Stress fractures are one of the most common and potentially serious overuse injuries reported in athletes, both elite and recreational. With this in mind, Andrew Hamilton looks at recent evidence linking stress fractures and nutritional status… MORE
Iron: why might an athlete need more?
You may have iron in your soul, but have you enough in your body?
Nutritional advice for female athletes is a growing area of interest, and indeed concern, for dieticians and sports nutritionists. Much of the concern is centred on the various subclinical eating disorders. However, a rather more subtle, yet no less important issue arises when one considers the often limited dietary intake by female athletes of two specific minerals -namely iron (Fe) and calcium (Ca) especially when heavy training loads are undertaken.
Levels of iron in the various body compartments – collectively known as ‘iron status’ – are of particular importance to the athlete given iron’s role in enzyme function, and especially considering the fundamental role of iron within haemoglobin and myoglobin (the muscle tissue’s equivalent of haemoglobin). Oxygen transport from the lung interface with the atmosphere to the muscle capillaries and subsequent delivery to the mitochondria within the muscle cells, removal of waste products including carbon dioxide, and the buffering of the metabolic acidosis (due to hydrogen ions) built up during intense exercise are all roles performed by haemoglobin and myoglobin. These chemical substances contain individual atoms of iron, each one of which can combine with one molecule of oxygen.Logically, one might think that simply consuming more iron in the diet would increase haemoglobin levels, but unless a severely inadequate diet has been the norm for a number of months or years, or in certain diseased states, this is not the case. However, an athlete’s iron status may be compromised, and the body stores may be far below the normal desired levels without producing a reduced haemoglobin level or even reducing performance capacity. The problem is, however, likely to appear as reduced performance in the future if the situation is allowed to get worse, when eventually the functional iron stores of certain enzymes, myoglobin and haemoglobin will be affected. So the aim of the serious athlete should be to maintain full iron status through adequate nutrition so that the possibility of worsening performance through lowered functional iron stores does not occur.
One complication for athletes is that their dietary iron requirements appear to be slightly higher than those of the general population, yet for some endurance athletes, those in events where bodyweight must be strictly controlled, and for female athletes in general, total energy and therefore other nutrient intakes may be habitually low (Hawley and co- workers, ‘Nutritional Practices of Athletes: Are they Sub-optimal?’, Journal of Sports Sciences, 1995, vol. 13, Special Supplement, ppS75-S87). Various scientific investigations, especially in the 1980s, discovered low iron intakes among female endurance athletes in particular. One such study conducted in Canada found that 91 per cent of female distance runners did not consume adequate amounts of iron in their diets (Clement and Asmundson, ‘Nutritional Intake and Haematological Parameters in Endurance Runners’, The Physician and Sportsmedicine, 1982, vol. 10, no. 3, pp37-43).
Why might an athlete need more iron?
Increased iron needs of athletes may be due to several factors, some of which have been backed up more than others through rigorous scientific investigation. Each of these may contribute to iron loss, and the type of athlete mentioned above, the female distance runner, may find many of the potential causes relevant to her own situation. It should be noted, however, that any athlete involved in heavy training can be affected.
The factors include:
- internal gastrointestinal bleeding
- minor damage to the wall of the bladder
- the physical breakdown of red blood cells, literally through being squashed between bone and an external object (eg, the floor) during high- impact activities
- over-frequent blood donation (the Good Samaritan Syndrome). At least one report shows that athletes donate more blood than the general population
- increased iron loss through heavy sweating (although relatively small amounts)
In addition, female athletes in general are likely to have a higher rate of iron loss than men, and also a higher daily requirement, largely because of blood loss through menstruation. Pregnancy and childbirth can also tax iron stores significantly.
Dietary factors that tend to limit iron intake include:
- Low total energy intakes. Low total amounts of food will lead to susceptibility to dietary deficiencies
- vegetarian diets. The iron found in meat and meat products is both more plentiful and far more easily absorbed than that found in plant foods, while dairy products and eggs (for those who eat them) do not compensate for iron as they do for other nutrients such as protein
- natural food diets. Certain food additives such as vitamins and minerals are beneficial; a good example is breakfast cereal fortified with iron
- fad diets. Diets with a limited range of food choices are unlikely to fulfil basic nutritional requirements, and athletes who follow unusual eating patterns may well be deficient in iron intake.
What is widely regarded as the most reliable single measure of iron status is ‘serum ferritin concentration’. Many laboratories specialising in exercise physiology and sports science support to athletes will be able to determine this measure and provide athletes with an indication of body iron stores. Appropriate action can be prescribed, often through dietary modifications.
How to tackle the problem
Sound nutritional advice with a particular emphasis on enhancing iron status must take into account the two forms of iron found in foods. The first, haem iron, is found in meat, seafood and poultry, and has an absorption rate from the gastrointestinal system of up to 23 per cent (meat sources being the most easily absorbed). While this may not sound very efficient, the importance of this form of iron is clear when one considers that most of the iron in many athletes’ diets (and all, in the case of vegetarians) is of the second form, non-haem iron (found in cereals, legumes and vegetables), which has an absorption rate of just 2-8 per cent on average.
The absorption rate of this form of iron is also that which is most heavily influenced by other dietary factors. For instance, high levels of vitamin C intake at mealtimes will increase iron absorption rates from other foods, as will small amounts of alcohol. Meanwhile, caffeine, and especially tannic acid (found mostly in tea) will reduce iron absorption rates dramatically.
The RDAs for iron intake in various countries are around 12-1 7mg/day for females, about 5mg/day more than the male RDAs. Given the complexity of the situation, the influence of an athlete’s dietary pattern and intake is a clear major factor in producing optimal iron status. The take-home lesson for any athlete (whether suspicious of iron deficiency or not) and especially female endurance athletes can be summed up in six short sentences (modified from Burke and Deakin (editors), Clinical Sports Nutrition, 1994, pl88):
- Increase total intake of iron-rich foods in regular diet
- If possible, eat at least 90g of lean meat, poultry or fish daily
- If vegetarian, ensure plan food choices are iron- dense (includes green leafy vegetables and legumes)
- Increase intake of bread (especially wholemeal) and iron-enriched cereals
- Include a good source of ascorbic acid (vitamin C) with each meal-such as salad, fruit or fruit juice 6. Avoid tea or coffee at mealtimes, particularly if plant-food sources or iron are included. Drinking tea at mealtimes is probably the worst single action an athlete can take for iron status.