Bioavailability of Iron

Uncategorized
29.05.2019

Dietary iron occurs in two forms in foods, heme and non-heme iron. The primary sources of
heme iron are haemoglobin and myoglobin from animal sources such as of meat, poultry,
and fish. Heme is part of haemoglobin in red blood cells and in muscles it is part of
myoglobin. Non-heme iron is from plant sources like cereals, pulses, legumes, fruits, and
vegetables. There is less than 5 grams of iron in the body and most of it is found in
haemoglobin.

Absorption of heme and non-heme iron
Heme iron from animal foods is better absorbed by the body than non-heme iron from plant
sources. Heme iron is highly bioavailable (15%-35%) and dietary factors have less effect on
its absorption. Non-heme iron absorption is lower (2%-20%) and the dietary components
present in food have a strong influence on its absorption.
Iron bioavailability was found to be in the range of 14–18% for mixed diets and 5–12% for
vegetarian diets in subjects with no iron stores in a study conducted.

Contribution of heme and non-heme iron towards common iron pool
But when the quantity of iron contributed by both heme and non-heme iron is compared
non-heme iron contributes more. That is because the amount of non-heme iron we
consume is more. Though bioavailability of non-heme is less it is a major contributor of iron
towards iron pool.

All non-heme food iron enters the common iron pool in the digestive tract in the similar
manner to that of heme iron. And then both are absorbed to the same extent, which
depends on the balance between the inhibitors and enhancers of absorption and the iron
status of the individual.
The principal inhibitors of iron absorption are
1.Phytic acid
2.Polyphenols
3.Calcium
4.Peptides from partially digested proteins.

Phytates
In diets which contain mostly plant-based foods, phytate is the major inhibitor of iron
absorption. Bran has a high amount of phytate content which inhibits iron absorption (3).
The negative effect of phytate on iron absorption has been see even at a very low
concentrations of 2–10 mg/meal. Sufficient amount of ascorbic acid can stop this inhibitory
action of phytates.

Polyphenols
Polyphenols are present in various amounts in plant foods and beverages, such as
vegetables, fruit, some cereals and legumes, tea, coffee, and wine.  The polyphenol quantity
and type influence iron absorption. In cereals polyphenols add to the inhibitory effect of
phytates. It is often suggested that tea should not be consumed before and after meals
containing iron so that iron is absorbed well.

Calcium
Evidence shows that calcium has negative effects on non-heme and heme iron absorption.
The other factors affect mainly non-heme iron absorption only. Calcium is also an essential
nutrient therefore cannot be taken out of the diet. A practical solution would be to eat
calcium rich and iron rich foods separately.

Proteins
It has been observed that animal proteins such as milk proteins, egg proteins, and albumin
inhibit iron absorption. Soy protein also reduces the absorption of iron.
Just like there are inhibitors there are enhancers such as ascorbic acid and muscle tissue
which may reduce ferric iron to ferrous iron and bind it in soluble complexes which are
readily available for absorption.

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