I would like to thank Dr. Dettwyler for bringing in additional information
on
the role of excessive iron in promoting infections.  I've always thought
that
the studies on increased infection incidence in populations treated with
iron
dextran injections were quite convincing, but Dr. Stuart-Macadam's
observational information also makes the point as well.

I'm not sure what eggs have to do with the discussion.  Egg whites also
contain avidin (which tightly binds biotin, an essential nutrient) and
lysozyme (a protein that lyses cells, including infectious ones).  As for
human vs. cow's milk, I think reduced infections in breastfed infants have
more to do with sIgA, lysozyme and other immunological factors in human milk

than they do simply with the iron witholding capacity of human milk
lactoferrin.  Lactoferrin is important, but it is one component of a whole
system.

Infectious organisms need iron to grow, as does every living cell.  Several
very successful species of these organisms have evolved specialized
molecules
to take up iron from their environment, called siderophores.  Human cells
produce a molecule called transferrin to bind and transport iron in the
body.  When an infection occurs, a series of proteins are produced by the
immune system to mobilize the body's defenses, including interleukins 1, 6
and 8 as well as tumor necrosis factor-alpha, collectively called acute
phase
proteins.  One effect of tumor necrosis factor-alpha is to reduce appetite
(a
global effect, not just against iron-rich foods).  In response to these
proteins, the body takes up circulating iron for rapidly proliferating
lymphocytes (which have a high need for iron) and as a reserve against
losses
(storing it in a form called ferritin).  In the process of saving itself,
the
body also keeps iron away from bacteria.  This is just to illustrate that
the
body reserves iron during infections for more reasons than just to keep
bacteria from growing.

I don't think anyone will argue that excessive amounts of iron promotes the
growth of infectious organisms.  The controversy is over the definition of
"excessive."  Surely, giving iron drops to a two month old full-term,
exclusively breastfed baby in the absence of other pathologies is
"excessive."  Surely, telling a 1st trimester pregnant vegan with iron-
deficiency anemia not to take supplemental iron is going too far the other
way.  Recommendations for iron intake needs are usually based on shifts in
hemoglobin distribution curves.  What this means is that you sample a group
of people before and after you give them iron and see if the population
levels of hemoglobin increase.  If they do, then they needed more iron to
make enough red blood cells for oxygen transport.  Based on these data and
population dietary intakes of iron, we can determine what iron needs are on
average for a population (pregnant women, infants, etc.)  The Food and
Nutrition Board currently adds a margin of safety (+2 SD) to this average in

making a population recommendation.  On an individual level, this means some

people will get too much and a few will get too little.  Until other
probability approaches are worked out, this is the best system we have for
making recommendations.  If someone misuses this information, well, that's
another problem entirely.

As a straw-man arguement, if the body is so wise, why doesn't a woman's iron

absorption rate reach 40% or 90% during pregnancy (usually 5-20%) to prevent

maternal death from anemia (one of the top 5 causes of maternal mortality in

childbirth worldwide)?

I hope this presents a more balanced view after the post of Drs. Dettwyler
and Stuart-Macadam on Jan. 18th.  I apologize for the length of the reply.

Paul Zimmer, Ph.D.