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.

Erythroferrone contributes to iron mobilization for embryo erythropoiesis in iron-deficient mouse pregnancies

Erythroferrone (ERFE) is an erythroblast-secreted regulator of iron metabolism. The production of ERFE increases during stress erythropoiesis, leading to decreased hepcidin expression and mobilization of iron. Pregnancy requires a substantial increase in iron availability to sustain maternal erythropoietic expansion and fetal development and is commonly affected by iron deficiency. To define the role of ERFE during iron-replete or iron-deficient pregnancy, we utilized mouse models expressing a range of ERFE levels: transgenic (TG) mice overexpressing ERFE, wild-type (WT), and ERFE knockout (KO) mice. We altered maternal iron status using diets with low or standard iron content and performed the analysis at E18.5. Iron deficiency increased maternal ERFE in WT pregnancy. Comparing different maternal genotypes, ERFE TG dams had lower hepcidin relative to their liver iron load but similar hematological parameters to WT dams on either diet. In ERFE KO dams, most hematologic and iron parameters were comparable to WT, but mean corpuscular volume (MCV) was decreased under both iron conditions. Similar to dams, TG embryos had lower hepcidin on both diets, but their hematologic parameters did not differ from those of WT embryos. ERFE KO embryos had lower MCV than WT embryos on both diets. The effect was exacerbated under iron-deficient conditions where ERFE KO embryos had higher hepcidin, lower Hb and Hct, and lower brain iron concentration compared to WT embryos, indicative of iron restriction. Thus, under iron-deficient conditions, maternal and embryo ERFE facilitate iron mobilization for embryonic erythropoiesis.

A social media intervention to improve nutrition knowledge and behaviors of low income, pregnant adolescents and adult women

Pregnant adolescents are at increased risk of adverse pregnancy outcomes compared to adult women, necessitating a need for early and comprehensive health care. This study aimed to evaluate the effectiveness of a social media intervention (i.e. weekly prenatal health messages) on improving diet quality, and health beliefs and knowledge. Participants (10 adolescents and 12 adults) completed pre-post intervention interviews, nutrition knowledge and health belief questionnaires, and 24-hour diet recalls. Participants entering pregnancy as overweight or obese were more likely to experience excessive GWG during the intervention. The adults had greater participation during the study despite high levels of social media access among both groups. Participants were able to identify sugar-sweetened foods and acknowledged the benefits of whole grains; however, overall knowledge of MyPlate Guidelines was limited. Social media-based education was well received by participants but did not result in large changes in dietary intake and knowledge. Although larger studies are needed, social media appears to have the potential to reach high-risk women.

Gestational iron deficiency is associated with pica behaviors in adolescents

A relation between pica (the craving and purposive consumption of nonfood items) during pregnancy and anemia is observed frequently. However, few studies related pica behaviors to biomarkers of iron status, and little is known about pica prevalence in U.S. pregnant adolescents. To address this, we undertook a longitudinal study examining iron status and pica behaviors among a group of 158 pregnant adolescents (aged ≤18 y). Approximately two-thirds of the participants were African American and 25% were Hispanic. Maternal iron status indicators [hemoglobin, soluble transferrin receptor, serum ferritin (SF), total body iron (TBI), and serum hepcidin] were assessed during pregnancy (18.5-37.3 wk) and at delivery. Pica behavior was assessed up to 3 times across gestation. Among the 158 adolescents, 46% reported engaging in pica behavior. Substances ingested included ice (37%), starches (8%), powders (4%), and soap (3%). During pregnancy, mean SF [geometric mean: 13.6 μg/L (95% CI: 11.0, 17.0 μg/L)], TBI (mean ± SD: 2.5 ± 4.2 mg/kg), and hepcidin [geometric mean: 19.1 μg/L (95% CI: 16.3, 22.2 μg/L)] concentrations were significantly lower (P < 0.05) in the pica group (n = 72) than values observed among the non-pica group [SF, geometric mean: 21.1 μg/L (95% CI: 18.0, 25.0 μg/L); TBI, mean ± SD: 4.3 ± 3.5 mg/kg; hepcidin, geometric mean: 27.1 μg/L (95%: 23.1, 32.1 μg/L); n = 86]. Although additional studies must address the etiology of these relations, this practice should be screened for, given its association with low iron status and because many of the substances ingested may be harmful. This trial was registered at clinicaltrials.gov as NCT01019902.

Nutrient Inadequacy Is Prevalent in Pregnant Adolescents, and Prenatal Supplement Use May Not Fully Compensate for Dietary Deficiencies

A longitudinal study was undertaken in 156 pregnant adolescents (≤18 years old) to characterize dietary intake and to determine the degree to which prenatal supplement use compensates for dietary deficits. The adequacy of dietary intake was assessed by comparing self-reported intake from up to three 24-hour dietary recalls with the dietary reference intakes. The majority of teens did not meet the estimated average requirements (EAR) for vitamin D (93%), vitamin E (94%), Mg (90%), Fe (76%), and Ca (74%). More than half of the adolescents in each gestational window (<23 weeks; 23-30 weeks; and ≥31 weeks of gestation) self-reported daily use of prenatal supplements, but the additional supplement contributions were not sufficient to meet the EAR for Mg (90%) or Ca (54%). Pregnant adolescents are at risk for insufficient intake of several essential nutrients from diet alone in spite of adequate or excessive energy intakes. Daily use of prenatal supplements reduces the prevalence of dietary inadequacy for many nutrients but may not be sufficient to meet the requirements for Mg and Ca. Practitioners should identify motivators and barriers to adequate diet and prenatal supplement use in order to address key nutrients of concern.

Supplementing a low-protein diet with dibasic amino acids increases urinary calcium excretion in young women

Increasing dietary protein within a physiologic range stimulates intestinal calcium absorption, but it is not known if specific amino acids or dietary protein as a whole are responsible for this effect. Therefore, we selectively supplemented a low-protein (0.7 g/kg) diet with either the calcium-sensing receptor-activating amino acids (CaSR-AAAs) L-tryptophan, L-phenylalanine, and L-histidine, or the dibasic amino acids (DAAs) L-arginine and L-lysine, to achieve intakes comparable to the content of a high-protein diet (2.1 g/kg) and measured intestinal calcium absorption. Fourteen young women took part in a placebo-controlled, double-blind, crossover feeding trial in which each participant ingested a 6-d low-protein diet supplemented with CaSR-AAAs, DAAs, or methylcellulose capsules (control) after an 11-d adjustment period. All participants ingested all 3 diets in random order. Intestinal calcium absorption was measured between days 5 and 6 using dual-stable calcium isotopes ((42)Ca, (43)Ca, and (44)Ca). There was no difference in calcium absorption between the diet supplemented with CaSR-AAAs (22.9 ± 2.0%) and the control diet (22.3 ± 1.4%) (P = 0.64). However, calcium absorption tended to be greater during the DAA supplementation period (25.2 ± 1.4%) compared with the control diet period (22.3 ± 1.4%) (P < 0.10). Larger and longer clinical trials are needed to clarify the possible benefit of arginine and lysine on calcium absorption.

Maternal hepcidin is associated with placental transfer of iron derived from dietary heme and nonheme sources

The determinants of placental transport of dietary iron remain largely uncharacterized. The objective of this research was to elucidate determinants of fetal Fe transfer from maternally ingested dietary heme and non-heme Fe. The study was undertaken in 19 pregnant females (16-32 y) who ingested intrinsically labeled (58)Fe-heme and a nonheme Fe source ((57)FeSO(4)) during the third trimester of pregnancy. At delivery, maternal and cord blood was obtained to assess neonatal (57)Fe and (58)Fe enrichment as a function of maternal/neonatal Fe status [serum ferritin (SF), transferrin receptor, hemoglobin (Hb), total body Fe, and hepcidin]. There was a greater percentage of maternally absorbed (58)Fe tracer present in the neonates compared to the (57)Fe tracer (5.4 ± 2.4 vs. 4.0 ± 1.6; P < 0.0001). Net dietary nonheme Fe (mg) and heme Fe (mg) transferred to the fetus were both inversely correlated with measures of maternal serum hepcidin (P = 0.002, r(2) = 0.43; P = 0.004, r(2) = 0.39) and SF (P = 0.0008, r(2) = 0.49; P = 0.003, r(2) = 0.41) and directly associated with neonatal Hb (P = 0.004, r(2) = 0.39; P = 0.008, r(2) = 0.35). The results of this study suggest that during pregnancy there appears to be preferential fetal use of maternally ingested Fe derived from a dietary, animal-based heme source compared to Fe ingested as ferrous sulfate. Maternal serum hepcidin and maternal/neonatal Fe status may play a role in placental uptake of dietary heme and nonheme Fe.

Utilization of iron from an animal-based iron source is greater than that of ferrous sulfate in pregnant and nonpregnant women

Heme iron absorption during pregnancy and the role of hepcidin in regulating dietary heme iron absorption remains largely unexplored. The objective of this research was to examine relative differences in heme (animal based) and nonheme (ferrous sulfate) iron utilization. This study was undertaken in 18 pregnant (ages 16-32 y; wk 32-35 of gestation) and 11 nonpregnant women (ages 18-27 y). Women were randomly assigned to receive both an animal-based heme meal (intrinsically labeled (58)Fe pork) and labeled ferrous sulfate ((57)Fe) fed on alternate days. Blood samples obtained 2 wk postdosing were used to assess iron status indicators and serum hepcidin and iron utilization based on RBC incorporation of iron isotopes. Heme iron utilization was significantly greater than nonheme iron utilization in the pregnant (47.7 ± 14.4 vs. 40.4 ± 13.2%) and nonpregnant women (50.1 ± 14.8 vs. 15.3 ± 9.7%). Among pregnant women, utilization of nonheme iron was associated with iron status, as assessed by the serum transferrin receptor concentration (P = 0.003; r(2) = 0.43). In contrast, heme iron utilization was not influenced by maternal iron status. In the group as a whole, women with undetectable serum hepcidin had greater nonheme iron utilization compared with women with detectable serum hepcidin (P = 0.02; n = 29); however, there were no significant differences in heme iron utilization. Our study suggests that iron utilization from an animal-based food provides a highly bioavailable source of dietary iron for pregnant and nonpregnant women that is not as sensitive to hepcidin concentrations or iron stores compared with ferrous sulfate.