The effect of iron status on vascular health

Associations Between Genetically Predicted Iron Status and Cardiovascular Disease Risk: A Mendelian Randomization Study

BACKGROUND

Mendelian randomization (MR) studies suggest a causal effect of iron status on cardiovascular disease (CVD) risk, but it is unknown if these associations are confounded by pleiotropic effects of the instrumental variables on CVD risk factors. We aimed to investigate the effect of iron status on CVD risk controlling for CVD risk factors.

METHODS AND RESULTS

Iron biomarker instrumental variables (total iron-binding capacity [n=208 422], transferrin saturation [n=198 516], serum iron [n=236 612], ferritin [n=257 953]) were selected from a European genome-wide association study meta-analysis. We performed 2-sample univariate MR of each iron trait on CVD outcomes (all-cause ischemic stroke, cardioembolic ischemic stroke, large-artery ischemic stroke, small-vessel ischemic stroke, and coronary heart disease) from MEGASTROKE (n=440 328) and CARDIoGRAMplusC4D (Coronary Artery Disease Genome Wide Replication and Meta-Analysis Plus the Coronary Artery Disease Genetics) (n=183 305). We then implemented multivariate MR conditioning on 7 CVD risk factors from independent European samples to evaluate their potential confounding or mediating effects on the observed iron-CVD associations. With univariate MR analyses, we found higher genetically predicted iron status to be associated with a greater risk of cardioembolic ischemic stroke (transferrin saturation: odds ratio, 1.17 [95% CI, 1.03-1.33]; serum iron: odds ratio, 1.21 [95% CI, 1.02-1.44]; total iron-binding capacity: odds ratio, 0.81 [95% CI, 0.69-0.94]). The detrimental effects of iron status on cardioembolic ischemic stroke risk remained unaffected when adjusting for CVD risk factors (all P<0.05). Additionally, we found diastolic blood pressure to mediate between 7.1 and 8.8% of the total effect of iron status on cardioembolic ischemic stroke incidence. Univariate MR initially suggested a protective effect of iron status on large-artery stroke and coronary heart disease, but controlling for CVD factors using multivariate MR substantially diminished these associations (all P>0.05).

CONCLUSIONS

Higher iron status was associated with a greater risk of cardioembolic ischemic stroke independent of CVD risk factors, and this effect was partly mediated by diastolic blood pressure. These findings support a role of iron status as a modifiable risk factor for cardioembolic ischemic stroke.

Associations between genetically predicted iron status and cardiovascular disease risk: A Mendelian randomization study

Background

Mendelian randomization (MR) studies suggest a causal effect of iron (Fe) status on cardiovascular disease (CVD) risk, but it is unknown if these associations are confounded by pleiotropic effects of the instrumental variables (IV) on CVD risk factors. We aimed to investigate the effect of Fe status on CVD risk controlling for CVD risk factors.

Methods

Fe biomarker IVs (total Fe binding capacity (TIBC, n=208,422), transferrin saturation (TSAT, n=198,516), serum Fe (SI, n=236,612), ferritin (n=257,953)) were selected from a European GWAS meta-analysis. We performed two-sample univariate (UV) MR of each Fe trait on CVD outcomes (all-cause ischemic stroke (IS), cardioembolic IS (CES), large artery IS (LAS), small vessel IS (SVS), and coronary heart disease (CHD)) from MEGASTROKE (n=440,328) and CARDIoGRAMplusC4D (n=183,305). We then implemented multivariate (MV) MR conditioning on six CVD risk factors from independent European samples to evaluate their potential confounding and/or mediating effects on the observed Fe-CVD associations.

Results

With UVMR analyses, we found higher genetically predicted Fe status to be associated with a greater risk of CES (TSAT: OR 1.17 [95%CI 1.03, 1.33], SI: OR 1.21 [ 95%CI 1.02, 1.44]; TIBC: OR 0.81 [95%CI 0.69, 0.94]). The detrimental effects of Fe status on CES risk remained unaffected when adjusting for CVD risk factors (all P<0.05). Additionally, we found diastolic blood pressure (DBP) to mediate between 7.1-8.8% of the total effect of Fe status on CES incidence. While UVMR initially suggested a protective effect of Fe status on LAS and CHD, MVMR analyses factoring CVD risk factors revealed a complete annulment of this perceived protective effect (all P>0.05).

Discussion

Higher Fe status was associated with a greater risk of CES independent of CVD risk factors, and this effect was partly mediated by DBP. These findings support a role of Fe status as a modifiable risk factor for CES.

Chronic Iron Overload Restrains the Benefits of Aerobic Exercise to the Vasculature

Physical exercise is a well-recognized effective non-pharmacological therapy for cardiovascular diseases. However, because iron is essential element in many physiological processes including hemoglobin and myoglobin synthesis, thereby playing a role on oxygen transport, many athletes use iron supplement to improve physical performance. Regarding this, iron overload is associated with oxidative stress and damage to various systems, including cardiovascular. Thus, we aimed to identify the vascular effects of aerobic exercise in a rat model of iron overload. Male Wistar rats were treated with 100 mg/kg/day iron-dextran, i.p., 5 days a week for 4 weeks, and then underwent aerobic exercise protocol on a treadmill at moderate intensity, 60 min/day, 5 days a week for 8 weeks. Exercise reduced vasoconstrictor response of isolated aortic rings by increasing participation of nitric oxide (NO) and reducing oxidative stress, but these benefits to the vasculature were not observed in rats previously subjected to iron overload. The reduced vasoconstriction in the exercised group was reversed by incubation with superoxide dismutase (SOD) inhibitor, suggesting that increased SOD activity by exercise was lost in iron overload rats. Iron overload groups increased serum levels of iron, transferrin saturation, and iron deposition in the liver, gastrocnemius muscle, and aorta, and the catalase was overexpressed in the aorta probably as a compensatory mechanism to the increased oxidative stress. In conclusion, despite the known beneficial effects of aerobic exercise on vasculature, our results indicate that previous iron overload impeded the anticontractile effect mediated by increased NO bioavailability and endogenous antioxidant response due to exercise protocol.

Effects of Genetically Determined Iron Status on Risk of Venous Thromboembolism and Carotid Atherosclerotic Disease: A Mendelian Randomization Study

Background Systemic iron status has been implicated in atherosclerosis and thrombosis. The aim of this study was to investigate the effect of genetically determined iron status on carotid intima-media thickness, carotid plaque, and venous thromboembolism using Mendelian randomization. Methods and Results Genetic instrumental variables for iron status were selected from a genome-wide meta-analysis of 48 972 subjects. Genetic association estimates for carotid intima-media thickness and carotid plaque were obtained using data from 71 128 and 48 434 participants, respectively, and estimates for venous thromboembolism were obtained using data from a study incorporating 7507 cases and 52 632 controls. Conventional 2-sample summary data Mendelian randomization was performed for the main analysis. Higher genetically determined iron status was associated with increased risk of venous thromboembolism. Odds ratios per SD increase in biomarker levels were 1.37 (95% CI 1.14-1.66) for serum iron, 1.25 (1.09-1.43) for transferrin saturation, 1.92 (1.28-2.88) for ferritin, and 0.76 (0.63-0.92) for serum transferrin (with higher transferrin levels representing lower iron status). In contrast, higher iron status was associated with lower risk of carotid plaque. Corresponding odds ratios were 0.85 (0.73-0.99) for serum iron and 0.89 (0.80-1.00) for transferrin saturation, with concordant trends for serum transferrin and ferritin that did not reach statistical significance. There was no Mendelian randomization evidence of an effect of iron status on carotid intima-media thickness. Conclusions These findings support previous work to suggest that higher genetically determined iron status is protective against some forms of atherosclerotic disease but increases the risk of thrombosis related to stasis of blood.

Increased serum ferritin levels are independently associated with carotid atherosclerosis in women

Previous studies have supported the theory that there is a positive association between ferritin and carotid atherosclerosis in Western people. Diet plays an important role in determining serum ferritin concentration. Asian dietary patterns are different from Western dietary patterns, implying that there may be a difference in the association of ferritin with carotid atherosclerosis between Asian and Western people. However, few studies focus on the association between ferritin and carotid atherosclerosis among Asians. The aim of this study was to investigate how serum ferritin levels are associated with carotid atherosclerosis in an Asian adult population. A cross-sectional assessment was performed in 8302 adults in Tianjin, China. Carotid intima-media thickness (IMT) and plaques were assessed using ultrasonography, and serum ferritin was measured using the protein chip-chemiluminescence method. Multiple logistic regression analysis was used to examine the association between quartiles of serum ferritin concentration and carotid atherosclerosis. In the present study, the overall prevalence of IMT and carotid plaques in participants is 29·2 and 22·7 %, respectively. In women, after adjustments for potentially confounding factors, the OR of IMT and carotid plaques by increasing serum ferritin quartiles were 1·00, 1·39 (95 % CI 0·98-1·99), 1·39 (95 % CI 0·99-1·97), 1·81 (95 % CI 1·30-2·55) (P for trend<0·001) and 1·00, 1·24 (95 % CI 0·89-1·73), 1·18 (95 % CI 0·85-1·65), 1·59 (95 % CI 1·15-2·20) (P for trend<0·01), respectively. However, no association was found between serum ferritin and carotid atherosclerosis in men. The study demonstrated that increased serum ferritin levels are independently associated with IMT and carotid plaques in Asian women but not in Asian men.

Serum ferritin levels are associated with arterial stiffness in healthy Korean adults

Although an association between serum ferritin and atherosclerosis has been suggested, limited epidemiologic data are available regarding the association between ferritin and arterial stiffness in healthy adults. A total of 2932 healthy subjects were enrolled in this study. Anthropometric and biochemical profiles including ferritin were measured. The arterial stiffness was measured using brachial–ankle pulse wave velocity (baPWV). Serum ferritin levels were classified into quartiles and baPWV values gradually increased with each ferritin quartile. Multiple regression analysis showed that ferritin levels were independently correlated with baPWV. After adjusting for multiple risk factors, as compared with the lowest quartile, the odds ratios for high baPWV (>75th percentile) were 1.15 (0.84–1.56), 1.37 (0.97–1.73), and 1.46 (1.29–2.17) among men ( p for trend < 0.05) and 1.24 (0.87–1.79), 1.53 (1.09–2.16), and 1.80 (1.25–2.82) among women ( p for trend < 0.05), for the second, third, and fourth quartiles of ferritin, respectively. In conclusion, serum ferritin levels are independently associated with arterial stiffness in healthy Korean adults.

Serum ferritin is associated with carotid atherosclerotic plaques but not intima-media Thickness in patients with abnormal glucose metabolism

BACKGROUND

We investigated the association between serum ferritin and carotid artery lesions in populations with abnormal glucose metabolism.

METHODS

We included 70 participants with abnormal glucose metabolism and 170 participants with normal glucose metabolism and measured their baseline serum ferritin levels. During follow-up carotid intima-media thickness and carotid plaque were evaluated.

RESULTS

Serum ferritin levels were higher in the participants with abnormal glucose metabolism (p<0.01). We further divided the patients with abnormal glucose metabolism into subgroups with and without intima-media proliferation, and found that ferritin was excluded from the final equation in the logistic regression. Furthermore, age, waist circumference, ferritin, 2h-PG, and total cholesterol were significantly different between the subgroups with and without carotid plaque. When the above data were included in a logistic regression model, the p values obtained for age, ferritin, and 2h-PG were 0.004, 0.032, and 0.011, respectively.

CONCLUSIONS

In the Chinese population, serum ferritin levels are significantly increased in patients with abnormal glucose metabolism. The carotid intima-media thickness showed no independent relationship with serum ferritin in patients with abnormal glucose metabolism. However, high serum ferritin is an important risk factor for carotid atherosclerosis in these patients.

The effect of dietary iron intake on the development of iron overload among homozygotes for haemochromatosis

Objective

To quantify the role of dietary Fe in total body Fe (TBI) accumulation among homozygotes for theHFEgene associated with haemochromatosis.

Design

A Monte Carlo model was built to simulate Fe accumulation based on findings from human feeding experiments and national dietary surveys. A hypothetical cohort of 1000 homozygotes with starting age 25 years was used in 39-year simulations. The impact of reducing dietary Fe intake on Fe accumulation was tested.

Results

In the baseline model without any dietary intervention, by age 64, the percentage of males with TBI > 10 g, >15 g and >20 g was 93·2 %, 49·6 % and 14·7 %, respectively. When the Fe intake of individuals in the cohort was reduced to ≤200 % of the recommended dietary allowance (RDA), the corresponding percentages were 92·0 %, 40·5 % and 10·2 %, respectively. The corresponding figures were 91·0 %, 40·0 % and 9·3 % for Fe defortification and 70·3 %, 21·3 % and 4·1 % when Fe intake was capped at 100 % RDA. Similar trends were seen with sexes combined, although the impact of interventions was less. Sensitivity analysis revealed that the rate of Fe accumulation and the impact of dietary interventions are highly dependent on assumptions concerning Fe absorption rates.

Conclusions

Variation in Fe intake as currently observed in the USA contributes to variation in Fe accumulation among homozygotes, when continued over an extended period. Lifelong dietary habits and national fortification policy can affect the rate of Fe accumulation, although the magnitude of the effect varies by gender, the TBI level of interest and factors affecting the Fe absorption rate.

Hereditary hemochromatosis: pathophysiology, diagnosis, and management

Hereditary hemochromatosis (HH) is an autosomal recessive genetic disease resulting in inappropriate intestinal iron absorption leading to iron overload and end-organ disease. The disease is most prevalent in white individuals of European descent. The C282Y mutation on the HFE gene accounts for most cases of HH; however, other genetic mutations have been identified. End-organ damage results in cirrhosis, diabetes mellitus, and cardiomyopathy. Therapeutic phlebotomy to deplete excessive iron stores is the standard treatment of HH and results in normal longevity if therapy is initiated before end-organ disease occurs.

Iron and thrombosis

Although essential for cell physiology, an increase or depletion of body iron has harmful effects on health. Apart from iron deficiency anemia and iron overload-related organ tissue damage, there are increasing evidences that body iron status is implicated in atherosclerotic cardiovascular diseases. The hypothesis formulated in 1981 that iron depletion may protect against cardiovascular events is intriguing and has generated a significant debate in the last two decades. Indeed, to study this phenomenon, several investigators have tried to design appropriate experimental and clinical studies and to identify useful biochemical and genetic markers of iron status. The results of the literature on the effect of iron deficiency and overload on vascular health are critically reviewed in this study from a pathogenic and clinical point of view.