Question

Discuss factors that influence the bioavailability of dietary iron. In your response identify factors that interfere with iron absorption, factors that enhance iron absorption, and the influence of a person's iron status on absorption?

Answer 1

Dietary iron is of 2 types:

1. Heme iron.

2. Non-Heme iron.

1. Heme iron:- That iron which comes from hemoglobin and myoglobin in animal source food. Heme iron contributes to approximately >40% of total absorbed iron.

2. Non heme iron​​​​​​:- That iron which comes from both plant food and animal tissue. Non-heme iron is usually less absorbed than heme iron.

FACTORS THAT INFLUENCE THE BIOAVAILABILITY OF DIETARY IRON :-

The bioavailability of dietary iron is influenced by the factors that interfere with iron absorption and the factors that enhance the iron absorption.

Factors that interfere with iron absorption are discussed below:-

1. Phytate:-  It effects non-heme iron. It is a salt or ester of phytic acid present in plant based diet like cereal grains and legumes. When we take a mixed meal in which iron and phytates are present, phytate tends to decrease the absorption of iron depending on the dose of phytate in meal. And very low levels of phytate 2-10 mg/meal are sufficient to influence the dietary iron absorption. The molar ratio of phytate to iron of < 1:1 or < 0.4 :1 should be maintained to improve iron absorption. Food preparation methods like milling, heat treatment, soaking, germination and fermentation of plant food can be used to remove or decrease phytate in plant food with varying degree.Vitamin C can also be used to enhance iron absorption in such meal.

2. Polyphenols:- Just like phytates, it effects non-heme iron only. These are organic chemicals present in various amounts in plant foods like vegetables, fruits, some cereals, legumes and beverages like tea, coffee, wine. Various studies suggest that type and quantity of polyphenol influence iron absorption. For instance black tea and herb tea show different influence, black tea is more inhibiting than herb tea. Polyphenols add to inhibitory effect of phytates in cereals and legumes and influence the iron absorption.

3. Calcium:- Unlike Polyphenol and phytate, calcium effects both heme and non-heme iron absorption. The inhibition takes place during the initial uptake of iron into enterocytes. Various studies suggest dose dependent inhibitory effects of calcium on iron absorption. Approximately 75-300 mg of calcium in a diet that contained iron as well, was found to be inhibitory. The effect of calcium also depends on either the meal is single or multiple meal. Single meal has more negative effect of calcium on iron absorption than multiple meal in which calcium has limited effect on iron absorption.

4.Proteins:- Animal proteins like milk proteins, egg proteins inhibit iron absorption. Two major bovine milk protein fractions that are casein and whey, and egg white inhibit iron absorption in humans.Proteins in soyabean also inhibit iron absorption.

FACTORS THAT ENHANCE IRON ABSORPTION ARE:-

1.Ascorbic acid:- Vitamin C (ascorbic acid) has dose dependent enhancing effect on iron absorption. The enhancing effect is largely due to the ability of ascorbic acid to reduce ferric to ferrous iron but is also due to its potential to chelate (bonding of metal ions or molecules) iron. Ascorbic acid also overcomes the inhibitory effect of phytates, polyphenols, calcium and protein in milk products and hence increase iron absorption. Ascorbic acid only enhances iron absorption from vegetarian diet.So inclusion of vitamin C in vegen meal helps in iron absorption. Cooking, processing and storage of ascorbic acid removes its enhancing effects.

2. Muscle tiseffect on iron  Meat, fish or poultry enhance the absorption of iron from vegetarian meals. Addition of chicken, beef or fish to a maize meal increases non-heme iron absorption 2-3 fold. Remember proteins like milk, egg proteins have inhibitory effect on iron absorption but protein tissue that is muscle tissue has enhancing effect on iron absorption.

INFLUNECE OF A PERSON'S IRON STATUS ON ABSORPTION:-

Iron status means the amount of iron present in body in different forms like serum iron in hemoglobin,transferrin,transferrin saturation,ferritin. Absorption of Non-heme iron is mainly influenced than heme iron by iron status. There is an inverse relation between iron status and iron absorption. The difference in iron absorption between different groups of people is defined by iron status, but the magnitude of enhancing or inhibiting effect is independent of iron status.