The present investigation revealed a significant and independent negative relationship between weight-adjusted Mg intake and HOMA-IR in a large Chinese population, independent of age, sex, smoking status, alcohol consumption, educational level, residence, energy intake, protein intake, carbohydrate intake, and fat intake. We also found that group with high Mg intake was less likely to have MetS. To the best of our knowledge, we first demonstrated that insulin resistance partly mediated the relationship between dietary Mg intake and MetS.
Consistent with the result of our research, previous studies demonstrated an inverse association between dietary Mg intake and insulin resistance and a dose-effect relationship [21–24]. However, there is a lack of relevant research in Asian population and most Asian studies focused on the relationship between dietary Mg and the risk of diabetes, which were inconsistent. Some studies showed high dietary Mg intake could reduce the risk of type 2 diabetes[5, 22, 25], whereas others could not reach a significant result [8]. To fill the gap, our study aimed to explore the relationship between Mg intake and insulin resistance in a national representative sample of Chinese adults. Our study found a significant association between total dietary Mg intake and HOMA-IR, but after adjusting for dietary factors, the results did not stay significant. Then, we decided to adjust the dietary Mg intake by body weight, and we found a significant association before and after the three-model adjustment. A study from Mexico also chose to adjust the Mg intake by body weight and found a significant result showing that the insulin sensitivity assessed by the Matsuda index was higher in the high dietary Mg intake group; however, the sample size of the study was small [26]. Given that the previous studies, which did not show a statistically significant correlation between dietary Mg intake and insulin resistance all used total dietary Mg intake instead of weight-adjusted dietary Mg intake, we assumed that the amount of dietary Mg an individual needed was related to his/her weight. In other words, the heavier the person, the more Mg might be needed in his/her diet. This is consistent with the Dietary Reference Intakes (DRI) from the French Food Safety Agency, which recommended 6 mg/kg body weight Mg intake per day [27]. A study from the United States also demonstrated that the dietary Mg intake requirement was related to body weight, which was 2.36 mg/kg per day [28]. More studies would be necessary to confirm whether weight-adjusted Mg intake was a better indicator for Mg intake.
Our study found that increased intake of dietary Mg was negatively associated with the prevalence of MetS and its five components in the Chinese population. Studies have reported the beneficial effect of dietary Mg intake or Mg supplementation in reducing the prevalence of MetS [12, 29, 30], which was consistent with our findings. In those studies, dietary Mg intake was only relative with one to no more than five components of MetS, and there were few correlation studies involving Chinese individuals. In addition to the abovementioned findings, we also further found that dietary Mg intake was significantly associated with all five components of MetS in a national representative sample of Chinese adults. According to our data, the risk of the metabolic syndrome was nearly 70% lower in the top quintile of the dietary Mg intake than in the bottom quintile.
Insulin resistance is the basis of MetS [9], and Mg has been shown to be a key factor in insulin action through the activation of the b-subunit of the insulin receptor and the activation of substrates and proteins in the insulin-signaling pathway [31]. It is easy to speculate that the effect of Mg on metabolic syndrome was mediated by insulin resistance. In this study, we explored the mediation of insulin resistance in the effect of dietary Mg on MetS, and the calculated percentage of mediation was 23.0%, indicating that there were other mechanisms in the relationship between dietary Mg intake and MetS besides insulin resistance. It has been previously suggested that higher Mg intake and intracellular Mg might have a role in insulin secretion by preserving pancreatic β-cell function via its effect on calcium homeostasis and oxidative stress [12]. Mg also acts as a cofactor for several critical enzymes involving lipid metabolism. Mg has been reported to raise HDL and reduce LDL cholesterol and TG by limiting the action of lecithin cholesterol acyltransferase and HMG-CoA reductase and by increasing the lipoprotein lipase activity [12]. It has been assumed that Mg, in the intestine, by forming an unabsorbable soap with fatty acids and cholesterol, can decrease their absorption, reduce energy intake from the diet, and may have advantages for weight maintenance because of this tendency [32]. Apart from the abovementioned mechanisms, the relationship between Mg and, MetS might also be affected by genes [33, 34].
The strengths of the present investigation include the following: 1) it is the first large-scale investigation of the relationship between dietary Mg intake, insulin resistance, and MetS in a national representative sample of Chinese population. 2) It is the first study to explore the mediating role of insulin resistance in the relationship between Mg and Mets, 3) in this study, standardized protocols, and data collection procedures were used, data collectors were well trained, and quality control was assured, which can largely avoid measurement bias. Potential perceived limitations would be that we applied a cross-sectional research design to investigate associations; thus, we cannot establish causality in the present study. The 24-hour recall, which was the method we used to calculate Mg intake, is considered to have a similar accuracy to that of semi-quantitative food frequency questionnaires, but it has the disadvantage of recall bias.