Iron - Nutrition

What is Iron?

Iron is one of the most abundant chemical elements on earth, and is most commonly found as iron oxide minerals. It is an essential nutrient in dogs and cats, but it is designated a trace element as it is required in much smaller amounts than major minerals such as calcium and phosphorus. Dietary iron is found in two forms: inorganic and organic, the latter mainly as haem iron where the iron is at the centre of an organic complex.

Why is it Important?

Iron was recognised as an essential dietary component more than 100 years ago. Its primary role is to serve as a component of haemoglobin and myoglobin which are the key for the transport and movement of oxygen around the body. It also functions in a number of enzymes such as the cytochromes, that are key regulators in energy metabolism.

Roles in the Body

Haemoglobin found in the red blood cells (erythrocytes) transports oxygen from the lungs to the tissues. Myoglobin is the primary oxygen transporter in muscle tissues. As a component of cytochromes, such as cytochrome c and cytochrome oxidase, iron is also essential for the functioning of the electron transport chain and the production of energy in the form of adenosine triphosphate (ATP). After haem, the liver contains the largest iron stores, as ferritin or haemosiderin, both of which are iron-containing proteins. Body stores of iron are tightly regulated to provide adequate supplies for nutritional requirements, while avoiding toxicity from excess. This is achieved primarily by regulating absorption. The biological availability of iron is affected by a number of factors, in particular the chemical form of the iron source. In addition, some minerals, especially calcium, can decrease the absorption of iron so if calcium supplements are being given to the animal it is particularly important to ensure that the dietary iron content is adequate.

Consequences of Iron Deficiency

Dog and Cat:

Reports of iron deficiency in dogs and cats are consistent with its involvement in haemoglobin function. The symptoms of iron deficiency are very similar for the two species. Although naturally-occurring dietary iron deficiency is rare, nursing puppies and kittens may have an inadequate intake due to the low iron concentration in milk.

Recognised Syndromes Related to Iron Deficiency

1. Haematological changes: Weanling puppies and kittens showed suboptimal blood haemoglobin concentration and low haematocrit values when fed diets containing less than 80 mg/kg iron on a dry matter (DM) basis. Other haematological changes reported in dogs include anaemia and low saturation of plasma transferrin – a beta-globulin acting as a carrier of iron.
2. Other clinical signs: The same researcher also reported clinical signs of iron deficiency in dogs and cats that included poor growth, pale mucous membranes, lethargy and diarrhoea. Haematochezia and melaena were also observed – both are signs of blood loss from the intestine, usually seen as blood in the faeces.

Toxicity

Dog:

There is little information on the toxic effects of excess iron in foods, however there are several reports of the adverse effects of iron when administered directly to dogs. D’Arcy and Howard found that a dose of ferrous sulphate supplying only 12 mg iron per kg bodyweight (BW) resulted in mild gastrointestinal damage. A higher dose of 600 mg iron per kg BW was fatal. Doses of ferrous sulphate as low as 200 mg iron per kg BW administered directly to the jejunum were fatal within 6 hours and ferrous sulphate supplying 250 mg iron per kg BW was fatal within 5 to 7 hours when administered by stomach tube. In contrast, ferrous carbonate did not produce any effects at 1.5 g iron per kg BW but did at 3 g iron per kg BW. Ferric oxide fed to dogs for 18 months at a dietary content of 1% produced no adverse effects. These variations in toxic effects are further examples of the differences in bio-availability of iron salts.

Cat:

As with dogs, the reports of adverse effects of iron are limited to studies of the administration of acute doses. Hoppe et al found that acute oral doses of ferrous sulphate or gluconate equivalent to 16 to 128 mg iron per kg BW resulted in vomiting within one hour of administration. The same researchers found that the median lethal dose of ferrous sulphate heptahydrate in cats was greater than 500 mg iron per kg BW. In relation to the degree of toxicity, these results are broadly in line with those from the dog studies.

Dietary Sources

Animal products are rich sources of iron, and typically manufactured dog and cat foods include materials containing organic iron in the form of haem, such as meat meals, meat and bone meals, fish meals and blood meals. Nevertheless, some mineral supplements such as ground limestone and calcium phosphate can contain high amounts of iron. Supplemental sources of iron commonly added to dog and cat foods include salts such as ferrous sulphate or fumarate. Iron can exist in two oxidation states, ferrous (Fe²⁺) and ferric (Fe³⁺), and the ferrous salts in general (especially the sulphate) have higher bio-availability. In contrast, ferric oxide has a very low bio-availability and is unsuitable as a dietary source. These differences in bio-availability are reflected in their degree of toxicity.

References

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