GSD is a common disease that can progress to severe cholecystitis; it is a strong risk factor for gallbladder cancer (GBC). Recent European studies have shown that hypertriglyceridemia, hypercholesterolemia and low levels of high-density lipoprotein cholesterol (HDL) are common in patients with cholelithiasis (24, 25). To verify our initial hypothesis, we performed this clinically-based, age-, sex-, diet-, nation-, blood pressure-, and FBG-matched case-controlled study of the risk factors for asymptomatic GSD, of the epidemiologic factors that are known risk factors for GSD. We found that elevated BMI, enlarged WC and WHtR, NAFLD, and HC are very likely important risk factors for hazard of new-onset asymptomatic GSD (24-27). Multivariate logistic regression further strengthened the notion that hypercholesterolemia may be an independent risk of new-onset asymptomatic gallstone diseases, and abdominal obesity and NAFLD might promote the occurrence of asymptomatic gallbladder stone disease.
First, we confirmed that there were no significantly positive correlations between GSD and WC, WHtR after further adjusted by hypercholesterolemia, though obesity has been recognized for its strong association with gallstone diseases. We found that obesity per se was not linked directly to cholelithiasis risk, and it was highly probable that both share several pathophysiologic and genomic pathways (28): On the one hand, hormone-sensitive lipase and adipose triglyceride lipase, discovered recently in adipose tissue, mediate the mobilization of stored triacylglycerol (29). Elevated triglyceride levels might decrease sensitivity to cholecystokinin (30), and might increase both biliary cholesterol saturation and increase bile viscosity by enhancing mucin production (31), thereby adding to the enhanced risk of gall stone disease. On the other hand, there is a vicious circle: Obesity might result in increasing levels of cholesterol secretion from liver, and bile supersaturation in biliary secretion of cholesterol. Meanwhile, there is a linear relationship between cholesterol production and body fat (32), and biliary cholesterol saturation, bile acid synthesis, turnover rates and bile acid pool sizes are increased in obesity (30, 33). In addition, it is clear that abdominal adiposity is associated with insulin resistance. Hepatic insulin resistance directly increases cholesterol secretion and reduces bile acid synthesis, leading to bile crystallization and stone formation. Hepatic insulin resistance has been identified as a key determinant of cholesterol gallstones formation by itself (34, 35).
Second, our study showed that the prevalence NAFLD was higher in GSD than in controls, suggesting a positive association between NAFLD and the risk of new-onset asymptomatic gallstone disease. A recent study reported that NAFLD was significantly associated with gallstone formation (36). It is worth noting that our multivariate regression showed that there was a weak positive correlation between GSD and NAFLD in hypercholesterolemia patients, and even based on the above, we tend to agree with the notion that NAFLD, possibly including hypertriglyceridemia, might be a secondary actor in overweight/obesity and hypercholesterolemia patients after eliminating blood pressure and diabetes (blood sugar, insulin resistance). These are the following supporting facts: first, NAFLD is typically characterized by atherogenic dyslipidemia featuring higher triglyceride levels with greater circulating very-low-density lipoprotein levels, lower levels of dense LDL-c and low levels of dysfunctional HDL-c (41). A recent report supported the notion that hypercholesterolemia is a major risk factor for the initiation and development of NAFLD (42), resulting from high-cholesterol diet, lipase deficiencies and liver inflammation (43, 44). Second, NAFLD, which affects approximately 12% to 40% of the general population, up to 95% of obese people, and nearly 70% of the overweight, is especially increased among middle-aged populations with obesity, type 2 diabetes, dyslipidemia, and other metabolic syndromes. NAFLD was observed in 96.5% (p<0.001) of subjects with overweight, hypertriglyceridemia, and hypercholesterolemia (37). Third, NAFLD is strongly associated with obesity, which might trigger decreased expression of leptin receptor in liver tissue and insulin resistance (38). The latter two interactions promote the occurrence and development of fatty liver (39).
We found that hypercholesterolemia may be an independent risk for new-onset asymptomatic gallstone diseases, and recent studies give strong backing to this notion. First, our logistic regression showed that TG and TC were not significantly correlated with GSD in patients with hypercholesterolemia, suggesting that the increased cholesterol and triglycerides play a integrally synergistic role in the pathogenesis of GSD: High levels of secretion of cholesterol in bile and the subsequent supersaturation of bile contribute to the formation of bile mud, increased triglycerides reduce gallbladder movement, reduce the sensitivity to cholecystokinin (CCK), and thereby reduce the gallbladder movement (30, 40). A recent report strongly supported the notion that sphingolipid ceramide accumulation in hypercholesterolemia causes inflammatory responses of the gallbladder (41). Second, we found that serum LDL-c levels were significant positively associated with the risk of new-onset asymptomatic GSD in participants with hypercholesterolemia. This is in agreement with findings of Ivanchenkova et al. (42), who found that serum LDL-c levels were increased in gallbladder cholesterosis and were a risk factor for gallbladder cholesterolosis, resulting in the minor particles of LDL being more rapidly penetrated into the gallbladder tissue, where the gallbladder wall is intensively involved with macrophages, participating in the formation of foamy cells. Lauridsen et al. (43) found that accelerating elevated LDL-c levels play a synergistic role with the continuous development and formation of gallstones in patients who have undergone cholecystectomy for symptomatic gallstones. Third, a study found that raised HDL-c levels were inversely associated with gallstone prevalence (44), implying that improvement HDL-c levels may be one of the mechanisms in the treatment and prevention of cholesterol gallstone disease (45). Probably, HDL-c, a precursor of bile acids, represents a major source of biliary cholesterol; however, elevated serum HDL increases primary bile acid formation which solubilizes cholesterol and reduces biliary cholesterol saturation (31), both which are important for reducing the lithogenicity of the bile (44). HDL might mediate the transport of excess cholesterol from lipid-laden macrophages within the vascular wall back to the liver for excretion into the bile, which represents the major route for irreversible removal of cholesterol from the body (46).
Interestingly, elevated weight was inversely associated with GSD in patients with hypercholesterolemia. We synthesize the relevant data with the following viewpoints: On the one hand, rapid weight loss leads to a change in cholesterol metabolism and consequently increases the concentration of cholesterol in the bile to a level at which not all cholesterol can be dissolved by bile salts. Undissolved cholesterol is prone to crystallize into stones, especially in the presence of calcium and mucin, a glycoprotein that stimulates cholesterol crystal aggregation (47). On the other hand, we consider lost weight to be a risk factor for GSD in patients with hypercholesterolemia, resulting from the participants including lean individuals, who may have diet models characterized by irrational vegetarian diets during weight loss. This results in not maintaining reasonably high intake of monounsaturated fats and fiber, olive oil and fish (ω-3 fatty acids), vegetable protein, fruit, coffee, or vitamin C (7, 48). A recent study showed that a supplementation with 5% chitosan oligosaccharide, a common food additive for weight loss, may cause liver damage via higher hepatic cholesterol accumulation and higher intestinal cholesterol uptake in high fat diet-fed rats (49). A carbohydrate-restricted, high-fat diet increased LDL-cholesterol concentrations because this effect of weight loss was related to the lack of suppression of both fasting and 24-h free fatty acids (50). However, weight loss or improper diet may cause imbalance of cholesterol homeostasis, and incomplete and slow gallbladder emptying may lead to cholestasis, increasing the risk of gallstone formation.
Strengths and limitations of this study. Strengths of this study include th large number of participants, a retrospective design, and complete follow-up for morbidities. The data came from participants who regularly undergo annual physical screening with long-term follow-up. We reviewed the case histories for more than five years, confirming that the patients had newly presented to the case group. The matched participants had the same occupations and came from similar environments. Our healthcare center, the largest institution in northwest China, is responsible for the health survey of multi-ethnic groups in the adjacent three provinces. The large scale as well as broad coverage of multi-ethnic people made the data in current study very informative and persuasive. Nevertheless, the present study has several limitations. First, this was not a multicenter and cohort study, and the statistics may be biased as a result. Second, the limited ethnic population may affect the generalizability of our findings. In particular, the association between BMI and gallbladder stone may differ among ethnic and regional groups with varying dietary structures and genetic diversity. Third, our study lacked data such as chemical composition analysis, cholesterol gallbladder stone-related risk factors, bile acid levels, and gastrointestinal dysfunction, because imbalance between bile salts and cholesterol in bile fluid cause bile fluid turn to become sludge, crystals and eventually gallstones.