Lack of iron is frequent in athletes, especially in females and for endurance sports. 

Only 10% of the iron taken through nutrition (10-20 mg/die) is absorbed by the organism: not more than 1-2mg per day in total. 
The majority of the inorganic and vegetable origin iron forms insoluble compounds that are not absorbed by the digestive system. 
Hemic iron (the one contained in hemoglobin and mioglobin) of animal origin, such as the one found in meat and fish, is absorbed much more effectively than non-hemic iron. 

Vitamin C and Citrate, taken together with food, increase the absorption of iron, even non-hemic, thanks to the formation of soluble compounds that are promptly absorbed. 

Other substances instead, like tannates in tea, phitates in plants and phosphates form compounds with iron, inhibiting its absorption. 

Once absorbed by the intenstine, blood iron is picked up by transferrin, a glycoprotein capable of transporting iron to all the tissues of the body. 

Every molecule of transferrin can bind 2 atoms of iron; its total iron binding capacity is labeled T.I.B.C.. 
Normally 20-45% of transferrin is loaded by iron atoms: percentages lower than 20% indicate a lack of iron. 

Iron in excess in the organism gets stored in the form of FERRITIN, a protein capable of releasing iron in case of need: a value in serum ferritin inferior to 20-40ng/ml suggests a latent iron deficiency. 

On average the stored iron the body is 4g; 2.7g in the form of Hb, 1g in the form of ferritin, 0.3g in mioglobina and mitochondrial enzymes. 

Every day 1 mg of iron is lost due to detachment of aged cells from the gastrointestinal and urinary systems and from skin. In women the menstrual cycle takes away an additional mg per day on average. 
2 cc of blood contain 1mg of iron: therefore a blood sample or loss of 12 cc takes away the iron absorbed in 3-6 days through normal nutrition. 

A heavy perspiration can bring upon a significative iron loss (1-3mg/die), as well as intestinal and urinary microhemorrhages, so frequent in athletes under effort. 

An overlooked lack of iron can determine ANEMIA, with negative effect on health and athletic performance. 

For such reasons athletes should monitor their values in ferritin, trying to maintain them around 60-100ng/ml, in order to guarantee adequate iron stores for heavy training loads or competitions.