Iron: keeping blood healthy

In the seventh article in our series on nutrition, Sarah Schenker explains how iron is essential to keep our bodies running smoothly and how to make sure you are getting enough

The body has a requirement for iron because of the central role that it plays in energy metabolism and in the production of red blood cells (erythropoesis). Iron is a transition metal and takes part in redox processes--for example, reduction by an organic substrate and reoxidation by oxygen. It binds with oxygen either on its own or as part of a complex. Iron is transported in the blood by the protein transferrin and is stored in the body as the proteins ferritin and haemosiderin. Functionally important forms of iron are haemoglobin, myoglobin, cytochromes, iron-sulphur proteins, iron enzymes, and lactoferrin.

Where is iron found in the body?

Approximately two thirds of the body's iron is in the pigment haemoglobin, which transports oxygen in red blood cells. Most of the 20-30 mg of iron needed each day for the synthesis of haemoglobin comes from the recycling of senile red blood cells. Red blood cells live for 120 days before their constituents are reprocessed: iron is recycled in the body maintaining a constant internal supply independent of diet.

Senile red blood cells are destroyed by macrophages in the spleen. Transferrin takes up the released iron and carries it to the bone marrow. Developing red blood cells remove this iron and incorporate it into haemoglobin. The body has no active mechanism to excrete unwanted iron, so little iron is lost from the body.

How much iron do we need?

Men lose about 1 mg of iron each day through sweating and shedding epithelial cells from the skin, intestine, and genitourinary tract. Women lose more iron because of menstruation: about 2 mg each day. Because obligatory loss of iron is small, only small amounts need to be absorbed from the diet to maintain balance. During times of increased iron need--for example, growth, pregnancy, or bleeding--the
intestine increases its absorption of iron to 4-5 mg each day. Absorption returns to baseline when the need declines. Despite the minimal requirements, anaemia due to iron deficiency is a common nutrient deficiency worldwide.¹

The dietary reference values set for iron allow for low absorption of iron but assume, in general, that absorption is about 15% from a mixed diet. The reference nutrient intake (RNI) for women of childbearing age is much higher (14.8 mg each day) than for other groups--for example, adult men have an RNI of 8.7 mg each day. This accounts for blood loss during menstruation, but women with very high menstrual losses may need intakes above the RNI, perhaps in the form of a supplement.

Anaemia

Men with less than 130 g/l of haemoglobin in their blood and women with less than 120 g/l have anaemia. About 500 million people are thought to be anaemic, but many more have absent iron stores or iron deficient erythropoesis. Prevalence is particularly high in developing countries, and in the United Kingdom it is high in infants, preschool children, adolescents, and young women.

On average, an adult male has 1000 mg of iron in storage. Iron stores are typically smaller in adult women (300-500 mg), marginal to absent in young women, and absent in adolescents and children. For new born babies the limited store of
iron is particularly important as a source of iron for the first six months of an infant's life because the amount of iron in breast milk is very small. Iron deficiency occurs through predictable stages of severity from depletion of iron stores to anaemia (box 1).

Who is iron deficient?

Intake of iron from food has been declining for some time mainly because people
are eating less bread. In the United Kingdom, the average iron intake from all sources each day is 14 mg for men and
12 mg for women. These values are above the RNI for men but not for women. Also, the distribution is skewed and it is of particular concern that a quarter of women in the survey of British adults had dietary iron intakes below the lower reference nutrient intake (LRNI).² Younger women are particularly at risk. A third of women between 16 and 18 years old had intakes below the LRNI: women obtained 15% of their total intake from dietary supplements, but men obtained almost all their iron from food.

Iron deficiency in early childhood can result not only in apathy and lethargy but in long term, possibly irreversible, effects on mental and motor development. It is important that mothers introduce the food sources of iron during weaning before iron fortified formula milks and infant foods are removed from the diet.

Low intake of iron and low iron status among young people were found in the recent national diet and nutrition survey of children aged 4-18 years.³ In the 4-6 year old group 3% of boys and 8% of girls had poor iron status. Among girls aged 15-18 years 9% had low haemoglobin levels. Thirteen per cent of all boys and 14% of all

girls, rising to 27% in the oldest group of girls, had low iron stores. In a previous study of nearly 400 white British schoolchildren aged 12-14 years living in south west London⁴ 3.5% of boys and 10.5% of girls were anaemic. In a different study, Nelson and colleagues studied a group of 11-14 year old north London schoolgirls with different ethnic backgrounds.⁵ Overall, 20% of girls were classified as anaemic; from 11% in white girls to 25% in Asian girls. The prevalence was higher amongst those from manual class backgrounds, vegetarians, and those trying to lose weight.

Where can we get iron from?

The major sources of iron in the United Kingdom are cereals and cereal products which together provide about half of iron intake: there is a statutory requirement for all white flour to be fortified with iron amongst other nutrients, and many breakfast cereal manufacturers voluntarily fortify their products with iron. Meat provides about 18% and vegetables, including potatoes, provide about 16%.

The extent to which iron is absorbed depends on whether it is in haem (Fe²⁺) or non-haem (Fe³⁺) form. The absorption of non-haem iron is influenced by the body's iron status and by the combination of foods ingested. Haem iron, the form found in red pigments of meat and offal, is relatively well absorbed (20-30%).

The non-haem iron found in cereals, pulses, vegetables, fruits, eggs, and dairy products is generally less well absorbed, but absorption increases when the body's stores are depleted and when the body requires more. Iron absorption also depends on other factors in the diet--for example, it is increased by the presence of meat and by vitamin C but decreased by tannins in tea and some forms of fibre.

What are the long term effects of iron deficiency?

A study of 600 British girls aged 11-18 years found that slight anaemia due to iron deficiency can affect cognitive function.⁴ Girls with poor iron status had significantly lower IQ (intelligence quotient) scores than in those with borderline or good iron statuses. The authors concluded that even in the absence of clinical signs poor iron status is associated with lower cognitive function and that this has important implications for educational achievement. If low iron status persists into early reproductive years there is an increased risk of low birth weight, stillbirths, and preterm delivery.⁵

Sports training--especially endurance training--and competitions or events spanning several days affect haemoglobin concentration and iron profile. Endurance training can expand plasma volume by as much as 10-20% diluting haemoglobin and serum ferritin. This can be misinterpreted as iron deficiency and anaemia in some athletes. Also, many athletes eat a modified vegetarian diet to keep their diet low in fat (erroneously avoiding red meat) and high in carbohydrate.

Such athletes should be advised to boost their iron intake (box 2).

Other causes of anaemia

Anaemia can also arise from a number of other causes including deficiencies in folic acid and vitamin B12; treatment is best effected medically and not nutritionally--for example by the use of iron salts which can be absorbed in much larger amounts than from iron in food.

1. British Nutrition Foundation. Task force report. Iron: nutritional and physiological significance. London: Chapman and Hall, 1995.
2. Gregory J, Foster K, Tyler H, Wiseman M. The dietary and nutritional survey of British adults. London: HMSO, 1990.
3. Gregory J, Lowe S, Bates CJ, Prentice A, Jackson LV, Smithers G, et al. National diet and nutrition survey: young people aged 4 to 18 years. London: HMSO, 2000.
4. Nelson M, White J, Rhodes C. Haemoglobin, ferritin, and iron intakes in British children aged 12-14 years: a preliminary investigation. Br J Nutr 1993;70:147-55.
5. Nelson M, Bakaliou F, Trivedi A. Iron-deficiency anaemia and physical performance in adolescent girls from different ethnic backgrounds. Br J Nutr 1994;72:427-33.
6. Nelson M. Anaemia in adolescent girls: effects of cognitive function and activity. Proc Nutr Soc 1996;55:359-67.
7. Nelson M. Childhood nutrition and poverty. Proc Nutr Soc 2000 59,307-15.