Sodium in relation with nonalcoholic fatty liver disease: A systematic review and meta‐analysis of observational studies

Abstract Findings on the association of sodium with nonalcoholic fatty liver disease (NAFLD) are conflicting. The present systematic review and meta‐analysis study aimed to assess the association between salt or sodium intake or serum sodium levels and NAFLD risk. Relevant articles were identified by searching PubMed, Web of Knowledge, Scopus, Proquest, and Embase databases through May 1, 2021, without language restriction. The pooled odds ratio (OR) and 95% confidence interval (CI) were estimated using Der‐Simonian and Laird method and random‐effects meta‐analysis. The certainty of the evidence was rated using the GRADE method. Out of 6470 documents, 7 epidemiological/observational (1 cohort, 1 case–control, and 5 cross‐sectional) studies on the relationship between dietary salt/sodium intakes and NAFLD risk met our inclusion criteria. The meta‐analysis of all studies showed a significant positive association between the highest salt/sodium intake and NALFD risk (OR = 1.60, 95% CI: 1.19–2.15) with a meaningful heterogeneity among studies (I2 = 96.70%, p‐value <.001). The NAFLD risk was greater in the studies with higher quality (OR = 1.81, 95% CI: 1.24–2.65) or using the equation‐based methods for NAFLD ascertainment (OR = 2.02, 95% CI: 1.29–3.17) or urinary sodium collection as a sodium intake assessment (OR = 2.48, 95% CI: 1.52–4.06). The overall certainty of the evidence was very low. In conclusion, high sodium intake seems to be related to increased NAFLD risk. Further well‐designed studies are needed to clarify this association and shed light on the underlying mechanisms.


| INTRODUC TI ON
Nonalcoholic fatty liver disease (NAFLD) is the most prevalent liver disorder affecting 25.24% of the general adult population and 59.67% of diabetic patients worldwide ( (Dai et al., 2017); (Younossi et al., 2016)). NAFLD represents excessive intrahepatic fat deposits (>5% of liver weight) in the absence of secondary causes of steatosis, including alcohol abuse, rapid weight loss, total parenteral nutrition, medications, viral infection, or other chronic liver diseases (Kneeman et al., 2012). The pathogenesis of NAFLD involves an imbalance between hepatic fatty acid influx from the diet, adipose tissue lipolysis, and de novo lipogenesis vs. hepatic fatty acid β-oxidation and very-low-density lipoprotein secretion (Nassir et al., 2015). NAFLD is recognized as the liver manifestation of metabolic syndrome and is associated with obesity, diabetes, hypertension, and cardiovascular disease (Wong and Lim, 2018). Along with a genetic predisposition, central obesity, and insulin resistance, dietary factors have critical roles in NAFLD development ((Birjandi et al., 2016); (Dongiovanni et al., 2013); (Nivukoski et al., 2020)).
Salt or sodium chloride is among the most widely used food additives at home and in food industries ((Bansal and Mishra, 2020); (United States Department of Agriculture Aimapp, [internet], 2015)).
No previous article has comprehensively summarized these studies and estimated the pooled effect size so far. Therefore, the present study aimed to conduct a meta-analysis on observational studies on both genders and all ages and races to seek whether dietary salt or sodium intake or serum sodium levels are associated with the risk of NAFLD.

| Search strategy
The preferred reporting items for systematic reviews and metaanalysis (PRISMA) guideline (Liberati et al., 2009)

| Data extraction and risk of bias assessment
The two authors independently extracted data, including the last name of the first author, year of publication, country, study design, age, gender, number of participants, dietary assessment tool, sodium intake measurement method, outcome assessment method, follow-up duration (for cohort studies), levels of salt or sodium, and the number of events in the highest and lowest categories. The data extraction sheet is summarized in Table 1.
The risk of bias was assessed by the two authors independently, using the Newcastle-Ottawa Scale (NOS) ( (Modesti et al., 2016); (Wells et al., 2000) HSI, FLI High sodium intake was associated with an increased risk of NAFLD and liver fibrosis Sodium intake (g/d) Yes/no Ultrasonography No association was found between sodium intake and NAFLD studies and from 0 to 4 for others), comparability (0-2), and exposure/outcome (0-3). Then, studies were tiered according to the total scores to the following categories: very high (0-3 points), high (4-6 points), and low risk of bias (7-10 points).

| Statistical analysis
The primary outcome of the present study was the pooled risk of NAFLD in subjects with high dietary salt or sodium intake or serum sodium levels than the reference category. We also assessed the effects of region, study design, risk of bias, methods of NAFLD ascertainment, and sodium intake measurement tools on the pooled estimates and heterogeneity.
Crude odds ratios (OR) with a 95% confidence interval (CI) were calculated for the included studies. When the data needed to estimate these values were not available in the article, a request for further information was sent to the authors by email. The summary ORs and 95% CI were also estimated for the highest vs. the lowest categories of salt or sodium using the Der-Simonian and Laird method.
A random-effects model also was selected to compensate for methodological between-study heterogeneities, that is, population and methodological diversities. Subgroup analysis was performed based on the region, study design, risk of bias, methods of NAFLD ascertainment, and sodium intake measurement tools to assess the source of heterogeneity across studies. Besides, sensitivity analysis was executed to evaluate the impact of removing each investigation on the pooled results. The presence of possible publication bias was not assessed in the current meta-analysis due to the small number of the included studies (<10) (Tarsilla, 2010). All tests were conducted using Stata MP Version 16 (StataCorp), and p-values <.05 were considered statistically significant.

| Certainty of the evidence
The have generalizability , respectively. Moreover, the imprecision downgrades when there is no statistically significant association, sufficiently narrow 95% CI, or optimal information size F I G U R E 1 PRISMA flow diagram of the literature search and study selection process (total number of participants >400) or the estimated point or upper or lower bounds of 95% CI is <1.25 .

| Study selection
The study flowchart is presented in Figure 1. After the title and abstract screening for 6470 publications, 31 articles were selected for full-text assessment based on the predefined inclusion and exclusion criteria. Ultimately, seven observational studies published from 2015 to 2020 were eligible for inclusion in the systematic review and meta-analysis. The list of excluded studies following full-text assessment is presented in Table S2. No additional articles were found through forwarding and backward citation tracking of eligible studies.

| Study characteristics
The main characteristics of the included studies are summarized in Table 1. Of the seven identified articles, one had a prospective cohort design (Shen et al., 2019), one was case-control (Emamat et al., 2021), and the others were cross-sectional ( (Choi et al., 2016); (Huh et al., 2015); (Portela et al., 2015); ingestion also was assessed through a three-item questionnaire in one study (Shen et al., 2019). A validated questionnaire was used in all studies, except one (Portela et al., 2015). The mean/median daily sodium intake in the lowest and highest categories were different between studies and ranged from 1077 (Choi et al., 2016) to 3183 (Emamat et al., 2021) and from 3310 (Choi et al., 2016) to 5143 (Emamat et al., 2021) mg, respectively. However, the cutoffs were not reported in one study (Portela et al., 2015).

| Risk of bias assessment
Based on the NOS, the total quality score of the included articles varied from 3 to 8. Therefore, four studies were categorized as ; (Huh et al., 2015)) in two studies and by self-reported questionnaire or unblinded interview for dietary sodium intake estimation in the others (Zhou et al., 2021;Emamat et al., 2021;Portela et al., 2015;Shen et al., 2019;Choi et al., 2016). The validity of the Tanaka formula, which uses spot urine specimens, was weak at the individual level (Zhou et al., 2017). Besides, the response rate was not reported in three articles (Emamat et al., 2021;Portela et al., 2015;Huh et al., 2015) (Table 2).

| Salt/sodium intake and risk of NAFLD
The result of the meta-analysis of seven identified epidemiological studies on the association between sodium and NAFLD risk is presented in Figure 2a. The pooled OR was 1.60 (95% CI: 1.19-2.15) for the highest category of sodium as compared to the lowest, with significant heterogeneity among studies (I 2 = 96.70%, p-value <.001).

| Sensitivity analysis
No individual study had a considerable influence on the pooled estimates ( Figure S2). The pooled ORs of the sensitivity analysis ranged from 1.70 (95% CI: 1.24-2.33) to 1.41 (95% CI: 1.14-1.75) upon omit-  Table S3 represents the GRADE assessments for the associations between salt/sodium intake and NAFLD risk. The overall certainty of the evidence was very low due to downgrades for risk of bias, indirectness, and imprecision.

| DISCUSS ION
As a result of the growing global incidence of NAFLD in recent years, numerous investigations have studied the potential risk factors and intervention strategies for its prevention and management. Excess salt intake is among the major health risks worldwide and a possible factor in the pathogenesis of some hemodynamic and metabolic abnormalities. The main sources of dietary salt appear to be bread and bakery goods, meat and its products, instant noodles, salted preserved foods, dairy products, and condiments (Menyanu et al., 2019;Campbell et al., 2012). To the best of our knowledge, this is the first comprehensive systematic review and meta-analysis study assessing the relation between salt/sodium intake and risk of NAFLD prevalence or incidence.
The present meta-analysis indicated a positive association between high salt/sodium consumption and NAFLD risk. The pooled OR indicates a 60% greater risk of NAFLD in high salt/sodium intake than the reference category (p-value <.001) (Figure 2). This association was significantly greater in the studies with higher quality (p-value =.031) or using equation-based method for NAFLD ascertainment (p-value =.023) or urinary sodium collection as a sodium intake assessment (p-value =.005) ( Table 3). Sensitivity analysis revealed that no particular study had a significant influence on the pooled estimates ( Figure S2). Due to the observed considerable heterogeneity, because of the differences in genetic backgrounds, age, health condition of participants, study designs, and methods of NAFLD ascertainment and sodium intake assessment, interpretation of findings should be made with caution. Our consequence was in line with previous investigations reporting higher NAFLD risk with adherence to salt-rich dietary patterns or higher salty snack intake (Choi et al., 2016;Chung et al., 2019;Yari et al., 2020). However, this relation has not been confirmed in all studies (Yang et al., 2015; Alferink et al., 2020). These contradictory results could be partially TA B L E 2 Detailed results of the risk of bias assessment of the included studies based on the Newcastle-Ottawa Scale (NOS)

Selection
An adequate definition of case

Ascertainment of exposure
Representativeness of the exposed cohort

Selection of the nonexposed cohort
Ascertainment that outcome was absent at the start and diet record) (Defagó and Perovic, 2015). In the present metaanalysis, the association between sodium consumption and NAFLD risk was modified by study quality, NAFLD ascertainment methods, and sodium intake measurement tools. So, a stronger association was observed in the studies with a lower risk of bias or using estimation equations method for NAFLD diagnosis or urinary sodium excretion for exposure assessment.
A high-salt diet boosted fat mass, plasma leptin concentration, and insulin-stimulated glucose oxidation in the adipocytes, probably because of elevating the lipogenic capacity of white adipose tissue in rats (Fonseca-Alaniz et al., 2007;Fonseca-Alaniz et al., 2008).
Leptin inhibits insulin secretion through several mechanisms including, suppression of glucose transport, inhibition of glucagon-like peptide-1-induced expression of preproinsulin mRNA, and induction of pancreatic β-cell apoptosis (Amitani et al., 2013). So, the potential relationship between sodium and NAFLD could partially be mediated by the effects of sodium on the NAFLD risk factors. In this regard, the association between sodium and NAFLD was attenuated and became nonsignificant after adjusting for BMI in two studies (Zhou et al., 2021;Choi et al., 2016). In another study, a strong positive correlation only was detected in overweight or obese, but not in normal-weight participants (Emamat et al., 2021). Besides, salty foods increase thirst and promote sugar-sweetened beverages and energy consumption (Moosavian et al., 2017). High salt intake can also represent unhealthy dietary patterns high in saturated fat, soft drinks, refined grains, and sweets, which have undeniable roles in NAFLD exacerbation (Moosavian et al., 2017;Azimi et al., 2020;Mirmiran et al., 2017). Nevertheless, there is some evidence on the independence of the mentioned association from relevant covariates (Shen et al., 2019;Huh et al., 2015).
There are few proposed mechanisms regarding the effect of sodium on NAFLD development. Uetake et al. reported aggravations of steatosis and NASH in high-fat/high-salt diet-fed LOX-1 transgenic/apoE knockout mice because of oxidative stress and inflammation induction (Uetake et al., 2015). High sodium intake also was positively related to serum C-reactive protein and tumor necrosis factorα levels, respectively, in hypertensive patients and healthy adolescents (Yilmaz et al., 2012;Zhu et al., 2014). Oxidative stress and chronic inflammation play pivotal roles in the pathogenesis and progression of NAFLD (Masarone et al., 2018;Tarantino et al., 2010). A high-salt diet also contributed to NAFLD development by the aldose reductase-fructokinase pathway activation in the liver and hypothalamus and subsequent stimulation of endogenous fructose production (Lanaspa et al., 2018 (Bielinska et al., 2018). TMAO is a metabolite of choline, betaine, and carnitine generated by gut microbiota that may aggravate steatosis by suppressing bile acid-mediated hepatic farnesoid X receptor signaling (Tan et al., 2019). However, the mediating influence of TMAO in the effects of sodium on NAFLD development should be more investigated. Another supposed mechanism regarding the presently observed association may be the effects of a high-salt diet on dysregulation of the renin-angiotensin system (RAS) (Emamat et al., 2021;Huh et al., 2015). Sodium loading typically inhibits renin synthesis and RAS but directly activates mineralocorticoid receptors. The RAS suppression by sodium is suggested to have weakened in diabetic and probably NAFLD patients. As a result, such patients have RAS activation that does not inhibit by a high-salt diet. RAS and mineralocorticoid receptors activation are linked to oxidative stress and inflammation and are involved in the pathogenesis of NAFLD.
Literature suggests that the sodium to potassium ratio may be a superior dietary metric than the separate sodium or potassium intakes for assessing their relation to cardiovascular disease risks (Iwahori et al., 2017). Choi et al. examined the association between sodium to potassium ratio and risk of NAFLD prevalence. They reported that those in the highest quintile of sodium and sodium to potassium ratio had a 25% and 13% higher NAFLD prevalence than ones in the lowest quintiles, respectively (Choi et al., 2016).
However, in another study, comparing the highest with the lowest quartile of sodium and sodium to potassium ratio intakes resulted in 1.35 (95% CI: 1.02-1.80) and 1.53 (95% CI: 1.15-2.03) times more risk of diabetes, respectively (Hao et al., 2020). More studies are warranted to clarify the modulatory effects of potassium in such associations.
F I G U R E 2 Forest plot (random-effects model) depicting the association of salt/sodium intake (highest vs.. lowest category) and risk of nonalcoholic fatty liver disease

| LIMITATI ON S OF THE ME TA-ANALYS IS
The present meta-analysis study has several limitations that should be considered during the interpretation of the findings. First, the observational design of the included investigations impeded the establishment of a causal link between salt/sodium intake and NAFLD.

| CON CLUS ION
In conclusion, the present meta-analysis revealed that higher sodium intake is associated with a higher NAFLD risk. This association tended to be increased in the studies with a lower risk of bias or using the predictive equation for NAFLD ascertainment or urinary sodium excretion for a sodium intake assessment. These findings provide evidence for another health hazard of excessive salt intake and further rationale for salt restriction. It is worth mentioning that our estimated pooled effect size is unadjusted and should be interpreted with caution. Because of high heterogeneity and significant subgroup differences, further large sample and long-term prospective cohort investigations using accurate methods are warranted to confirm the independent association between high salt intake and NAFLD risk. Besides, the effects of a low-salt diet on NAFLD management and the potential underlying mechanisms should be investigated in future studies.

ACK N OWLED G EM ENTS
We thank the Vice Chancellor's Office for Research Affairs of Shiraz University of Medical Sciences, Shiraz, Iran.

CO N FLI C T O F I NTE R E S T
The authors report no conflict of interest.

TR A N S PA R EN C Y D ECL A R ATI O N
The lead author affirms that this manuscript is an honest, accurate, and transparent account of the reported study. The reporting of this work is compliant with PRISMA guidelines. The lead author affirms that no important aspects of the study have been omitted and that any discrepancies from the study as planned (PROSPERO registra- F I G U R E 4 Forest plot (random-effects model) depicting the association of salt/sodium intake (highest vs.. lowest category) and risk of nonalcoholic fatty liver disease subgrouped by sodium intake measurement tools