Effect of Taking a Single Tablet of Iron on Insulin Secretion

Overview

Oral supplementation with highly bioavailable forms of iron, such as ferrous sulphate, is the treatment of choice for iron-deficiency anemia. Iron from ferrous sulphate is efficiently absorbed in the duodenum, resulting in a rapid increase in transferrin saturation and appearance of "free iron" or non-transferrin bound iron (NTBI) in blood. NTBI is highly reactive and can catalyze the generation of reactive oxygen species and cause oxidative tissue damage. Human pancreatic beta cells are known to express ZIP14, a transporter that has been implicated in uptake of NTBI from blood. In vitro and animal studies have shown that iron loading in beta cells can result in impaired insulin secretion. However, there are no human studies that have looked at the acute effects of oral iron intake on insulin secretion. In this study, we plan to look at the effect of a single oral dose of ferrous sulphate on insulin secretion kinetics in healthy individuals. A single arm before-and-after (pre-post) study design will be used. Consenting individuals who meet the participation criteria will undergo a 75g oral glucose tolerance test (OGTT) to document baseline insulin secretion kinetics. One week later, OGTT will be repeated after administering a single dose of ferrous sulphate (120 mg of elemental iron) 2 hours prior to the test. Iron-induced change in insulin secretion kinetics will be documented. In addition, we will determine changes in glucose tolerance, insulin resistance and insulin clearance rates.

Oral iron supplementation is the treatment of choice in patients with iron deficiency anemia. In several developing countries, including India, iron is routinely supplemented to pregnant women, especially during the second and third trimesters of pregnancy owing to the increased iron requirement for the placenta and growing fetus. Oral administration of iron is preferred to intravenous administration because it is effective, relatively cheap and safe. There are many different oral iron preparations and most of them contain iron in the ferrous form (ferrous sulphate, ferrous fumarate, ferrous gluconate, ferrous ascorbate etc.). Although it has been shown that all these preparations are equally effective in increasing hemoglobin levels, ferrous sulphate, being easily available and economical, is the most prescribed iron preparation. Iron is absorbed in the duodenum. Dietary iron is usually in the ferric form and must be reduced to the ferrous form prior to absorption. This reduction reaction is catalyzed by duodenal ferrireductases (such as duodenal cytochrome b) and is aided by gastric HCl and other reducing substances in the diet, such as vitamin C (ascorbic acid). Administration of iron in the ferrous form (e.g., ferrous sulphate) circumvents this step, thus making it readily bioavailable. Ferrous iron is transported across the luminal membrane of the enterocytes via divalent metal transporter-1 (DMT-1). Iron is then transported across the basolateral membrane (into blood) by another transporter, ferroportin. Hepcidin, a peptide hormone synthesized and secreted by the liver, binds to and degrades ferroportin, thus reducing intestinal iron absorption. In the blood, iron is transported bound to the plasma protein, transferrin, which binds iron with high affinity. Transferrin is normally saturated to about 30 to 35% of its total iron binding capacity, leaving a large reserve to bind additional iron. In conditions of iron overload, such as hemochromatosis or in patients with thalassemia, transferrin saturation can increase significantly. When transferrin saturation increases beyond 60% and especially as it approaches 80%, a small but significant amount of iron circulates in blood that is not bound to transferrin. This fraction, called "labile iron" or non-transferrin bound iron (NTBI), is highly reactive and can cause oxidative tissue damage. NTBI is rapidly cleared from circulation, mainly by hepatocytes. It has been shown that ZIP14 is physiologically the most important transporter that transports NTBI into hepatocytes. Recently, it was shown that ZIP14 is also expressed on human pancreatic beta cells and that it may mediate NTBI uptake by these cells. Several in vitro and animal studies have shown that iron overload impairs pancreatic beta cell function. Patients with hemochromatosis are known to accumulate iron in the beta cells, resulting in diabetes due to decreased insulin secretory capacity. On the other hand, iron chelation or dietary iron restriction improves insulin secretion in mouse models of diabetes. Similarly, iron chelation in hemochromatosis and thalassemia also improved insulin secretion. These studies prove a strong link between increased iron and impaired beta cell function. It has been shown that, following a single dose of ferrous sulphate (containing 60-100 mg of elemental iron), transferrin saturation increases rapidly and peaks (at \~ 80%) 2 hours after administration. This is associated with a significant increase in NTBI, which also peaks at 2 hours. Given that oral iron administration increases NTBI in blood and that pancreatic beta cells take up NTBI via ZIP14, we hypothesized that oral iron may lead to increased beta cell iron levels which may then cause impaired insulin secretion In order to test this hypothesis, we plan to conduct a quasi-experimental single arm before-and-after study, where insulin secretion kinetics will be determined at baseline and after a single dose of iron (ferrous sulphate, 120 mg elemental iron) in healthy men. Healthy male volunteers will be recruited from among the staff of Christian Medical College, Vellore after obtaining written informed consent.Participants will undergo a 75g oral glucose tolerance tests (OGTT) to document baseline insulin secretion kinetics. One week later, the OGTT will be repeated after a single dose of ferrous sulphate (120 mg of elemental iron) given 2 hours before the test. Serum levels of glucose, insulin, C-peptide, serum iron and transferrin saturation will be measured during both OGTT. The effects of iron on insulin secretion kinetics will be documented. In addition, we will determine if changes occur in glucose tolerance, insulin resistance and insulin clearance rates.