Iron deficiency is common in strenuous exercise i.e in endurance athletes.
In runners "GI iron loss"is frequently mentioned as a possible cause of the iron deficiency frequently observed in runners but those publications rarely describe or explain exactly what they mean by GI iron loss.
Do they mean that the incidence of well recognized causes of GI bleeding (colon cancer,polyps,peptic ulcer etc) are common in runners. I find little evidence of that.Perhaps the reference is to some type micro bleeds. Or do they mean there is some physiological mechanism by which there is GI iron loss without gross GI bleeding namely occult GI bleeding ( ie. normal stool color with a positive hemoccult test).
Actually there are data indicating occult GI iron loss with at least one study with positive guaiac tests post marathons and another pre and post marathon upper endoscopy study showed some small lesions in the stomach that could possibly cause small amount of blood loss.
There is a physiologic mechanism by which runners ( and other endurance athletes) loose blood in the GI tract.The mechanism is the sloughing off of iron loaded duodenal luminal lining cells (aka enterocytes) Actually this is a physiologic process that happens in everyone but is believed to be significantly increased in endurance athletes particularly runners because they may have more exercise induced hemolysis which in turn leads to more iron loss due to duodenal cell sloughing which is intensified by hepcidin release for exercise which traps the iron in the duodenal cells and the macrophages. Enteroyctes have short ( about 3 day) life span.
Here is an "as if" story of how that might work. Strenuous exercise leads to the release of IL6 which in turn stimulates the release of hepcidin, the hepatic hormone that is the master regulator of iron absorption and transfer and storage. (The putative release of hepcidin by IL6 is not crucial to the story it is the release of hepcidin that is important ).Hepcidin blocks the release of iron from the enterocytes and from macrophages slowing down the transfer of recycled iron to the bone marrow.
The Assocation of iron deficiency and running is well discussed in the sports medicine literature and in the lay runners press but much less so the hematology journals.