Sodium status and the metabolic syndrome: A systematic review and meta-analysis of observational studies

The prevalence of metabolic syndrome (MetS) has been greatly increased, worldwide. In recent years, investigators have proposed that sodium might contribute to the development of metabolic syndrome; however, the published data were conflicting. The present systematic review aimed to summarize the evidence from observational studies in this regard. We conducted a systematic search for relevant observational studies investigating the association between sodium status and MetS, published until June 2017 in electronic databases including PubMed, EMBASE, Scopus and Google Scholar. Summary effects were derived using random effects model. After screening the records, seventeen publications with 66,274 participants were eligible to be included in the systematic review and meta-analysis. The analysis revealed that subjects with MetS have significantly higher levels of sodium compared to healthy controls (Hedges' g = 0.21, 95% CI: 0.12, 0.29, I² = 68.6). Subgroup analyses revealed that the difference was significant when the sodium status was assessed using urinary sodium levels. The random effects meta-regression analysis also revealed that body sodium level increases with the number of MetS components. Furthermore, participants with highest dietary/urinary or serum sodium levels had 37% higher chance of developing MetS when compared with participants with the lowest sodium levels (OR = 1.37 95%CI: 1.31, 1.42, I² = 86.9). The current meta-analysis revealed that higher sodium input into the body is directly associated with the likelihood of MetS. Prospective cohort studies and well-designed randomized clinical trials considering the effect of sodium restricted diets on the risk of MetS as an outcome are necessary to represent the causal association.