IRON ABSORPTION FROM FOODS (METHODS AND RESULTS)

.

The different stages in the development of knowledge are connected to the methods which have been available.

Optional: IRON BALANCE(-+-)

(balance = equilibrium Norweg. likevekt)

B = In - out.

• The balance is of a very small order of magnitude, 1-2 mg, large sources of error, in addition difficult to do
• Advantages: mixed diets, long-lasting experiments

Results

• Total absorption of iron from one meal varied from 7-21 % (in experiments with several individuals there will be differences in iron status)

.

Optional:BIOLOGICAL TAGGING

1 BIOLOGICAL TAGGING WITH ONE RADIOACTIVE ISOTOPE (-+) (incorporation with ⁵⁹ Fe)

Plants fertilized with e.g. ⁵⁹Fe-EDTA complex; animals fed an iron-tagged diet.
Testing of single food items

Results

• Variation between individuals; largest uptake with iron deficiency, i.e. iron status influences the absorption.
• The absorption from vegetable foods is poorer than from animal foods.
  Moreover, the absorption varies between the different vegetable food items and between the different animal food items.

2 BIOLOGICAL TAGGING WITH TWO RADIOACTIVE ISOTOPES (incorporation with ⁵⁵Fe and ⁵⁹Fe)(-+-)

E.g. testing of ⁵⁵Fe-tagged vegetable and ⁵⁹Fe-tagged animal foods.

Results

• Vegetable and animal food influence each other mutually

.

EXTERNAL TAGGING

Isotope is added to the food item on the plate.

3 COMBINED EXTERNAL AND INTERNAL TAG (extrinsic and intrinsic tag)
(-+-)

The absorption of one isotope (e.g. ⁵⁹Fe) biologically incorporated in the food item and the absorption of another (e.g. ⁵⁵Fe) added to the food item on the plate are measured.

Results

• The absorption of the isotopes is equal, i.e., all the non-haem iron is mixed in one pool irrespective of a plant having an incorporated tag (internal) or a tag (external) added to the vegetable diet. There is complete exchange between the isotopes. This is valid for both types of iron and has been shown for a series of foods.
• Haem iron and non-haem iron are absorbed differently; there are 2 pools (shown by tagging haem iron and non-haem iron with different isotopes.
• Complicated interaction; nutrients and compounds function as inhibitors (tannic acid, fytic acid, and others) and enhancers (ascorbic acid, 'meat factor'), respectively.

Biological tagging is no longer in use. The external tag method has made it possible to study the absorption of iron from compound meals and thereby how the different food items may influence each other.

Figure Iron stores are positively correlated with serum ferritin, but negatively correlated with iron absorption. A: Correlation between iron stores and serum ferritin [Ref 8] B: Negative correlation between iron stores (measured as S-ferritin) and iron absorption [Ref 9]

.

Methodology
For research purposes absorption and bioavailability has been measured by evaluating the iron absorption, either by balance studies (direct measure of the difference between ingested and excreted iron) or by using (one or two) radioactively labelled iron isotopes. The knowledge of iron absorption has been (and still is) the result of the currently available methodology.

.

Optional: FACTORS INFLUENCING IRON ABSORPTION

• Factors connected to the individual (such as body iron stores and bone marrow activity).

• Factors connected to the diet (such as organic acids).

While the condition of the body iron status determines the absorption of both haem iron and non-haem iron, the composition of the diet only influences the absorption of non-haem iron.