1. Introduction
Central obesity has become a worldwide problem within the last several decades, and the prevalence of obesity remains high. More than 33% of adults and 17% of youths in the United States [
1], and 26.3% of adults in Germany, where the prevalence is among the highest in Europe [
2], are obese. Furthermore, the rest of the world is quickly catching up, particularly developing countries [
3]. Metabolic syndrome (MetS), which is known as a cluster of obesity-driven alterations, such as central obesity, insulin-resistance, dyslipidaemia, and hypertension [
4], has received increasing attention in recent years as its prevalence and public health burden have increased worldwide [
5]. To date, substantial evidence shows that MetS is associated with diabetes [
6,
7], dyslipidaemia, cardiovascular disease [
7,
8], specific types of cancer [
3,
9], and many other conditions.
Potassium is anintracellular cationic electrolyte that is necessary for normal cellular function. Because it is easily excreted by the kidneys rather than stored in the body, humans need a constant supplement of potassium. However, the average potassium consumption is inadequate, only 54% and 58% compared to the recommended amount in the U.S. and Korean populations [
10]. According to recent studies, the associations between potassium and obesity/MetS were investigated, although these observational studies are considered controversial. Several studies have shown that higher potassium could alleviate obesity and MetS risk, while other studies have found that potassium has a null effect on obesity and MetS.
To resolve the controversies and to improve the generalizability of these results, we conducted a systematic review and meta-analysis to combine information to estimate the overall effect of potassium on the risk of obesity and MetS.
4. Discussion
The results of meta-analysis and systematic review indicated potassium intake should not be investigated simply, and sodium-to-potassium ratio seemed to be a more sensitive index to obesity for the joint consideration of both sodium and potassium. In addition, the participants in the highest category of potassium intake had lower odds for metabolic syndrome compared to those in the lowest category (pooled OR = 0.75; 95% CI: 0.50–0.97). Consistent with these findings, our nonlinear analysis revealed a protective effect of adequate potassium intake on obesity and MetS. Additionally, high potassium intake may play a more important role in females than males.
Recently, the relationship between potassium intake and obesity/MetS was controversial. A study performed by Murakami
et al. [
16] indicated that higher potassium intake is associated with a lower risk of obesity, while studies performed by Shin
et al. [
10] and Lee
et al. [
19] found that potassium intake had a null effect on obesity. According to metabolic syndrome, Shin
et al. [
10] showed that high potassium intake was a protective factor, and this finding was consistent for both sexes. Nevertheless, Teramoto
et al. [
20] revealed that potassium was more correlated with MetS in females than in males. The urinary sodium-to-potassium ratio was found to be associated with obesity robustly, which also influenced blood pressure [
23,
24]. However, no articles reported the association between sodium-to-potassium ratio and MetS.
Our meta-analysis included 51,702 subjects from four different countries ranging from 2011 to 2015 and consisted of 15,527 obesity and 10,482 MetS cases. Nevertheless, most studies were performed in Asia (China, Japan and Korea) were in Asia. There are no relevant studies in the European countries and other areas that have high rates of obesity and metabolic syndrome. However, all of the included studies in our meta-analysis were published within the past five years. This finding indicates that the association between potassium and obesity/MetS appears to be a relatively new topic for nutrients and public health. However, these recent studies were confined to Asia and USA, and have small-scale limitations. We propose that it is necessary to perform global research studies using a larger number of participants in the future with prospective design or random controlled trials. Sodium-to-potassium ratio is probably also a sensitive index to evaluate the risk of obesity and MetS in addition to potassium intake, although more studies are needed to verify the results.
In this study, four studies employed dietary surveys to assess potassium intake, while two studies used serum potassium as exposure and the last two used urinary sodium-to-potassium ratio as exposure. We could not conduct meta-analysis for serum potassium and sodium-to-potassium ratio for the lack of sufficient relevant studies. Hence, a systematic review was performed to describe the relationship between serum potassium/sodium-to-potassium ratio and obesity/MetS. Murakami
et al. [
16] showed that high potassium assessed from 24 h urinary excretion was also associated with obesity. Nevertheless, the results of Ge
et al. [
25] did not indicate a significant association between 24 h urinary potassium excretion and MetS. Due to the lack of sufficient relevant studies, we did not perform the meta-analysis between 24 h urinary potassium excretion and obesity/MetS.
The precise mechanism between potassium intake and obesity/MetS is unclear. Central obesity is a component of metabolic syndrome, and the mechanisms of obesity and MetS are homogeneous. Obesity has been shown to be associated with potassium channel function [
26,
27]. Potassium can affect carbohydrate accumulation and glucose homeostasis [
28,
29] and plays a critical role in insulin secretion and carbohydrate metabolism [
28,
30]. Moreover, it is also a predictor of incident diabetes [
31]. Dietary potassium intake has also been inversely correlated with blood pressure [
24,
32,
33]. However, the protective effect of high potassium intake on obesity may be due to the high intake of fruits and vegetables, which are major sources of potassium [
34] and have also been shown to be beneficial to metabolic syndrome [
35].
This study has several limitations. First, a small number of studies were included in the study. Only four relevant studies and seven estimates were incorporated into the pooled analysis, and all of the relevant articles included in the meta-analysis are cross-sectional studies, which are not as scientific as prospective observational studies and random controlled trials. Second, an insufficient number of relevant estimates weaken the accuracy of the results of nonlinear dose-response analysis, and we have limited power to perform a subgroup analysis of some potential confounding factors. Third, 24 h dietary recall may not be accurate to assess the precise dietary intake due to errors in estimating the nutriment size and faults in memory. In addition, the relevant studies did not apply the same food items in the frequency questionnaire to survey the dietary potassium intake. Although recent studies revealed the inverse relationship between serum potassium and obesity/MetS, this condition may be contributed by the subclinical elevations of cortisol, and be the results of hypercortisolism or hyperaldosteronism rather than potassium intake. Prospective studies with a large sample size and accurate assessment of potassium intake are needed to confirm or update our results. Additionally, the effects of sodium-to-potassium ratio on obesity and MetS are recommended to be researched. We will update the meta-analysis in the future when high quality studies (prospective cohort studies and RCTs) are reported.
Our study also has several strengths. To the best of our knowledge, this is the first systematic review and meta-analysis to investigate the overall effect of potassium on the risk of obesity and MetS, and our study reviewed the effect of different measurement of potassium (intake, serum and sodium-to-potassium ratio) on obesity and MetS in detail. Furthermore, we performed nonlinear does-response analysis using the restricted cubic splines method to test the potential nonlinear relationships. These results suggest a daily intake of potassium that is larger than 2200 mg/Kcal to reduce the risk of obesity and MetS. Moreover, we applied strict inclusion criteria to reduce selection bias and improve the reliability of our conclusions.