The low solubility of iron at physiologic pH precludes urinary excretion as a major mechanism of maintaining iron homeostasis. Thus, in contrast to most other trace minerals whose homeostasis in maintained by excretion, the primary mechanism of maintaining whole body iron homeostasis is to regulate the amount of iron absorbed so that it approximates iron losses. Iron losses can vary considerably with the gender of the individual. In male humans, total iron losses from the body have been calculated to be 1 mg/day. For premenopausal female humans, this loss is slightly higher. The predominant route of loss is from the GI tract and amounts to 0.6 mg/day in adult males (28). Fecal iron losses derive from shed enterocytes, extravasated red blood cells, and biliary heme breakdown products, which are poorly absorbed. Urogenital and integu-mental iron losses have been estimated to be greater than 0.1 mg/day and 0.3 mg/day, respectively, in adult males (27). Menstrual iron loss, estimated from an average blood loss of 33 mL/month, equals 1.5 mg/day, but may range as high as 2.1 mg/day (31). Oral contraceptives reduce this loss and intrauterine devices increase it (32,33).
Pregnancy is associated with losses approximating 1 g, which consists of basal loss of 230 mg iron, increased maternal red cell mass of 450 mg iron, fetal needs of 270-300 mg iron, and placenta, decidua, and amniotic fluid iron content of 50-90 mg. All maternal iron stores are called upon to provide Fe for these needs. Women at partum are often iron deficient and sometimes also quite anemic. Because iron need and distribution are different while pregnant, assessment of iron status can be difficult in this population.
A number of clinical and pathological conditions are attended by variable amounts of blood loss. These include hemorrhage, hookworm infestation, peptic gastric or anastomotic ulceration, ulcerative colitis, colonic neoplasia, cow's milk feeding to infants, aspirin, nonsteroidal anti-inflammatory drugs or corticosteroid administration, and hereditary hemorrhagic telangiectasia [see (4) for review]. In addition to these conditions, a significant amount of iron (210-240 mg/unit) can be lost with regular blood donation.
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