Iron

Functions

Iron is an important component of a number of proteins and enzymes that perform essential roles in the human body. Iron transports oxygen from the lungs to tissues throughout the body; it is part of the transport mechanisms for electrons inside cells and is also an important part of numerous enzymes. Well known examples of proteins and enzymes containing iron include haemoglobin, myoglobin, cytochromes and various redox enzymes.

Haemoglobin and myoglobin are molecules intricately involved in oxygen transportation and storage in the body. Around two-thirds of our bodies iron is found in haemoglobin within red blood cells that deliver oxygen around the body. Haemoglobin collects oxygen from the lungs and then transports it around the body to various tissues. Myoglobin is responsible for the transport and short-term storage of oxygen within muscle cells. Cytochromes, which contain iron, are involved in the electron transport system that is crucial to energy production within body cells.

Iron requirements during periods of rapid growth are high, such as during fetal development and infancy. At birth an infant has sufficient iron stores to last for approximately 4-6 months of life, thereafter iron requirements rise markedly towards one year of age. Between one year and six years of age a child’s iron stores double in size. Iron requirements are also relatively high during adolescence, especially during periods of rapid growth. Furthermore, for young women the onset of menstruation and the associated blood loss increases iron intake requirements.

Role in Disease Prevention

Iron and Children's Brain Development

A number of observational studies have shown a relationship between iron deficiency anaemia in children and both delayed development and behavioural problems. Anaemic children tend to explore their environment less than children with good iron stores and this may possibly cause the developmental delays seen in these children. 

Iron and Immune Function

Iron is an essential component of a number of immue responses in the human body. Iron is needed for the production of T cells involved in cell-mediated immunity, and also for the production of reactive oxygen species (ROS) that are used to kill pathogens.

Food Sources

The main sources of iron in New Zealand and Australian diets are meats, fish, poultry and wholegrains. Iron exists in two different forms in food: haem iron and non-haem iron. The iron found in animal-derived foods such as red meat and poultry is a combination of haem and non-haem iron, whereas iron sourced from plant-based foods is non-haem iron. Each type of iron is absorbed in a different manner from the gut. Haem iron is more bioavailable than non-haem iron. In most industrialised countries haem iron accounts for approximately 5-10% of dietary iron intake, non-haem iron the remainder.

Nutrient Interactions with Iron

The absorption of iron from foods is affected by a number of nutrient and food interactions. Some factors increase iron absorption from the gut, while others decrease iron absorption. When considering food sources of iron consideration must also be given to other foods and nutrients that can affect the absorption of iron.

Factors That Enhance Iron Absorption

• Vitamin C or ascorbic acid is a potent enhancer of non-haem iron absorption
• Citric, lactic and malic acid increase non-haem iron absorption
• Consumption of meat, fish and poultry increases non-haem iron absorption from plant foods

Factors That Inhibit Iron Absorption

• Calcium (alone or within dairy products) and zinc decrease haem and non-haem iron absorption. Note: One glass of milk reduces iron absorption by more than half. Conversely, high iron intakes can inhibit the absorption of zinc and calcium too. As both calcium and iron are essential minerals a solution is necessary: a. increase iron intake or b. separate intake of calcium and iron, i.e. avoid calcium-rich foods at main meal time, instead increase calcium intake at breakfast and snack time. Main meal time is a time to increase iron intake.
• Phytates found in legumes, rice and some grains, seeds, nuts, fruits and vegetables inhibit absorption of haem and non-haem iron (90% of phytates in western diets originate from cereals). Vitamin C (ascorbic acid) can counteract the inhibitory effect of phytates.
• Polyphenols found in tea, coffee, cocoa, some vegetables such as spinach, herbs and spices such as oregano, inhibit non-haem iron absorption. Studies have shown that drinking coffee with a test meal inhibited iron absorption by between 25-70%. Drinking tea with main meals can also cause problems with iron absorption and is most likely an important factor contributing to iron deficiency.
• Vegetable protein, such as that found in soy, can inhibit non-haem iron absorption. It should be noted that soy is a source of phytates (see above) which inhibit iron absorption, however the inhibition is in addition to the effect of the contained phytates.

Note: Non haem iron absorption is also affected by iron status – more replete individuals will absorb less non-haem iron from a given meal than someone who has lesser iron status.

Cooking Time and Temperature

Cooking at high temperatures or for a long time can cause haem iron to degrade and convert into non-haem iron.

Typical Contributions from Food

Food	Iron (mg)
Beef, sirloin steak, grilled (100g)	3.8mg
Beef, mince, fried (100g)	3.4mg
Chicken breast, grilled (100g)	1.9mg
Snapper, flesh, baked (100g)	0.7mg
Fettucine, fresh, cooked (1/2 cup)	1.1mg
Brown rice, boiled (1/2 cup)	0.5mg
Bread, white (1 medium slice)	0.3mg
Bread, wholemeal (1 medium slice)	0.5mg
Weetbix, breakfast cereal (2 biscuits)	4.4mg
Rice bubbles, breakfast cereal (1/2 cup)	1.05mg
Baked beans (1/2 cup)	1.35mg
Peas, green, frozen, boiled (1/2 cup)	1.15mg
Silverbeet, boiled (1/2 cup)	1.05mg

Source: The Concise New Zealand Food Composition Tables 5th Edition.

Recommended Dietary Intake (RDI)

Following are the recommended dietary intake (RDI) levels for New Zealand and Australia. 

Iron RDIs – New Zealand and Australia

Life Stage	Age	Males (mg/day)	Females (mg/day)
Children	1-3 yr	9	9
	4-8 yr	10	10
	9-13 yr	8	8
	14-18 yr	11	15
Adults	19 - 50 yr	8	18
	51+ yr	8	8
Pregnant	14-18 yr	-	27
	19+ yr	-	27
Breastfeeding	14-18 yr	-	10
	19+ yr	-	9

Suggested Dietary Target for Reducing Chronic Disease Risk

The New Zealand and Australian governments have not set a suggested dietary target (SDT) for iron intake.

Deficiency

Symptoms of iron deficiency include reduced physical work capacity as evidenced by fatigue and rapid heart rate and breathing on exertion, palpitations, impaired immunity and cognitive function. Iron deficiency can result in adverse pregnancy outcomes and the delayed development of infants.

Insufficient iron intake at the lower end of the scale can cause low iron stores (indicated by low serum ferritin and decreased iron-binding capacity), followed by iron deficiency (decreased serum transferring saturation; increased erythrocyte protoporphyrin concentration and increased serum transferring receptor) and finally iron-deficiency anaemia (indicated by low haemoglobin and haematocrit, reduced mean corpuscular haemoglobin and volume).

Toxicity

The highest level of average daily intake of iron for adults aged 19 years and over, that is believed to not cause any adverse health effects, is 45mg/day.

References

Garrow JS, James WPT & Ralph A. (2000). Human Nutrition and Dietetics (10th Ed). London: Churchill Livingstone.

Ministry of Health. (2006). Nutrient reference values for Australia and New Zealand. Canberra: Commonwealth of Australia.

Last Updated: 10 October 2008

The material provided by Thinking Nutrition Ltd on this website is for information purposes only. It is not a substitute for appropriate health advice from a qualified medical practitioner.