2.9 Iron (Fe)
The ability of blood to carry oxygen is due to the presence of the red pigment, haemoglobin, present in red blood cells. Haemoglobin is a protein formed from four polypeptide chains called globins, in the centre of each of which is a small non-protein part called a haem group ( haima is Greek for ‘blood’). Each of the haem groups has an iron atom within it ( Figure 4 ).
The majority of iron in the body is in the form of haemoglobin. The total amount present depends on a number of factors, including gender and body weight, as well as general health.
If a woman weighing 60 kg has 40 mg of iron for each kilogram of body weight (mass), how much iron will her body contain?
If two-thirds of the iron in the body is contained in haemoglobin molecules, how many grams of iron will be in the woman's haemoglobin?
The total amount of iron in her body is 60 x 40 mg = 2400 mg = 2.4 g. This quantity is significantly less than the average amount of iron in the human body which is usually given as 4 g. Women of reproductive age are often short of iron due to menstrual losses or to the needs of pregnancy.
If two-thirds of this quantity is in the haemoglobin, then the amount of iron in the haemoglobin is 2.4 g x 2 divided by 3 = 1.6 g, i.e. just over one and a half grams.
Iron is also found in another molecule, that also binds to oxygen, called myoglobin. A myoglobin molecule is very similar to a quarter of a haemoglobin molecule, i.e. one globin chain with its associated haem. The red colour in the meat that we eat (the muscles of animals) is due to myoglobin. There are several types of fibres in muscles, including red fibres and white fibres, and whether the meat has a light or dark colour depends on the amount of each type present. Red muscle fibres predominate in those muscles that sustain long periods of activity. Fat may be stored around these muscles as an energy source, and the oxygen needed to combine with the fat to provide energy when the muscles are active is obtained from myoglobin. The myoglobin constantly replenishes its oxygen by picking it up from the haemoglobin in the blood. So these muscle fibres are high in myoglobin, which gives them their red colour. White fibres are found in muscles that are only required to be active for a short time. They use glucose from the blood as an energy source, and so there is little fat around the muscles. Less oxygen is needed to combine with glucose, and so less myoglobin is present and the muscles are lighter in colour.
Where do you find red meat and white meat in a cooked chicken? Can you relate this distribution to the requirement for myoglobin?
Chickens have red meat on the legs and white meat on the breast. Since the legs are used almost continuously for standing and walking, you would expect more myoglobin to be present and therefore the meat would be red. The breast muscles are used for flight and, since flights are rare and short in chickens, the myoglobin levels are low and so the breast meat is much paler. In birds that fly more, such as wild ducks, the breast meat is much darker in colour.
Liver is another type of meat that is very rich in iron, since the liver is where all mammals, including ourselves, store their iron, bound to a protein called ferritin. Cereals contain iron, though it is usually bound to a substance called phytate which is also found in nuts and some vegetables, and which also binds to calcium and zinc. Being bound in this way means that the iron from vegetables and cereals is less easily absorbed than the iron from animal products. However, phytate is removed by milling and then white bread flour is fortified with iron to ensure that bread is a useful source of the mineral. Various other substances in the diet can also bind iron and therefore prevent its absorption, while others, including vitamin C, can enhance the amount that is absorbed. The complex interaction with other foods makes it extremely difficult to predict just how much iron will be absorbed into the body from a particular meal. The average amount of iron in the diet is about the same as the RNI, which hides the fact that many individuals, especially women, are deficient in iron.
Red blood cells do not have a nucleus and so, without a set of chromosomes, they are unable to make new components or repair any damage. Since they spend a lot of their time being squeezed through tiny blood capillaries, often not much wider than themselves, they are easily damaged and this results in their limited lifespan of about 120 days. So, there is a huge daily turnover of red blood cells. In fact, every second, one and a half million (1 500 000) red blood cells are destroyed, and the same number of new ones are produced. If the iron was not recycled from the red cells that are destroyed, 240 mg of iron would be needed every day. Red cells are broken down mostly in the liver (and spleen), where the iron is stored attached to ferritin, as mentioned earlier, and new red cells are made in the bone marrow. So there is a requirement for iron to be transported in the blood between these sites, which it does attached to a protein called transferrin. Small amounts of the storage protein, ferritin, are present in the blood and a fall in ferritin level is the first sign of iron deficiency. If the deficiency persists, the red cells that are produced are smaller and contain less haemoglobin than normal and gradually the person develops the symptoms of anaemia, with tiredness and lack of appetite. It is thought that anaemia may affect one-tenth of the world population. A change in diet or taking iron supplements should correct the condition.