Study population
The Tianjin Chronic Low-grade Systemic Inflammation and Health (TCLSIH) cohort study is a dynamic prospective cohort study and the details of the cohort study have been described elsewhere [13]. In brief, the study participants were randomly recruited between January 2010 and December 2019 from the general population who took part in annual health checkups in Tianjin, China. The study protocol was approved by The Institutional Review Board of Tianjin Medical University. All participants provided written informed consent.
Figure 1 showed the flowchart for the selection of the study population. A total of 31,821 participants received at least one health examinations, including blood tests and liver ultrasonography. To be eligible for the present study, we included participants with alcoholic fatty liver disease (n = 1,010), other liver diseases (n = 209), CVD (n = 1,508), and cancer (n = 230) at baseline. Moreover, baseline NAFLD (n = 10,887, 37.7%) was excluded. Finally, participants who did not undergo health examinations during follow-up were excluded (n = 2,229).
Assessment of serum ferritin levels
Fasting blood samples were taken in the morning after a 12-h overnight fast from venipuncture of the cubital vein. Serum ferritin levels were measured using the Quantitative Kit for Tumor Markers (Huzhou Shukang Biological Technology) by the protein chip-chemiluminescence method and expressed as ng/mL. The measurement range of the assay was 5-600 ng/ml. The coefficients of variation of intra- and inter-assay were less than 15%. In the present analysis, serum ferritin levels were divided into four groups (quartiles).
Assessment of NAFLD
Fatty liver disease (FLD) was tested by abdominal ultrasonography, which was carried out by experienced sonographers using a TOSHIBA SSA-660A ultrasound machine (Toshiba, Tokyo, Japan), with a 2-5 MHz curved array probe. Images were also assessed by an experienced hepatologist. FLD was diagnosed if participants had two or more abnormal findings of liver ultrasonography as follows: diffusely increased echogenicity liver, liver echogenicity greater than kidney or spleen, or vascular blurring and the gradual attenuation of ultrasound signal. NAFLD was diagnosed without a history of heavy alcohol drinking (>210 g/week in males and >140 g/week in females) [14] on the basis of the presence of fatty liver. Abdominal ultrasonography was conducted every year, and NAFLD was diagnosed yearly during follow-up.
Assessment of covariates
Sociodemographic characteristics (age and sex), lifestyle factors (smoking and alcohol drinking status), and personal and family disease history were obtained by using a detailed and constructed questionnaire. Body height (m), weight (kg), and waist circumference (WC) (cm) were measured during annual health checkups by trained nurses. Body mass index (BMI) calculated from anthropometric measurements (kg/m2).
Blood tests including total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), triglycerides (TG), fasting blood glucose (FBG), alanine aminotransferase (ALT), aspartate transaminase (AST), gamma-glutamyl transpeptidase (GGT), and hemoglobin (Hb), which were measured from fasting blood samples. Diabetes was defined as fasting blood glucose ≥7.0 mmol/L or a self-reported history of diabetes [15]. Hyperlipidemia was defined as total cholesterol ≥5.17 mmol/L, triglycerides ≥1.7 mmol/L, or low-density lipoprotein cholesterol ≥3.37mmol/L, or taking lipid-lowering drugs [16]. Anemia was defined as Hb <120 g/L in nonpregnant adult females and <130 g/L in males [17]. Blood pressure (BP) was measured 2 times using the TM-2655 oscillometric device (A&D) and estimates from the 2 measurements were averaged. Hypertension was defined as systolic blood pressure (SBP) ≥140 mm Hg or diastolic blood pressure (DBP) ≥90 mm Hg or having a history of hypertension [18].
Statistical analysis
The distribution of all continuous variables was checked by one-sample Kolmogorov-Smirnov test (n ≥2,000). For the baseline participants characteristics, descriptive data were expressed as geometric mean (95% confidence interval, 95% CI) for continuous variables due to skewed distribution and percentages for categorical variables. The differences among the incident NAFLD categories were examined by using analysis of covariance (ANCOVA) for continuous variables, and multiple logistic regression analysis for proportional variables after adjustment for age and sex.
The Cox proportional hazards regression models were used to assess the association between serum ferritin levels and the incidence of NAFLD. Three multivariable models were fitted in our analyses. Model 1 was a crude model. Model 2 was adjusted for baseline age (continuous variable), sex (males or females), and BMI continuous variable). Model 3 was further adjusted for baseline serum ferritin, hypertension (yes or no), hyperlipidemia (yes or no), diabetes (yes or no), anemia (yes or no), smoking status (current, former, or never), alcohol drinking status (everyday drinker, sometime drinker, ex-drinker, or non-drinker), family history of diseases (including cardiovascular disease, hypertension, hyperlipidemia, and diabetes [each yes or no]). Hazard ratios (HRs) and 95% CI were calculated. Multicollinearity among the covariables in the fully adjusted models was diagnosed using variance inflation factor, and the results showed that collinearity was acceptable (all variance inflation factors are less than 10).
We stratified the participants by potential effect modifiers including age (<50 or ≥50 years), sex (males or females), BMI (<24 or ≥24 kg/m2), smoking status (current, former, or never), alcohol drinking status (everyday drinker, sometime drinker, ex-drinker, or non-drinker), hypertension (yes or no), hyperlipidemia (yes or no), diabetes (yes or no), anemia (yes or no) and then examined the interactions between these variables and serum ferritin levels.
To assess the robustness of the results, we performed a sensitivity analysis by excluding participants diagnosed with NAFLD for less than one year from baseline survey. Moreover, since anemia was associated with serum ferritin level, we further performed a sensitivity analysis after excluding participants with anemia.
Receiver operating characteristic (ROC) curves were used to assess the ability of the serum ferritin levels in diagnosing NAFLD. The optimal cut-off points used were the peaks of the curve, where the sum of sensitivity and specificity is at maximum.
All statistical analyses were performed using SAS software, version 9.3 (SAS Institute Inc., Cary, NC, USA). All tests were two-sided, and P <0.05 was considered statistically significant.