What we need to know about eggs
Concerns about cholesterol and the iron content of eggs incorrectly relegate this fine source of nutrition to lists of foods we should avoid or eat less frequently. As for the cholesterol issue, ample evidence exists in defense of the egg as the culprit for wreaking havoc in our arteries. When it comes to iron, however, the relationship with the egg is more complex.
The egg is a cost to benefit bonanza. It is a good source of vitamin A and a great source of protein (considering an egg costs about 10-15 cents and contains roughly 70 calories). The egg also contains iron, but interestingly, a component in the egg impairs absorption of the metal.
A phosphorprotein called phosvitin lowers the bioavailability of iron. One boiled egg can reduce absorption of iron in a meal by as much as 28%. This “egg factor” must be considered when organizing a diet for iron balance. A person with hemochromatosis (too much iron) can enjoy eggs freely since the goal for this patient’s diet is iron reduction. For the iron deficient, bumping up the bioavailability of iron from a meal with eggs might be as simple as adding a tall glass of vitamin C-rich juice. If taking oral iron supplements, choose one with EDTA-bound iron, since this compound is among the few that can release iron bound to phosvitin.
Nature knows the beneficial and destructive capabilities of iron. As such elaborate systems are put into place to assure iron is properly bound, delivered, and impaired from getting free. In the following bullet points Dr. Eugene Weinberg illustrates some of the amazing facts about nature’s ability to control iron.
Eggs and Iron
Eugene Weinberg, Ph.D., Professor Microbiology Emeritus Indiana University, Member Iron Disorders Institute Scientific and Medical Advisory Board
- Eggs are deposited by birds and reptiles in places that are heavily contaminated by all sorts of bacteria.
- Eggshells need to have air holes for exchange of oxygen and carbon dioxide for the developing embryo.
- Bacteria can enter the air holes and, if allowed to feed on the yolk, will cause the egg to rot and the embryo to die.
- Therefore, the egg white has to contain a material that will prevent the bacteria from growing.
- There is no possible antibody or antibiotic that the hen could produce that would be able to prevent all bacteria from growing.
- Mercury would be able to kill all the bacteria but it would be far too dangerous for the embryo to have the hen to try to put it in the egg white.
- The hen can keep the bacteria from growing by starving them of iron because the harmful bacteria need iron to produce their own DNA. The hen puts a large amount of iron into the yolk so that the embryo can produce its own DNA, but she puts no iron whatsoever into the egg white.
- The hen puts a protein into the egg white that traps any iron that might enter from the outside.
- The iron-trapping protein works best under alkaline conditions, so the hen makes the egg whites alkaline.
- If the shells of the eggs are washed with water that contains iron, the eggs will spoil sooner, even if stored in a refrigerator.
- For many centuries, people have known that raw egg white can be used as an antiseptic. In Shakespeare’s play King Lear, a character who has had his eyes torn out is treated by having raw egg whites put on his bleeding eye-sockets.
- Iron trapping proteins are also present in our blood, tears, and many other body fluids to help prevent bacteria from growing within us.
Himali Samaraweera, Wan-gang Zhang, Eun Joo Lee, Dong U Ahn. “Egg Yolk Phosvitin and Functional Phosphopeptides—Review” Journal of Food Science 2011; 76: R143-R150.
Online reference: DOI: 10.1111/j.1750-3841.2011.02291.x
Posted on Thu, July 18, 2013
by David Garrison