Who hasn’t heard of Popeye the Sailor Man? His strength came from a can of spinach — or did it? Spinach contains nonheme iron, the form of iron that is not readily absorbed unless a person is deficient or it is consumed in combination with heme iron (animal products) or vitamin C. Not only is the form of iron not easily absorbed in spinach, but spinach contains compounds that bind with iron and interfere with its absorption. This article will further explore iron’s absorption and distribution.
Interference
The absorption of nonheme iron is affected by many dietary factors. Oxalic acid in vegetables and phytic acid as well as additional factors found in grain fibers can bind iron and decrease its absorption. In tea, tannins (polyphenols) reduce the absorption of nonheme iron. If someone has an iron deficiency, the intake of tannins should be watched and fiber intake should be kept within current recommendations. Additionally, zinc supplements will interfere with the absorption of nonheme iron since zinc and iron compete for absorption. (1) (2) (3)
Iron Needs & Absorption
The body’s need for iron is the single most important element affecting nonheme iron absorption. Iron needs increase:
- During growth and pregnancy;
- During a state of iron deficiency; and
- At high altitude due to the air’s lower oxygen concentration resulting in the increased concentration of hemoglobin in the blood. (2) (3)
Colorado Rockies – High Altitude & Iron Absorption
Where there are inadequate iron stores, the main protein that transports iron in the blood easily binds additional iron from intestinal cells, moving this iron into the bloodstream. On the other hand, when there are adequate iron stores and the protein that binds iron in the blood is totally saturated with iron, little is absorbed from the intestinal cells where iron stays bound. (2) (3)
It is through this mechanism that iron, and the nonheme form in particular, is only absorbed as needed under normal circumstances. Intestinal cells have a two to five day life cycle. If iron is not needed, it will be excreted from the iron that is stored in intestinal cells. Even though high doses of iron is toxic, under normal dietary conditions in the majority of people it is carefully regulated. (2) (3)
Distribution
The hemoglobin molecules of red blood cells contains the majority of iron in the body. The bone marrow stores some, and a small amount is found in other body cells, for example the liver, to store it. The iron found in these body stores can be mobilized if iron is needed. However, these iron stores can be depleted if there is a chronically inadequate intake of dietary iron. (2)
In light of the fact that iron deficiency remains a world wide problem, its absorption is an important factor. Even though a food may be rich in iron does not mean it will be absorbed. The form of iron as well as interference with anti-nutrient compounds and mineral interactions all affect absorption. Taking a specific mineral supplement will interfere with the absorption of other essential minerals. It is for this reason natural supplements are most effective. (2) (3)
References:
(1) https://www.cdc.gov/immpact/micronutrients/index.html
(2) https://www.cdc.gov/nutritionreport/99-02/pdf/nr_ch3.pdf
(3) https://www.cdc.gov/mmwr/preview/mmwrhtml/00051880.htm