Body iron stores and heme-iron intake in relation to risk of type 2 diabetes: a systematic review and meta-analysis

Insulin resistance modulates iron-related proteins in adipose tissue

OBJECTIVE Circulating markers of iron overload are associated with insulin resistance. Less is known about the impact of iron overload on adipose tissue (AT). We hypothesized that gene expression markers of iron metabolism in AT could be associated with insulin action. RESEARCH DESIGN AND METHODS The AT expression of ferroportin (SLC40A1), transferrin (TF), TF receptor (TFRC), ferritin (FT) heavy polypeptide 1 (FTH1), and FT light polypeptide (FTL) was analyzed cross-sectionally in three independent cohorts and also after weight loss-induced changes in insulin sensitivity (clamp M value) in an independent fourth cohort. RESULTS In human AT, TF mRNA and protein levels were decreased with obesity and insulin resistance in the three cohorts and were positively associated with adipogenic mRNAs and insulin action. Otherwise, FTL mRNA and protein and SLC40A1 transcripts were positively associated with BMI and negatively linked to adipogenic genes and insulin action. Bariatric surgery-induced weight loss led to increased TF and decreased TFRC, FTH1, FTL, and SLC40A1 in subcutaneous AT in parallel to improved insulin action. CONCLUSIONS These results suggest that iron overload impacts on AT in association with insulin resistance.

Heme, an essential nutrient from dietary proteins, critically impacts diverse physiological and pathological processes

Heme constitutes 95% of functional iron in the human body, as well as two-thirds of the average person’s iron intake in developed countries. Hence, a wide range of epidemiological studies have focused on examining the association of dietary heme intake, mainly from red meat, with the risks of common diseases. High heme intake is associated with increased risk of several cancers, including colorectal cancer, pancreatic cancer and lung cancer. Likewise, the evidence for increased risks of type-2 diabetes and coronary heart disease associated with high heme intake is compelling. Furthermore, recent comparative metabolic and molecular studies of lung cancer cells showed that cancer cells require increased intracellular heme biosynthesis and uptake to meet the increased demand for oxygen-utilizing hemoproteins. Increased levels of hemoproteins in turn lead to intensified oxygen consumption and cellular energy generation, thereby fueling cancer cell progression. Together, both epidemiological and molecular studies support the idea that heme positively impacts cancer progression. However, it is also worth noting that heme deficiency can cause serious diseases in humans, such as anemia, porphyrias, and Alzheimer’s disease. This review attempts to summarize the latest literature in understanding the role of dietary heme intake and heme function in diverse diseases.

Associations between red meat intake and biomarkers of inflammation and glucose metabolism in women

BACKGROUND

Greater red meat intake is associated with an increased type 2 diabetes and cardiovascular disease risk. However, the relation of red meat intake to biomarkers of inflammation and glucose metabolism has not been investigated thoroughly.

OBJECTIVE

We hypothesized that greater red meat intake would be associated with biomarkers of inflammation and glucose metabolism, which would be partly explained by body mass index (BMI).

DESIGN

We analyzed cross-sectional data from diabetes-free female participants in the Nurses' Health Study (n = 3690). Multiple linear regression was conducted to assess the associations of total, unprocessed, and processed red meat intakes (quartile categories) with plasma C-reactive protein (CRP), ferritin, adiponectin, fasting insulin, and hemoglobin A1c (Hb A1c).

RESULTS

Greater total, unprocessed, and processed red meat intakes were associated with higher plasma CRP, ferritin, fasting insulin, and Hb A1c and lower adiponectin after adjustment for demographic information (P-trend ≤ 0.03 for all). Adiponectin was not associated with any type of red meat intake when further adjusted for medical and lifestyle factors. After adjustment for BMI, most of these associations with inflammatory and glucose metabolic biomarkers were substantially attenuated and no longer significant. BMI accounted for a statistically significant proportion of associations with CRP, Hb A1c, and fasting insulin (P-contribution ≤ 0.02 for all) but not with ferritin. Substituting a serving of total red meat intake with alternative protein food in a combination of poultry, fish, legumes, and nuts was associated with significantly lower CRP (β ± SE: -0.106 ± 0.043), ferritin (-0.212 ± 0.075), Hb A1c (-0.052 ± 0.015), and fasting insulin (-0.119 ± 0.036) (all P ≤ 0.02 for comparison of extreme quartiles for all).

CONCLUSIONS

Greater red meat intake is associated with unfavorable plasma concentrations of inflammatory and glucose metabolic biomarkers in diabetes-free women. BMI accounts for a significant proportion of the associations with these biomarkers, except for ferritin. Substituting red meat with another protein food is associated with a healthier biomarker profile of inflammatory and glucose metabolism.

Diabetes and hemochromatosis

The common form of hereditary hemochromatosis is an autosomal recessive disorder most prevalent in Caucasians that results in excessive iron storage. The clinical manifestations of hemochromatosis are protean. HFE genotype, which determines the degree of iron overload and duration of disease have profound effects on disease expression. The prevalence of diabetes in this population has likely been underestimated because of studies that include a broad range of ethnicities and associating diabetes with allele frequency in spite of the decreased risk of diabetes in heterozygotes compared with homozygotes. Loss of insulin secretory capacity is likely the primary defect contributing to development of diabetes with insulin resistance playing a secondary role. Phlebotomy can ameliorate the defects in insulin secretion if initiated early. Screening a select population of individuals with type 2 diabetes may identify patients with hemochromatosis early and substantially impact individual clinical outcomes.

Is oxidative stress induced by iron status associated with gestational diabetes mellitus?

Gestational diabetes mellitus (GDM) is a common pregnancy complication in high risk populations, and is associated with increased perinatal and long term outcomes for both mothers and newborns. Both its prevention and early management can be reinforced by identifying risks factors, particularly those factors influencing glucose metabolism. On the other hand, several epidemiological studies have shown an increased oxidative stress (OS) in pregnant women with GDM. Elevated OS was also reported in pregnant women supplemented with iron, which can generate OS and may also influence insulin resistance. This review summarizes the current state of knowledge, highlighting the potential relationship between OS induced by iron status and the development of GDM.

Heme iron intake and risk of new-onset diabetes in a Mediterranean population at high risk of cardiovascular disease: an observational cohort analysis

BACKGROUND

Several epidemiological studies have observed an increased risk of type 2 diabetes mellitus (T2DM) among subjects with a higher consumption of red and processed meat. Heme iron intake has been directly associated with a higher risk of T2DM in healthy adult Chinese and U.S populations. The objective of the present study was to evaluate the association between heme iron intake and the incidence of T2DM in a Mediterranean population at high cardiovascular risk.

METHODS

We assessed a subset of participants in the PREDIMED trial as an observational cohort, followed up for a maximum of eight years. We initially included 1073 non-diabetic subjects (57.1% women) aged 67.3 ± 6.0 years, at high cardiovascular risk. Diet was assessed at the study baseline using a validated, semi-quantitative food frequency questionnaire.

RESULTS

During the follow-up period 131 diabetics were newly diagnosed. The risk of developing T2DM was assessed using baseline heme iron intake and proportional hazard models, first unadjusted, then adjusted for energy, and finally adjusted for dietary, anthropometric, socio-demographic and lifestyle variables. Significant direct associations with the incidence of T2DM were found for heme iron (Hazard Ratio [HR] 1.30, 95% confidence interval [CI], 1.02 to 1.66). Secondarily, we have also observed that coffee (HR:0.93, 95% CI, 0.89 to 0.98) and alcoholic beverages (HR: 1.02, 95% CI, 1.01 to 1.04) were also found to reduce and increase the risk of T2DM, respectively.

CONCLUSION

High dietary intake of heme iron was associated with an increased risk of developing T2DM in a Mediterranean population at high cardiovascular risk.

TRIAL REGISTRATION

Identifier: ISRCTN35739639.

When less is more: novel mechanisms of iron conservation

Disorders of iron homeostasis are very common, yet the molecular mechanisms of iron regulation remain understudied. Over 20 years have passed since the first characterization of iron-regulatory proteins (IRP) as mediators of cellular iron-deficiency response in mammals through iron acquisition. However, little is known about other mechanisms necessary for adaptation to low-iron states. In this review, we present recent evidence that establishes the existence of a new iron-regulatory pathway aimed at iron conservation and optimization of iron use through suppression of nonessential iron-consuming processes. Moreover, we discuss the possible links between iron homeostasis and energy metabolism uncovered by studies of iron-deficiency response.

Elevated serum ferritin level is associated with the incident type 2 diabetes in healthy Korean men: a 4 year longitudinal study

Background

Elevated ferritin concentration has been implicated in the etiology of type 2 diabetes. Accumulating evidence, mostly from studies conducted on western populations, has demonstrated a strong association between the elevated ferritin concentrations and incident type 2 diabetes. In Asian populations, however, the longitudinal studies investigating the association of elevated serum ferritin levels and type 2 diabetes are lacking. In present study, we aimed to determine whether elevated serum ferritin levels are related to the incident type 2 diabetes in healthy Korean men.

Methodology/Principal Findings

This 4 year longitudinal observational study was conducted at the Asan Medical Center, Seoul, Republic of Korea. The study population consisted of 2,029 men without type 2 diabetes who underwent routine health examination in 2007 (baseline) and 2011 (follow-up). Baseline serum ferritin concentrations were measured by chemiluminescent two-site sandwich immunoassay. In multiple-adjusted model, the relative risk (RR) for incident type 2 diabetes was significantly higher in highest compared with the lowest ferritin quartile category, even after adjusting for confounding variables including homeostasis model assessment of insulin resistance (RR = 2.17, 95% confidence interval 1.27–3.72, P for trend = 0.013).

Conclusions/Significance

These results demonstrated that elevated level of serum ferritin at baseline was associated with incident type 2 diabetes in an Asian population.

Systematic review of clinical studies related to pork intake and metabolic syndrome or its components

Globally, both the incidence of type 2 diabetes and the consumption of meat, in particular pork meat, have increased, concurrently. Processed meats have been associated with an increased risk for diabetes in observational studies. Therefore, it is important to understand the possible mechanisms of this association and the impact of meats from different species. The goal of this systematic review was to assess experimental human studies of the impact of pork intake compared with other protein sources on early markers for the development of diabetes, ie, insulin resistance, glucose intolerance, and the components of the metabolic syndrome. A systematic review was conducted searching PubMed and EMBASE and using the Cochrane and PRISMA guidelines. Eight studies were eligible and critically reviewed. Five studies were based on a single meal or single day exposure to pork, as compared with other sources of protein. The glucose-insulin response following the pork meals did not differ compared with beef, shrimp, or mixed sources of proteins. However, compared with eggs, ham (processed meat) led to a larger insulin response in nonobese subjects. Compared with whey, ham led to a smaller insulin response and a larger glucose response. These findings suggest possible mechanisms for the association between processed meat and the development of diabetes. Nonprocessed pork meats were not compared with eggs or whey. The three longer interventions (11 days to 6 months) did not show a significant impact of pork on the components of the metabolic syndrome, with the exception of a possible benefit on waist circumference and high-density lipoprotein cholesterol (one study each with significant limitations). Most of the findings are weak and there is a lack of solid evidence. The literature on the topic is limited and important research gaps are identified. Considering recent trends and projections for diabetes and pork intake, this is an important global public health question that requires more attention in order to provide improved evidence-based dietary recommendations.

Elevated plasma ferritin is associated with increased incidence of type 2 diabetes in middle-aged and elderly Chinese adults

Epidemiological studies suggest that elevated circulating ferritin is associated with heightened incident diabetes in mainly Western populations, although the results were not entirely consistent. We aimed to prospectively investigate the ferritin-diabetes association in an Asian population for the first time, to our knowledge, and also to examine this association with an updated meta-analysis. Our prospective study included 2198 community-living Chinese between 50 and 70 y of age in 2005. All individuals participated in a 6-y follow-up survey in 2011. Fasting plasma ferritin, high-sensitivity C-reactive protein (hsCRP), adiponectin, and γ-glutamyltransferase (GGT) were measured at baseline. A total of 538 incident diabetes cases were documented by self-reports and/or fasting glucose ≥7.0 mmol/L at the follow-up survey. After multiple adjustments, the RR of type 2 diabetes was 1.90 (95% CI: 1.37, 2.65) when comparing the highest with the lowest sex-specific ferritin quintile. The association remained significant after further controlling for BMI, hsCRP, adiponectin, and GGT. To update the evidence reported in previous meta-analyses, we searched all prospective studies evaluating the association between blood ferritin and incident diabetes on PubMed prior to October 24, 2012. Besides our prospective study, 9 additional studies were also included. The pooled RR was 1.60 (95% CI: 1.25, 2.04) when comparing the highest with the lowest category of ferritin with a moderate heterogeneity (I(2) = 49.0%; P = 0.03). A significant linear dose-response relationship was detected in this meta-analysis. Overall, our results indicate an independent and significant positive association between higher plasma ferritin, a marker of elevated body iron stores, and increased risk of developing type 2 diabetes in middle-aged and elderly Chinese adults, which is similar to Western populations.

Red meat, dietary heme iron, and risk of type 2 diabetes: the involvement of advanced lipoxidation endproducts

There is growing evidence of disordered iron homeostasis in the diabetic condition, with links proposed between dietary iron intakes and both the risk of disease and the risk of complications of advanced disease. In the United States, Britain, and Canada, the largest dietary contributors of iron are cereals and cereal products and meat and meat products. This review discusses the findings of cohort studies and meta-analyses of heme iron and red meat intakes and the risk of type 2 diabetes. These suggest that processed red meat is associated with increased risk, with high intakes of red meat possibly also associated with a small increased risk. Historically, humans have relied on large quantities of heme iron and red meat in their diets, and therefore it is paradoxical that iron from meat sources should be associated with the risk of type 2 diabetes. A reason for this association may be drawn from studies of dietary advanced glycation and lipoxidation endproducts present in processed food and the mechanisms by which insulin output by pancreatic islet cells might be influenced by the protein modifications present in processed red meat.

Processing of meats and cardiovascular risk: time to focus on preservatives

Dietary guidelines emphasize selecting lean (low-fat) meats to reduce saturated fat and cholesterol, but growing evidence suggests that health effects may relate to other ingredients, such as sodium, heme iron, or L-carnitine. Understanding how meats influence health, and on which nutrients this relationship depends, is essential to advise consumer choices, set guidelines, and inform food reformulations. A recent study published in BMC Medicine involving 448,568 participants in 10 European countries, provides important evidence in this regard. After multivariate adjustment, intake of unprocessed red meat was not significantly associated with total or cause-specific mortality; conversely, intake of processed meat was associated with a 30% higher rate of cardiovascular disease (CVD) (per 50 g/day, relative risk 1.30, 95% confidence interval 1.17 to 1.45) and also higher cancer mortality. These findings are consistent with our previous meta-analysis, based on smaller studies, showing strong associations of processed meats, but not unprocessed meats, with CVD. Preservatives are the notable difference; the calculated blood-pressure effects of sodium differences (around 400% higher in processed meats) explain most of the observed higher risk. Although unprocessed red meats seem to be relatively neutral for CVD, healthier choices are available, including fish, nuts, legumes, fruits, and vegetables. Public-health guidance should prioritize avoidance of processed meats, including the low-fat deli meats currently marketed as healthy choices, and the food industry should substantially reduce sodium and other preservatives in processed meats.See related research article here http://www.biomedcentral.com/1741-7015/11/63.

Unprocessed red and processed meats and risk of coronary artery disease and type 2 diabetes--an updated review of the evidence

Growing evidence suggests that effects of red meat consumption on coronary heart disease (CHD) and type 2 diabetes could vary depending on processing. We reviewed the evidence for effects of unprocessed (fresh/frozen) red and processed (using sodium/other preservatives) meat consumption on CHD and diabetes. In meta-analyses of prospective cohorts, higher risk of CHD is seen with processed meat consumption (RR per 50 g: 1.42, 95 %CI = 1.07-1.89), but a smaller increase or no risk is seen with unprocessed meat consumption. Differences in sodium content (~400 % higher in processed meat) appear to account for about two-thirds of this risk difference. In similar analyses, both unprocessed red and processed meat consumption are associated with incident diabetes, with higher risk per g of processed (RR per 50 g: 1.51, 95 %CI = 1.25-1.83) versus unprocessed (RR per 100 g: 1.19, 95 % CI = 1.04-1.37) meats. Contents of heme iron and dietary cholesterol may partly account for these associations. The overall findings suggest that neither unprocessed red nor processed meat consumption is beneficial for cardiometabolic health, and that clinical and public health guidance should especially prioritize reducing processed meat consumption.

Ferritin levels and risk of type 2 diabetes mellitus: an updated systematic review and meta-analysis of prospective evidence

BACKGROUND

Emerging evidence suggests that a strong link that exists between elevated baseline body iron stores and high risk of incident type 2 diabetes mellitus (T2DM) in general populations, but the precise magnitude of the associations remains uncertain.

METHODS

We conducted a systematic review and meta-analysis of published prospective studies evaluating the associations of baseline ferritin (a biomarker of body iron stores) levels with risk of T2DM in general populations. A subsidiary review of dietary heme iron status and T2DM risk associations was also conducted. Studies were identified in a literature search of PubMed, EMBASE, and Web of Science up to October 2012.

RESULTS

Of the 730 studies reviewed for eligibility, 12 published studies involving 185 462 participants and 11 079 incident T2DM events were included in the analyses. Nine studies assessed associations between ferritin levels and T2DM with heterogeneous findings (I(2)  = 58%, 12-80%, p = 0.014). The pooled fully adjusted relative risk (RR) with (95% confidence interval) for T2DM was 1.73 (1.35-2.22) in a comparison of extreme fifths of baseline ferritin levels. Three studies evaluated associations between dietary heme iron status and T2DM with a corresponding RR for T2DM of 1.28 (1.16-1.41). In dose-response analyses, the pooled RRs for an increment of 5 ng/mL in ferritin levels and 5 mg/day in dietary heme iron were, respectively, 1.01 (0.99-1.02) and 3.24 (2.05-5.10).

CONCLUSION

Elevated levels of ferritin may help identify individuals at high risk of T2DM. Further research is warranted to establish causality of these associations and to ascertain which patients are likely to benefit from lifestyle or therapeutic interventions.

Intakes of heme iron and zinc and colorectal cancer incidence: a meta-analysis of prospective studies

BACKGROUND

Epidemiologic findings concerning the associations between intakes of heme iron and zinc and colorectal cancer (CRC) incidence yielded conflicting results. We aimed to investigate the associations by performing a meta-analysis of prospective studies.

METHODS

We conducted a literature search on PubMed and EMBASE databases up to December 2012 to identify the prospective studies that investigated the relationships between heme iron or zinc intake and risk of CRC. We also reviewed the bibliographies of the retrieved articles to identify additional studies. We used a random-effects model to calculate the summary relative risks (RRs) with 95 % confidence intervals (CIs).

RESULTS

Eight studies on heme iron intake and six studies on zinc intake met the inclusion criteria. The summary RR of CRC for the highest versus the lowest intake was 1.14 (95 % CI = 1.04-1.24) for heme iron and 0.83 (95 % CI = 0.72-0.94) for zinc, respectively. The observed associations were not significantly modified by subsites within the colorectum, sex, geographic area, study duration, the number of cases, or the range of intakes. In the dose-response analyses, the summary RR of CRC was 1.11 (95 % CI = 1.03-1.18) for heme iron intake of 1 mg/day, and 0.86 (95 % CI = 0.78-0.96) for zinc intake of 5 mg/day, respectively. There was little evidence of publication bias.

CONCLUSION

This meta-analysis suggests a significant positive dose-response association of heme iron intake and a significant inverse dose-response association of zinc intake with risk of CRC.

Meat consumption, diabetes, and its complications

Several prospective studies have reported that risk of type 2 diabetes (T2DM) is elevated in meat consumers, especially when processed meats are consumed. Elevated risks of coronary heart disease (CHD) and stroke in meat consumers have also been reported. In this overview, the evidence regarding meat consumption and the risk of diabetes, both type 1 diabetes (T1DM) and T2DM and their macro- and microvascular complications, is reviewed. For T2DM, we performed a new meta-analysis including publications up to October 2012. For T1DM, only a few studies have reported increased risks for meat consumers or for high intake of saturated fatty acids and nitrates and nitrites. For T2DM, CHD, and stroke, the evidence is strongest. Per 100 g of total meat, the pooled relative risk (RR) for T2DM is 1.15 (95 % CI 1.07-1.24), for (unprocessed) red meat 1.13 (95 % CI 1.03-1.23), and for poultry 1.04 (95 % CI 0.99-1.33); per 50 g of processed meat, the pooled RR is 1.32 (95 % CI 1.19-1.48). Hence, the strongest association regarding T2DM is observed for processed (red) meat. A similar observation has been made for CHD. For stroke, however, a recent meta-analysis shows moderately elevated risks for meat consumers, for processed as well as for fresh meats. For the microvascular complications of diabetes, few prospective data were available, but suggestions for elevated risks can be derived from findings on hyperglycemia and hypertension. The results are discussed in the light of the typical nutrients and other compounds present in meat--that is, saturated and trans fatty acids, dietary cholesterol, protein and amino acids, heme-iron, sodium, nitrites and nitrosamines, and advanced glycation end products. In light of these findings, a diet moderate to low in red meat, unprocessed and lean, and prepared at moderate temperatures is probably the best choice from the public health point of view.