Introduction

Diabetes mellitus is a complex entity; it includes a number of syndromes with distinct etiologies having hyperglycemia as a common feature. The consequences of diabetes mellitus can be severe affecting several organs. Two major types of diabetes are recognized, type 1 diabetes (T1D, previously known as insulin-dependent diabetes mellitus (IDDM)) and type 2 diabetes (T2D, previously known as non-insulin-dependent diabetes mellitus (NIDDM)). Type 1 diabetes appears in childhood and adolescence, and it has a major autoimmune component leading to destruction of pancreatic insulin-producing cells. Five to ten percent or less of diabetes cases are thought to be due to type 1 diabetes, with the remaining 90–95 % attributed to type 2 diabetes [1]. This form is often associated with obesity, and it is characterized by insulin resistance in early stages, and β-cell exhaustion in its later stages [2].

In the last decade, interesting advancements have been made regarding the role of zinc (Zn) in insulin secretion and action; perhaps, the most relevant was the identification of the Zn transporter ZnT8 as a key element in β-cell function [3, 4], and later, the findings obtained from genome-wide association studies focused on of genetic variant of this transporter and its association with risk of diabetes [5, 6]. This review focuses on the recent epidemiologic evidence between Zn and type 2 diabetes risk/course.

Zinc in Biology

Zinc is an essential element in key aspects of mammalian cell metabolism. Zn is required for the function of more than 300 enzymes [7, 8], and for a long time, this was thought to be the principal role of zinc in mammalian biology. It is now clear that regulatory roles of Zn are also highly relevant. For instance, Zn is crucial to form Zn fingers, which allow protein–DNA interactions. A significant number of transcription factors and nuclear receptors contain zinc fingers. Thus, Zn participates in the regulation of expression of a large number of genes and activity of selected hormone and vitamins. Zinc participates in DNA synthesis and DNA transcription, translation of mRNA into proteins, and also in the structure and stabilization of proteins [9]. Even though Zn is redox-inert, it presents relevant antioxidant effects [10]. Free or loosely bound zinc is able to take part in cell signaling processes [11]. As a result, Zn is involved in growth, immunity, cell and tissue repair, hormone (including insulin) action, vitamin A metabolism, and neuropsychological functions, among others.

Zn homeostasis is highly complex and requires the compartmentalization of this element into cellular organelles. Twenty-four proteins (Zn transporters) have been identified, and knowledge of their roles is incomplete. There are 14 members of the family gene solute-linked carrier SLC39 family that encodes for Zn transporters ZIP 1–14. They are mainly importers of Zn into the cytoplasm. There are ten members of the family gene solute-linked carrier SLC30 that encode for Zn transporters ZnT1–10. They are mainly exporters of Zn from the cytoplasm to organelles and to the extracellular space. Zn transporter expression is regulated by cytokines, hormones, and Zn itself, among others [12]. In the context of Zn and diabetes, there are interesting data on the role of ZnT8.

The assessment of Zn status remains a difficult task because despite the large number of indices proposed, all have problems affecting their validity [13, 14]. Zinc presents very effective homeostatic mechanisms that respond to modifications in Zn intake; in addition, there are no specific Zn stores in mammals, including humans. Lowe et al. [14] carried out a systematic analysis of 32 methods to assess Zn status in humans. Their main conclusions were that plasma Zn may be a useful indicator but with many limitations. Similar conclusions have been reached by King et al. [8, 15]. This issue is highly relevant because, with few exceptions, most epidemiologic studies have used plasma/serum Zn as the only Zn status parameter.

Biological Basis for the Interaction Zn–Type 2 Diabetes

Zn is important for a number of processes related to insulin secretion and insulin activity in peripheral tissues, making this element an interesting potential co-adjuvant in the treatment of type 2 diabetes patients. This issue has been reviewed by Ruz et al. [16]. Briefly, Zn is essential for the correct processing (formation of insulin hexamers), storage, and secretion of insulin by β-cells [17]. Zn is co-secreted along with insulin and is involved in the paracrine regulation of glucagon secretion by α-cells [18, 19]. In addition, Zn that reaches the liver through the portal circulation suppresses hepatic insulin clearance by a clathrin-dependent mechanism [20]. Studies in animal models have shown that Zn can reduce both fasting glucose and insulin in db/db mice [21], and lessen oxidative changes in the retinas of diabetic rats [22].

Among the mechanisms explaining the effects of Zn on diabetes, the seminal work by Chimienti and colleagues [3, 4] clarifying the role of Zn transporter ZnT8 was crucial. This work showed that ZnT8, a zinc transporter, is responsible for the transport of Zn into insulin granules in beta cells. In cellular and animal models, it was observed that overexpression of ZnT8 was associated with increased intracellular Zn [23], and that deletion of this Zn transporter altered insulin secretion [24]. In addition, Zn has been shown to inhibit release of inflammatory cytokines involved in β-cell destruction [25], has anti-apoptotic effects in a number of cells and tissues [26], and presents antioxidant functions that are protective to β-cells [27]. More recently, participation of Zn in cell signaling processes has been described [11], suggesting Zn insulin-mimetic roles: Zn may be involved in the activation of the insulin receptor by increasing its β subunit phosphorylation and activation of several key components of insulin pathways, which include the extracellular signal-regulated kinase 1/2 (ERK1/2) and phosphoinositide 3-kinase (PI3K)/Akt signaling pathways [26, 28••].

This biology linking Zn to glucose homeostasis has raised much interest in the possible role of Zn in the development of diabetes and as a therapeutic agent in diabetes. In this review, we synthesize the current evidence from epidemiologic and supplementation studies in humans on the association between zinc and diabetes mellitus.

Analysis Strategy

Using the Pubmed electronic database (search dates: June 2011–2016), we identified 798 articles using the string “Zinc OR Zn” AND “diabetes.” After we restricted to studies in humans with type 2 diabetes and epidemiologic studies and clinical trials, we identified 26 publications for this review. We synthesized the evidence from epidemiologic (non-intervention) studies separately from Zn supplementation trials. Of the epidemiologic studies, we discuss studies evaluating zinc status (i.e., measured zinc levels) separate from those evaluating zinc intake. We included additional key/landmark studies outside of the search dates based on expert opinion to provide additional support to available evidence.

Non-Supplementation Studies in Type 2 Diabetes

Twelve observational studies evaluating the relationship between Zn status by a measured Zn level and T2D were analyzed (Table 1). Among these, 11 studies evaluated plasma/serum Zn concentration as Zn status parameter, 3 used urinary Zn excretion, 1 whole blood Zn, and 1 evaluated erythrocyte Zn concentration and erythrocyte superoxide dismutase (SOD) activity. It is often indicated that plasma Zn is reduced in T2D as consequence of increased urinary losses [29]. Nevertheless, available evidence analyzed here only partially supports this conclusion because of the 11 studies that evaluated plasma/serum Zn, only 6 found significantly reduced concentrations in T2D patients compared to controls, 4 did not find significant differences, and 1 reported increased levels. In terms of urinary Zn, two studies reported increased values in T2D compared to controls. In addition, a cross-sectional analysis found an association between higher urinary Zn and increased prevalence of T2D [30]. Of note, in 5 of the 12 studies, participants with diabetes had a baseline mean plasma/serum Zn in the Zn deficiency range (below 70 μg/dL or 10.7 μmol/L). We identified several issues with quality in these studies: For instance, one of the studies that reported significant differences between groups did not provide the units of plasma/serum Zn used [31], and in another study, authors used whole blood Zn, which is not an appropriate index of Zn status [32].

Table 1 Summary of observational studies on the relationship between zinc status/intake and type 2 diabetes (T2D) and related traits in the last 5 years
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Few studies have evaluated the association between Zn status and insulin action (e.g., HOMA-IR) and/or glucose homeostasis (e.g., fasting glucose). Yerlikaya [33•] in Turkey evaluated the relationship between plasma/serum Zn values and insulin resistance by HOMA-IR in non-diabetic obese and T2D obese subjects and found an inverse association between Zn status and insulin resistance in both groups, although such association was stronger in diabetic individuals. A similar result was found by Islam [34] in a group of glucose-intolerant patients. Vashum [35] reported a positive association between Zn status and insulin sensitivity in persons with prediabetes. In terms of glucose homeostasis, as shown in Table 1, only two studies provided information in this regard [33•, 36].

Gender may be a confounding variable when evaluating Zn status and some metabolic outcomes (e.g., positive correlation between serum Zn and insulin in men but not in women) [37, 38]. The relevance of gender in the context of the association between Zn and T2D has not been evaluated. Among the studies in Table 1, only three considered this potential confounder [30, 32, 39].

Similarly, another relevant biologic variable that is not always considered in the designs is body weight/composition (e.g., BMI). Among the studies evaluated here, it is worthy to mention the findings of Yerlikaya et al. [33•], which showed no differences in plasma/serum Zn between obese subjects with and without diabetes, but both obese groups showed reduced plasma Zn relative to non-obese non-diabetic subjects. Likewise, Islam et al. [34] found a significant interaction between plasma/serum Zn and BMI for insulin resistance.

Zinc Intake and Risk of Type 2 Diabetes

Of the studies that we identified evaluating the association between Zn intake and risk of type 2 diabetes, two cohorts found an inverse association: Compared to the lowest quintile of Zn intake, the highest Zn intake quintile was associated with lower risk of type 2 diabetes in the Australian Longitudinal Study [40] and the Nurses Health Study [41]. However, in a third cohort, the Multi-Ethnic Study of Atherosclerosis (MESA) study in the USA, there was no association between dietary Zn or supplements use at baseline and risk of T2D in the results of the 10-year follow-up [42, 43].

Another factor related to risk of T2D development, currently under intense study, is the genetic component, particularly, the presence of genetic variants in SLC30A8, the gene encoding ZnT8 [44–46, 47•]. While these studies suggest an interaction between zinc status or intake and glucose homeostasis, these studies have either between cross-sectional or small studies of brief duration and are unable to address the clinical relevance of SLC30A8 variation and zinc status to diabetes risk at this time.

Zinc Supplementation Studies in Type 2 Diabetes

A valuable tool to assess translation of mechanisms associated to Zn action is response to Zn supplementation.

Prevention of Type 2 Diabetes

El Dib et al. [48] carried out a systematic review on Zn supplementation for the prevention of T2D in adults with insulin resistance. They were able to identify three trials with a total of 128 subjects. Duration of intervention was from 4 to 12 weeks. Nevertheless, none of the studies reported on key outcome measures (incidence of type 2 diabetes mellitus, adverse events, health-related quality of life, all-cause mortality, diabetic complications, and socioeconomic effects). Thus, there is no enough evidence to properly assess usefulness of such intervention as a preventive action.

Effects of Zn Supplementation among Persons with Type 2 Diabetes

During the past 5 years, the number of Zn supplementation studies carried out in diabetic populations is very limited, although two meta-analyses were available in this period. In our analysis of studies evaluating the effect of Zn supplementation on diabetes, we considered not only the recent supplementation trials but also those included in the meta-analyses.

The meta-analysis conducted by Jayawardena et al. [49] included studies in which Zn was supplemented, alone or combined with other vitamins and minerals, in patients with T2D (22 studies), and 3 studies in patients with T1D. The second meta-analysis developed by Capdor et al. [50••] reviewed nine studies on T2D, one trial in T1D, another trial with both types of diabetes, and seven trials in non-diabetic populations including metabolic syndrome patients or healthy subjects.

In the meta-analysis of Jayawardena et al. [49], 12 studies in T2D considered as outcomes the change in fasting blood glucose levels [51–62], and 8 of them also evaluated changes in HbA1C [54–56, 58–62]. Among other observed results were the effects on plasma lipids, antioxidant effects, and neuro-physiological parameters and symptoms of neuropathy. Eight of the 12 trials were randomized, double-blind and controlled [51, 52, 54–56, 58, 60, 61], 3 studies were single-blind [57, 59, 62], and 1 was a case–control study [53]. Intervention periods in these 12 studies, ranged from 3 weeks to 6 months, with supplementation doses of elemental Zn between 20 and 240 mg/day. In seven of these trials, with a mean time of intervention of 2.5 months, a significant reduction of fasting glucose concentration (FG) compared with the control group was observed [51–54, 58, 59, 62]. In the other five trials, with a mean of 3.9 months of intervention, no significant effect of supplementation was detected [55–57, 60, 61]. In studies with positive results of Zn supplementation, cumulative Zn dose was 4170 mg per patient, and in studies with negative results was 4463 mg per patient.

Changes in HbA1C were analyzed only in studies with at least 3 months of intervention, four of them had a significant reduction in the Zn arm [54, 58, 59, 62] and the remaining four studies showing no significant change [55, 56, 60, 61]. The cumulative Zn doses were 4059 and 3125 mg per patient, respectively.

In the meta-analysis by Capdor et al. [50••], most of the studies in T2D considered by Jayawardena et al. were also included, and they added the study by Oh et al. [63], which was a 4-week intervention with 50 mg of Zn, in which a reduction in fasting glucose and increase in C peptide was observed only in diabetic patients with Zn deficiency. Another study, added in the meta-analysis by Capdor et al., conducted by Kajanachumpol et al. [64], was a randomized, controlled trial in 25 T2D patients with Zn deficiency, which showed a significant reduction in fasting glucose after 12 weeks of intervention.

More recently, Foster et al. [65] published the results of a controlled trial in 43 postmenopausal T2D women without Zn deficiency, observing no significant effects on glucose metabolism after 12 weeks of intervention with 40 mg of Zn alone or associated with alpha-linolenic acid.

It is intriguing that despite diabetes is a chronic condition, Zn supplementation studies have been rather short-term duration, of just a few months. Our group in Chile is finishing a 2-year Zn supplementation trial in T2D individuals; results should be available shortly.

Examining changes in fasting blood glucose levels, and/or HbA1C, beyond the point of view of Zn supplemented/dose, but also considering initial Zn status, 12 of 15 trials available provided such information (Table 2). In five studies, which observed metabolic improvement, a significant proportion of T2D patients had Zn deficiency at the beginning of intervention [51, 52, 59, 62, 64]. Instead, six trials in which there was no improvement in fasting blood glucose levels or HbA1C were conducted in patients with T2D with absence of Zn deficiency [55–57, 60, 61, 65]. The case of the study of Oh et al. [63] is illustrative, showing metabolic improvement only in the subgroup of T2D patients with Zn deficiency.

Table 2 Summary of glycemic control outcomes and presence of zinc deficiency at the beginning of zinc supplementation
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Conclusions

Despite the host of mechanisms through which Zn may have beneficial effects on prevention and/or therapy of T2D, results of human studies are not consistent. While some positive results have been reported in some, in others such effects have not been observed. Potential causes of such discrepancy are varied. Diabetes is a complex disease, with a number of factors involved in its development and evolution; therefore, elements such as stage of the disease, degree of metabolic disturbance, co-morbidities, type and duration of medication, among others, have to be considered when developing study protocols. On the Zn side, despite the limitations plasma/serum zinc may present, it is a useful tool to assess Zn status. Moreover, it was noteworthy that only supplementation trials with a significant proportion of subjects with subnormal Zn status showed positive responses in terms of glucose control outcomes. Certainly, including better Zn indices, such as the size of the rapidly exchangeable Zn pool, in addition to plasma/serum Zn, would strengthen this literature. Longer supplementation periods are also needed for conclusive evidence. We hope to make soon available the results of a 2-year Zn supplementation study that recently completed its data collection phase. Also, genetic factors, such as variation in SLC30A8, should not be overlooked. In conclusion, the effect of Zn on type 2 diabetes remains an open question, and better study designs are needed to clarify the real impact and characteristics of this interaction.