Haemoglobin and red blood cells are topics that many cyclists know a bit about, unfortunately that is mostly because of the incessant news stories about EPO and blood transfusions being used among cyclists as ways of increasing red blood cell number and thus oxygen carrying capacity. However, in this column I will focus more on what happens when the opposite occurs, that is "anaemia" or iron deficiency and the role of diet in maintaining iron stores.
When talking about energy metabolism, in many ways humans are analogous to an automobile in the sense that both combust fuel for energy to propel movement. In the automobile to combust petrol, there needs to be oxygen intake and a spark. The resultant release of energy from the combustion moves a piston helping to propel the auto. In humans, food substitutes for petrol, and the combustion of the energy contained in chemical bonds of the food creates ATP for movement and other processes. In humans, like autos, oxygen is a necessary component of this process in order to have sustainable energy production. Therefore, oxygen has to somehow travel from the air we breathe to the energy-producing units in the body, which are called mitochondria. The process of transporting oxygen throughout the body is what makes iron invaluable.
When we breathe in air we pass the air to the lungs. Oxygen from the air then crosses a barrier from the lungs to the blood. However, only a small amount of oxygen actually diffuses into the blood. The majority of oxygen is carried through the blood by haemoglobin, a protein structure contained within red blood cells (Figure 1), and at the core of haemoglobin is iron. To put it another way, without iron we would not have functional haemoglobin, without functional haemoglobin there would be no way to carry oxygen in red blood cells, and therefore no way to get the necessary amount of oxygen from the air we breathe to the mitochondria. It could be said that iron is fundamental to sustaining any aerobic (oxygen utilizing) species, thus it is easy to see why maintaining sufficient amount of iron in our diet is important.
Iron is not created in the body, and therefore has to be taken in from our diet. Most people know that red meat is a good source of iron. The reason for this is that muscle contains another iron containing protein that binds oxygen called myoglobin. So when someone eats red meat, they are essentially consuming the iron contained within the myoglobin of that animal's muscle. In fact, myoglobin is responsible for making red meat red. Red meat has the most readily available source of iron. However, it is possible to get iron from plant sources as well such as dark, leafy greens. Although these greens have a lot of iron, this iron is in a form that is less available for the human body so that only a portion of the iron actually makes it into the system. Iron metabolism in people seems to be quite variable and while someone may need a red meat source for iron, others maintain sufficient iron stores without red meat. For those who have found that they cannot sustain iron in the body on an adequate diet, or who simply consume an inadequate diet, iron supplementation is a popular option.
There has been a fair amount of misinformation spread among athletes regarding iron deficiency. Whereas it is true that prolonged physical activity can contribute to loss of iron, the frequency of iron deficiency (or anaemia) in athletes is not greater than the general population. But if a deficiency is present in an athlete it is likely to have more of an effect then on the average person. For instance, if a person has a slight deficiency, that deficiency may only become apparent at high intensity exercise when the demand for oxygen delivery by iron is greatest. But, the average person does not regularly approach the intensities at which the deficiency becomes apparent, and therefore will not observe any of the affects related to iron deficiency. So, although iron deficiency is not desirable in any population, it may only be readily apparent in the athlete.
A second point to consider is that a female athlete is more likely to suffer from iron deficiency or anaemia than a male athlete. The primary reason for the increased deficiency in women is the additional iron loss from monthly menstrual bleeding. It has actually been reported that the incidence of anaemia in males is less then the incidence of having too much iron (iron toxicity). Therefore, standard advice is that when iron deficiency is suspected in women, iron supplementation should be started, but in men a more thorough evaluation is necessary and iron supplementation should not be started without good evidence of a deficiency.
How does one determine if they are iron deficient? This involves regular blood screening, something your general practitioner can do for you. It used to be that a standard value was used to determine whether one is deficient in iron or iron stores. However, it was realized that among some people above the cut-off level, iron supplementation actually improved performance. Therefore, although the people were not below the cut-off level for anaemia, they were functionally anaemic. It has since been determined that we are anaemic only when compared to our own values. That is, everyone has a value at which they become functionally anaemic and iron must be maintained above that level. Having said that, it is VERY important to note that if you are not iron deficient or anaemic, iron supplementation will NOT enhance performance. As mentioned before, too much iron can be toxic in humans and has major consequences.
In conclusion, it is very important to maintain iron and iron stores in the body so that oxygen can be delivered efficiently to muscle tissue to sustain aerobic energy production. A strategy of regular blood screening can help determine whether iron stores are sufficient. If a deficiency is suspected, males should have further screening while females should consult with a properly trained nutritionist on strategies to increase usable iron in to the diet.
Ben Miller is Senior Lecturer in Exercise Physiology. Ben did a PhD at the University of California – Berkeley and a Post-Doc at the Institute for Sports Medicine, Copenhagen before arriving in New Zealand. As a departure from his life in a closed scientific box safe from the realities of the world, he is a cyclist regularly taking his life in his own hands on the streets of Auckland and in the local club racing and criteriums. Ben's wife is much more successful at cycling having competed full-time in Europe and the US for the last 4 years.