**3. Deteriorative effects of iron against NASH/NAFLD**

Iron overload is one of the important risk factors for diabetes. The relationship between iron and diabetes was first recognised in pathologic conditions − namely hereditary hemochromatosis and thalassemia − but high levels of dietary iron also enhance the risk of diabetes. It is generally recognised that iron plays a direct and causal role in diabetes pathogenesis, mediated both by β-cell failure and insulin resistance.

Iron is capable of generating hydroxyl radicals from peroxide and can also inhibit antioxidant defences such as SOD2 [55]. Highly elevated iron levels have been linked to oxidative damage to DNA, lipids and proteins that, in turn, have been implicated in cardiovascular disease, diabetes, atherosclerosis and neurological degeneration, as seen in Alzheimer's disease [56].

Iron homeostatic pathways are tightly associated with inflammatory stressors. Inflammation causes significant upregulation of hepcidin, largely through interleukin-6 (IL-6), and results in large increases in serum ferritin levels.

There is a greater prevalence of iron deficiency in obese (39%) and overweight (12%) children and adolescents than in normal-weight children, the prevalence of iron deficiency in whom is only 4% [28]. The association of iron deficiency with obesity has been confirmed in other populations, which include children and adults of both sexes [57]. The conceivable cases for causality, in turn, can be made in both directions: normal or high iron stores might be required to support higher rates of fatty acid oxidation so that iron-deficient individuals are less able to mobilise and use high fat, or, conversely, the inflammatory nature of obesity might trigger increased hepcidin levels, which limit the absorption of dietary iron.

In the progression of diabetes, ROS can cause both β-cell failure and insulin resistance. β-cells are particularly sensitive to ROS because of low expression of antioxidants such as catalase and SOD2, overexpression of which has been associated with increased β-cell viability [58]. ROS can cause β-cell dysfunction by multiple mechanisms including decreased insulin gene expression secondary to decreased expression of transcription factors necessary for β-cell differentiation, maintenance and insulin gene transcription.

ROS have also been reported to directly affect circulating human insulin by hydroxylation of phenylalanine residues that result in lower affinity binding to the insulin receptor [59]. Finally, ROS can induce insulin resistance through multiple mechanisms; for example, through the activation of FOXO1, which prevents downregulation of gluconeogenesis even in the presence of insulin signalling [60]. Hypoxia-inducible factors 1 and 2 (HIF-1 and HIF-2) regulate cellular responses against low oxygen by upregulating transcription of a diverse set of proteins involved in angiogenesis, erythropoiesis and glycolytic flux [61]. HIFs also regulate iron metabolism, and under conditions of low iron levels, HIF-2 upregulates DMT-1 and DCYTB, whereas HIF-1 upregulates DMT-1 and decreases ferritin [62]. Conversely, cellular iron levels regulate HIF protein levels through the control of prolyl hydroxylase (PHD) activity [63].

Emerging data demonstrate that iron plays an important role in metabolic regulation and the pathophysiology of diabetes. Iron overload is common in T2DM [64, 65]. On the contrary, iron depletion seems to be protective for the development of diabetes. Rats with iron-deficiency anaemia are more insulin sensitive than controls [66], and phlebotomy improves the insulin sensitivity and glycemia, both in nondiabetic subjects [67] and T2DM subjects with high ferritin [68]. These studies suggest that iron plays an important role both in the development and improvement of diabetes. However, the precise molecular mechanisms of iron-associated diabetes are not well understood at present [69].
