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Received Oct 25, 2017; Revised Feb 10, 2018; Accepted Mar 12, 2018
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1. Introduction
Manganese (Mn) is an essential element in the human body that is mainly obtained from food and water. Mn is absorbed through the gastrointestinal tract and then transported to organs enriched in the mitochondria (in particular the liver, pancreas, and pituitary) where it is rapidly concentrated [1]. Furthermore, Mn is involved in the synthesis and activation of many enzymes (e.g., oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases); metabolism of glucose and lipids; acceleration in the synthesis of protein, vitamin C, and vitamin B; catalysis of hematopoiesis; regulation of the endocrine; and improvement in immune function [2]. Moreover, Mn metalloenzymes including arginase, glutamine synthetase, phosphoenolpyruvate decarboxylase, and Mn superoxide dismutase (MnSOD) also contribute to the metabolism processes listed above and reduce oxidative stress against free radicals (Figure 1).
[figure omitted; refer to PDF]However, environmental or occupational Mn overexposure is harmful to human health, especially in at-risk populations such as miners, welders, and steel makers. According to data from the Mineral Commodity Summaries released by the US Geological Survey in 2016, South Africa, China, and Australia accounted for 67% of the total Mn mined (18 million tons) in the world in 2015. Mn ore mining and its processing cause air and water pollution, threatening the health of workers and general populations residing near factories through oral ingestion and inhalation as well as dermally and intravenously. Acute Mn exposure can lead to manganism, and chronic Mn exposure causes an extrapyramidal syndrome with features resembling those found in Parkinson’s disease and postencephalitic parkinsonism [3].
The prevalence of metabolic diseases, including type 2 diabetes mellitus (T2DM), obesity, insulin resistance, atherosclerosis, hyperlipidemia, nonalcoholic fatty liver disease (NAFLD), and hepatic steatosis, has increased dramatically over the past few decades [4]. These metabolic disorders are usually caused by the clustering of metabolic syndrome (MetS). The criteria for identifying MetS include three of five markers: abdominal obesity, impaired carbohydrate metabolism, high blood pressure, and dyslipidemia, including elevated levels of triglycerides and decreased levels of high-density lipoprotein (HDL) [5]. In...