Iron
To maintain the balance of iron levels in the body, the body adjusts iron absorption; it has a limited capacity to excrete iron.
The iron present in food can occur as heme iron, found in hemoglobin and myoglobin, and as non-heme iron, which is present in plant material.
The absorption of heme iron is not strongly dependent on iron status or other nutrients in the diet; however, there are exceptions regarding heme iron absorption. For example, meat increases heme iron absorption, while calcium inhibits the absorption of both heme and non-heme iron.
The absorption of non-heme iron is strongly influenced by certain nutrients in the diet. The amounts of phytate, tannins, excess calcium, phosphorus, manganese, zinc, copper, and ascorbic acid all play a role in non-heme iron absorption. Additionally, iron status also affects the uptake of this form of iron.
From the above information, it can be concluded that the amount of iron a dog or cat can absorb from food depends on the body's iron status, the amount of heme and non-heme iron in the diet, and the availability of dietary iron (with other nutrients in the diet playing an important role).
Iron transport occurs via plasma, and its uptake takes place in the bone marrow. In the bone marrow, iron is then used for the synthesis of hemoglobin – a protein necessary for the transport of oxygen and carbon dioxide.
The amounts of iron present in urine are negligible. The amounts found in feces are mainly the non-absorbed forms of iron. Other ways iron is lost include sweat, nails, and fur.
Iron is an essential nutrient for dogs and cats because it is present in various enzymes and other proteins responsible for oxygen activation (oxidase and oxygenase), electron transport, and oxygen transport (hemoglobin and myoglobin).
Iron is found in all cells of the body. However, the majority of iron is present as a component of the protein molecules hemoglobin and myoglobin; respectively >65% and about 4%. Hemoglobin is found in red blood cells and is responsible for oxygen transport. Myoglobin binds oxygen so it can be directly used by muscle cells. Iron is also a cofactor for various enzymes; without iron, these enzymes cannot perform their functions.
Healthy animals obtain iron only through the food they consume. The absorption of iron from food, whether heme or non-heme iron, takes place in the duodenum.
The maintenance of iron levels in the body is tightly regulated by iron absorption in the intestines, the amount of iron needed for erythropoiesis – blood formation –, the recycling of iron from old, aged body cells, and the storage of iron by hepatocytes – liver cells – and macrophages – white blood cells, a defense cell of the body.
