Association between Gallstone Disease and Statin Use: A Nested Case—Control Study in Korea

The correlation between statin use and the development of gallstone disease remains controversial. Existing data, primarily based on Caucasian populations, are biased, thus necessitating validation studies involving Asian cohorts. We conducted a nested case–control study using data from the Korean National Health Insurance Service Health Screening Cohort (2002–2019) to determine the likelihood of gallstone disease according to periods of previous statin use and type of statin. Among the 514,866 participants, 22,636 diagnosed with gallstones at ≥2 clinic visits (using the International Classification of Diseases, 10th revision, code K80) were matched 1:4 to 90,544 controls for age, sex, income, and residential area, and their statin prescription history for 2 years prior to the index date was examined. Propensity-score-weighted odds ratios (ORs) for gallstone disease were calculated using conditional logistic regression. Long-term use (>545 days) of any statin or lipophilic statins was associated with lower odds of incident gallstones (OR = 0.91, 95% confidence interval [CI] = 0.86–0.96, p < 0.001 and OR = 0.88, 95% CI = 0.83–0.93, p < 0.001, respectively) after adjusting for confounders. Short-term use (180–545 days) of any statin or hydrophilic statins was not statistically related to incident gallstones. In summary, prior statin medication, particularly long-term lipophilic statin use, may confer a preventive advantage against gallstone disease.


Introduction
Gallstone disease (cholelithiasis) imposes a significant epidemiological and economic burden worldwide [1][2][3]. Patients with gallstone disease are at increased risk of developing cholecystitis, cholangitis, biliary pancreatitis, and even gallbladder cancer, all of which lead to increased risk of mortality [1]. Considering the close relationship between gallstone disease and a high-saturated fat and meat diet [4], this disease is characterized by considerable variations in prevalence rates among different geographic areas and ethnicities; these range from 10.7% in Asia to 21.9% in Europe and 28.5% in North America [2,[5][6][7][8][9]. Gallstone disease is a significant cause of surgery that has substantial socioeconomic costs in most Western countries, with annual spending of $6.5 billion reported in the United States (US) [3,10] and $0.51 million in 2013 in a single city in Brazil [11]. Despite a relatively low prevalence (2.0-3.43%) in the Republic of Korea (hereafter, Korea) [12,13], the pattern

Baseline Characteristics of the Study Participants
The current study included 22,636 patients with gallstones and 90,544 control participants. When the patient and comparison cohorts were propensity-score matched, the groups had similar demographic features (income, region of residence, sex, and age; standardized difference = 0) ( Table 1). The proportion of patients with a Charlson Comorbidity Index (CCI) score ≥2 and a history of dyslipidemia was higher in the gallstone group than in the counterpart group. The remaining features, including weight status, smoking status, alcohol consumption, systolic blood pressure (SBP), hemoglobin, fasting blood glucose, diastolic blood pressure (DBP), and total cholesterol, were also similarly balanced between the gallstone and non-gallstone counterpart groups (standardized difference ≤0.2).

Odds Ratios of Incident Gallstones for the Duration of Use and Type of Statin
The odds of subsequently occurring gallstones were investigated with respect to the duration of use of any statin and statin type (hydrophilic vs. lipophilic) ( Table 2 and Figure 1). Long-term medication (>545 days) with any statin or lipophilic statins was associated with a lower likelihood of developing gallstones (odds ratio [OR] = 0.91, 95% confidence interval [CI] = 0.86-0.96, p < 0.001 and OR = 0.88, 95% CI = 0.83-0.93, p < 0.001, respectively). In contrast, short-term use (180-545 days) of any statin or hydrophilic statins exhibited no statistical relationship with incident gallstones.

Discussion
Although a growing number of studies have demonstrated the influence of statin use on the risk of gallstones in the European population [28,29,31,37], large-scale nationwide studies based on Asian patient groups are limited [36], and the effects of statin use on incident gallstones remain unclear [31,36]. In this large nationwide cohort study, we ob-

Discussion
Although a growing number of studies have demonstrated the influence of statin use on the risk of gallstones in the European population [28,29,31,37], large-scale nationwide studies based on Asian patient groups are limited [36], and the effects of statin use on incident gallstones remain unclear [31,36]. In this large nationwide cohort study, we observed that previous statin prescription, especially lipophilic statin use for a long period (>545 days), was associated with reduced risk of incident gallstones after adjusting for potential confounders. Thus, this finding seems to support a potential relationship between statin administration and the decrease in incident gallstones in the adult Korean population. This is supported by the results of an earlier large study in the United Kingdom (UK) published in 2009 that comprised 27,035 patients in the gallstone cohort and 106,531 controls (OR = 0.78, 95% CI = 0.73-0.83) [28], and also by the results of a 2010 case-control study in Israel comprising 1465 patients with gallstones and 5860 controls (OR = 0.69, 95% CI = 0.57-0.84) [26]. Statin use was associated with an 18% decline in the risk of cholecystectomy for gallstone disease in a female cohort study (n = 2479) carried out as part of a prospective nurses' health study in the US (95% CI = 0.70-0.96) [27]. Furthermore, in the latest clinical trial in the Netherlands involving patients who had undergone bariatric surgery, which revealed that postoperative weight loss predisposed them to gallstone disease, patients receiving statins had a 58% lower likelihood of gallstone development than those who did not receive statins (95% CI = 0.20-0.90) [29]. These findings suggest the potential disease-prevention and -inhibition implications of statin use.
However, previous population-based studies in France and Taiwan did not observe a meaningful association between incident gallstones and statin use [31,36]. Furthermore, another large cohort study based on American military healthcare data (13,626 statin users and 29,812 controls) reported that statin administration was unrelated to the risk of gallstones [39]. The differences in results can be attributed to variations in the study cohorts, study design, and/or methods of statistical analysis. To reduce the effects of confounding factors, our study used well-structured nationwide data with adjustment for possible confounding variables. Therefore, we were able to reproduce previous findings showing the preventive potential of prior statin medication against gallstones.
Notably, long-term users were less likely to develop gallstones than short-term users or non-users. In the present study, participants using any statin or lipophilic statins exhibited 9% and 12% reductions (95% CI = 0.86-0.96 and 95% CI = 0.83-0.93) in the probability of incident gallstones, respectively, only when the statins were administered for a long duration (>545 days); conversely, short-term use (180-545 days) of statins showed no statistically significant association with gallstones. Despite the different duration cut-off values, a case-control study in Switzerland showed that statin administration for >180 days was related to a 23% lower risk of gallstone disease in the treatment group than in the counterpart group (95% CI = 0.65-0.92), with no such effect observed for statin use for ≤180 days [25]. A study based on a northern Denmark population, one of the most extensive cohort studies involving gallstone (n = 32,494) and control (n = 324,925) groups, indicated that statin use for either 1-2 years or ≥3 years had a protective effect, with 11% and 24% lower likelihoods of gallstone disease, respectively, in the patient group than in the comparison group, revealing a correlation between increased duration of statin use and decreased odds for gallstones [37]. In contrast, individuals who had recently started taking statins exhibited a 1.17-fold greater risk of gallstone development (95% CI = 1.06-1.30) [37]; opposite correlations were noted in the UK case-control study (OR = 1.19, 95% CI = 1.07-1.32) [28]. Overall, statin use seems to have a cumulative protective effect on patients who have undergone bariatric surgery; the dose-effect relationship between statin use and the formation of gallstones is remarkable, depending on the surgery method, possibly because of imbalanced digestive absorption in the operated gastrointestinal tract [29,41]. The relatively low ORs in our study compared with those reported in European populationbased studies might be due to the higher prevalence of cholesterol gallstones and the lower proportion of pigment gallstones in Western populations compared with Asian people [42]. Although the pattern of gallstone disease in Korea is similar to that observed in Western countries in the last 20 years [14], the ratio of cholesterol stones is slightly lower than that of pigment stones, at 0.96:1 [16], which may have contributed to the low ORs in the present study.
Cholesterol gallstones can develop due to various factors, including hypersecretion of bile, genetic factors, inflammation, mucin accumulation in the gallbladder, sluggish intestinal motility, enhanced cholesterol absorption in the gut, and changes in gut microbiota [24]. Metabolic conditions such as insulin resistance, type 2 diabetes, and obesity can also lead to inflammation and an imbalance in inflammatory cytokines [24,43], which may contribute to the development of gallstones [44]. By inhibiting HMG-CoA reductase, statins can reduce hepatic cholesterol biosynthesis and biliary cholesterol secretion, thereby preventing cholesterol gallstone disease [19,20,45]. Additionally, statins possess pleiotropic properties such as anti-inflammatory, immunomodulatory, antifungal, antimicrobial, antioxidant, and anticancer effects that can further stabilize risk factors for gallstone formation in patients with metabolic conditions [43,44,46]. Combination therapy with simvastatin and ezetimibe reduces plasma levels of remnant cholesterol, apolipoprotein B/lipoprotein, triglycerides, and cholesteryl esters in patients with gallstones, potentially due to the anti-inflammatory effects of statins [44].
The association of statins with incident gallstones may vary between the different statin types. We noted that previous use of lipophilic statins reduced the probability of gallstones, but no such link was observed with hydrophilic statin use. Scarce data are available on the preventive effects of different statin types against gallstones. The possible reasons for this are illustrated by a few experimental and clinical studies. Although an animal study failed to demonstrate a significant reduction in total cholesterol and gallstone formation in animals treated with simvastatin (lipophilic statin) [45], a preliminary clinical study suggested that lovastatin (lipophilic statin), either alone or in combination with bile acid agents (e.g., ursodeoxycholic acid), may help dissolve gallstones in humans, which may reduce biliary cholesterol levels via multiple mechanisms; lovastatin induced a 28% response rate to dissolution therapy [21]. Additionally, a clinical trial showed that adding simvastatin to ezetimibe reduced cholesterol particles in gallstones [44]. Lipophilic statins may have more beneficial outcomes than hydrophilic statins in terms of anti-inflammatory effects and immune suppression, which may be effectually related to a reduction in the risk of gallstone development [43,44,47]. However, Erichsen et al. [37] reported no significantly different ORs according to statin subtype, although the authors did not describe their results in detail. Patients with type IIa hyperlipidemia medicated with 10 mg/day pravastatin (hydrophilic statin) for 12 months showed enhanced bile pathogenicity even at a relatively low dose, and pravastatin could reduce the incidence and complications of cholesterol gallstones [43]. Nevertheless, further research on this topic is necessary.
The strengths of this study include its extensive and balanced gallstone (n = 22,636) and control cohorts (n = 90,544) extracted from a well-organized nationwide healthcare database, which helped minimize the risk of selection bias and mimic the rigor of a randomized trial. As the Korean National Health Insurance Service Health Screening Cohort (KNHIS-HSC) database contains data from each hospital and clinic in the country, no medical history was excluded during the follow-up period, which may enhance the generalizability of our findings. This study used universally applied nationwide data, while previous studies were based on comparatively small regions or limited sets of medical records [25,27,31,38]. To minimize selection bias and the effects of confounding factors, the non-gallstone counterpart group was randomly chosen using propensity-score matching. Furthermore, we included many important potential confounders. The main strength of this study is the balanced distribution of socioeconomic factors (e.g., income and residential region) as well as lifestyle factors and chronic medical conditions possibly connected to gallstone or statin use (e.g., smoking status, blood pressure, fasting blood glucose levels, total cholesterol, alcohol consumption, and weight) between the patient and control groups. Large-scale, refined observational studies that adjust for the effect of confounding factors may provide important evidence on the association between statin use and gallstone disease. To our knowledge, this is the first Korean cohort study to evaluate the association between statin use and gallstone disease using propensity-score matching. The results may support the protective effect of statin use on incident gallstones.
The limitations of the present study must be considered. First, we used days of prescription to assess duration of treatment, but actual medication intake was not monitored as patient compliance could not be assessed. Second, although we tried to include confounding factors as adjustments, some factors may have been missed. Because we could not control whether participants used hydrophilic or lipophilic statins, we adjusted for each medication as a covariate in the analysis to remove possible confounding effects. Third, we did not consider statin dose or adjustment for other cholesterol-lowering drugs such as ezetimibe. Fourth, information about family history or genetic factors, type of gallstone (cholesterol vs. pigment), co-medication, diet, type or quantity of alcohol consumed, and physical activity level were not considered in the health insurance data.
In summary, our results indicate that long-term medication with statins (>545 days), particularly lipophilic statins, may reduce the likelihood of gallstone onset in the Korean population. Using Korean insurance data, this nationwide cohort study adds supportive evidence for the protective effect of statins against gallstone disease.

Ethical Consideration and Data Source
All analyses adhered to the guidelines and regulations of the ethics committee of Hallym University, which approved this study (2019-10-023). The need for written informed consent was waived by the Institutional Review Board.
The present retrospective, nested case-control study was based on analysis of the KNHIS-HSC database, which offers population-based data on Koreans for research purposes, by means of a random sampling method. The KNHIS is a mandatory nationwide health insurance policy in Korea that has included registration of over 98% of all Korean citizens since 1999. The medical data files and all individuals' information were de-identified and anonymized [48]. The KNHIS-HSC database uses the diagnostic codes of the International Classification of Diseases, 10th Revision, Clinical Modification (ICD-10-CM). A detailed description of the KNHIS-HSC data has been reported in previous studies [46,49].

Study Population and Participant Selection
Participants (n = 25,036) were selected from 514,866 adult patients (age >40 years) with 895,300,177 medical claim codes in the KNHIS-HSC database from 2002 through 2019 ( Figure 2). To reduce the incidence of false positives, records of patients with gallstones diagnosed using ICD-10 code K80 following more than two clinic visits were retrieved. The index date for every patient with gallstones was established as the day when the ICD-10 code was electronically assigned to the participant in the health insurance claims dataset. To avoid including in the analysis individuals with pre-existing gallstones that occurred before the index date, we excluded patients who were diagnosed with gallstones in 2002 and 2003 (2 years of washout period, n = 2397). Participants with no data for body mass index (kg/m 2 , n = 1) or fasting blood glucose level (n = 2) were discounted from the gallstone group. The counterpart non-gallstone group included participants without gallstones between 2002 and 2019 (n = 489,830). Comparison participants were discounted if they had been diagnosed with ICD-10 code K80 (gallstone) at least once (n = 12,471).

Independent Variable (Prescription Date of Statin Medication)
We retrospectively identified the statin prescription dates prior to the index date, for both groups. Patients were considered statin users if they had a statin prescription history within 2 years of the index date [27,29]. Those with any previous statin history >2 years To optimize the balance of baseline characteristics between the gallstone and comparison groups, propensity-score matching was applied based on residential region, income, age, and sex, and the data were sorted employing a random number order, followed by selection from top to bottom to prevent possible selection bias. As the index date of the control participants depended on the index date of their matched counterparts with gallstones, each patient with gallstones and each matched control member had the same index date. Therefore, we ruled out participants in the control group who died before the index date. During the matching step, 386,815 control members were discounted. Thus, 22,636 patients with gallstones were matched 1:4 to 90,544 comparison individuals. We then surveyed patients in both groups who had a history of statin prescription before the index date.

Independent Variable (Prescription Date of Statin Medication)
We retrospectively identified the statin prescription dates prior to the index date, for both groups. Patients were considered statin users if they had a statin prescription history within 2 years of the index date [27,29]. Those with any previous statin history >2 years from the index date in their medical records were ruled out. To estimate the cumulative impact of statin exposure for each individual, the total duration of drug use was computed as the total number of prescription days in the 2 years preceding the index date and was categorized into three classes: Class 1: <180 days, Class 2: 180-545 days, and Class 3: >545 days. Statin users were defined as patients who had statin prescriptions for a minimum of 180 days [50,51]; those with prescriptions for <180 days, 180-545 days, and >545 days were deemed statin non-users, short-term users, and long-term users, respectively, as previously explained [29,46].
Simvastatin, atorvastatin, lovastatin, pravastatin, fluvastatin, and rosuvastatin were the prescribed statins in this study, and were grouped as lipophilic (fluvastatin, lovastatin, simvastatin, and atorvastatin) or hydrophilic statins (rosuvastatin and pravastatin) in order to evaluate the impact of statin type on the occurrence of gallstones according to the statins' lipid affinity.

Dependent Variable (Gallstone Formation)
Patients with more than two clinic visits for treatment of gallstones defined according to ICD-10 code K80 (cholelithiasis) were deemed as having gallstones.
The CCI is commonly utilized to calculate disease burden by evaluating the presence and severity of 17 comorbidities. The CCI for each participant was summed as the total score of continuous variables from 0 (no comorbidities) to 29 (multiple comorbidities) in order to quantify the severity and number of diseases among the 17 comorbidities [52]. For patients with gallstones, history of dyslipidemia (ICD-10 code: E78) was added to the CCI if they had been treated ≥2 times.

Statistical Analyses
Categorical data were summarized as percentages, and standardized differences were exploited to compare the general characteristics between the cohorts. Propensity-score matching was conducted to facilitate exact matching between the gallstone and control groups for age, sex, income status, and region of residence [53]. During propensity-score matching, a greedy, nearest-neighbor matching algorithm was applied to form dyads of gallstone cases and counterparts whose propensity scores were closest to the patients' [54]. To reduce the possibility of intergroup bias, we assessed whether the matched data were balanced in terms of absolute standardized differences in the covariates. A standardized difference of ≤0.20 in the ascertained covariate indicated an appropriate balance.
To analyze the OR with 95% CI for gallstones with statin prescriptions, conditional logistic regression was applied in the matched groups. For the logistic regression analysis, model 1 (SBP, DBP, total cholesterol, fasting blood glucose, hemoglobin, and dyslipidemia history) and model 2 (model 1 plus obesity, smoking status, alcohol consumption, statin type, and CCI scores) were considered (when a lipophilic statin was analyzed, hydrophilic statin was adjusted as a covariate, and vice versa). Additionally, the effect of statin prescription was analyzed by division into hydrophilic and lipophilic statins. Two-tailed analyses were performed, and statistical significance was defined as p < 0.05. SAS version 9.4 (SAS Institute Inc., Cary, NC, USA) was used for all statistical analyses.

Conclusions
In summary, our results may indicate that long-term use of statins (>545 days), particularly lipophilic statins, could have a beneficial effect against the onset of gallstone disease in the Korean population. Based on Korean insurance data, this nationwide cohort study adds supportive evidence for the protective effect of statin use against gallstone disease.