again after 5 min, and the average of the two measurements was recorded. Early morning fasting blood samples were collected from the subjects to measure blood glucose and lipids. Fasting blood glucose level was measured using the hexokinase/glucose-6-phosphate dehydrogenase method, and the coefficient of variance of blind quality control samples was < 2.0%. Triglycerides were determined by glycerol phosphate oxidase assay (coefficient of mutual variation < 10%). After the precipitation of apolipoprotein B with dextrose sulphate and magnesium chloride, high-density lipoprotein cholesterol level was measured in the supernatant [26].
Definition of Variables
MetS was defined according to the harmonized International Diabetes Federation criteria [27], as shown in Table 1. Obesity statuses were categorized according to the Working Group on Obesity in China into normal weight (BMI < 28 kg/m2) and obesity (BMI ≥ 28 kg/m2) [28]. MO status was classified into four phenotypes according to whether the subject had MetS or obesity (Table 4).
According to MetS and obesity status at baseline (2006–2007) and 2008–2009 survey, transitions in MetS were divided into four phenotypes: non-MetS to non-MetS, non-MetS to MetS, MetS to non-MetS and MetS to MetS. Transitions in MO status were classified into seven phenotypes: MHN to MHN, MHO to MHO, MHO to MUO, MUN to MUO, MUO to MHO, MUO to MUN and MUO to MUO.
Smoking status was classified as follows: never smoker, former smoker (has quit smoking for more than 12 months) and current smoker (smokes one or more cigarettes per week for not less than 12 consecutive months). Alcohol consumption status was divided into the following categories: never, former (abstained from alcohol for more than six months) and current alcohol consumption (one or more drinks per month for no less than six months in a row).
Collection of Endpoint Event Information
The follow-up period started when the participants were recruited and the baseline survey was performed (2006–2007). The last follow up was completed on December 31, 2020. The follow-up endpoint event was a new UC or death in the observed subject (whichever came first). Firstly, the information of participants’ medical records was obtained from the Tangshan City Health Insurance System. Professionally trained investigators then went to the hospitals to collect information on the subjects’ medical history. Clinicians verified the pathology, imaging (including magnetic resonance imaging, computed tomography and colour Doppler ultrasonography) and blood biochemical examination results to confirm and refine the diagnosis of UC. Tumour cases were encoded according to the International Classification of Diseases-10. UC included prostate cancer, kidney cancer, carcinoma renal pelvis, ureteral cancer, bladder cancer and urethral cancer with codes C61 and C64–C68. Information on death events was obtained from the Kailuan Group Social Insurance System.
Statistical analysis
The baseline characteristics of MetS, MO phenotypes, and transitions in MetS and MO status were determined. Continuous variables were expressed as mean ± standard deviation (SD), whereas categorical variables were expressed as the number of cases (percentage). Multiple imputation methods were used to replace the missing data for covariates.
Multifactorial Cox proportional risk regression models were used to analyse the relationships between MetS (non-MetS group as the reference) and MO phenotypes (MHN group as the reference) at baseline (2006–2007) and the risk of developing UC. The association between the transitions in MetS (compared with non-MetS to non-MetS individuals) and MO status (compared with MHN to MHN individuals) from baseline (2006 − 2007) to 2008 − 2009 survey and the risk of UC incidence during the follow up (after 2008 − 2009) was also assessed in the same way. Model 1 was adjusted for age (at the start of follow up) and gender, whereas Model 2 was further adjusted for smoking status, alcohol consumption, occupation, education level, income, marital status, salt intake and sitting time.
Subgroup analyses were conducted to explore the associations of MetS, MO status and their transitions with the risk of UC according to age, gender and smoking status, Multiplication interactions were performed as well.
Several sensitivity analyses were performed to examine the robustness of the results excluding participants who (1) had a history of myocardial infarction and stroke, (2) developed UC within the first 2 years of follow up and (3) had less than 1.5 years between baseline (2006–2007) and 2008–2009 surveys (only for transitions in MetS and MO status ).
All statistical analyses were performed using SAS 9.4 and were considered statistically significant at P < 0.05 (two sided).