While the role of iron in forming haemoglobin in red blood cells (required to transport oxygen around the body to produce energy) is widely understood, many people struggle to meet their iron requirements.
This is something that can seriously dent performance during activity.
This article attempts to answer two important questions:
- Why is iron nutrition such a challenge?
- How can you make sure that you do not fall into an iron insufficiency trap?
Three Challenges to Optimum Iron Nutrition
There are three key reasons why maintaining optimum iron nutrition can be difficult, especially for those who are very active and training on a regular basis:
- Absorption: The iron in many foods (especially non-animal foods) tends to be ‘locked up’ very tightly indeed, making it hard to release it for absorption into the bloodstream.
- Losses: Iron losses are often higher in those who do regular endurance training, particularly women.
- Sensitivity: Those who are involved in endurance training may be more sensitive to iron insufficiency than sedentary folk.
We can now look at each of these factors in further detail.
Without delving too much into the biochemistry, the key thing to understand is that absorbing iron is actually quite difficult.
In living systems, iron carries a positive electric charge, which means that it readily becomes tightly bound to any molecules nearby containing a negative charge, for example some carbohydrates.
So, while many carbohydrate foods contain iron, the iron may be bound so strongly that the process of digestion is not able to ‘pluck them away’.
The iron stays joined to these carbohydrates as they pass through the digestive tract and, therefore, passes out largely unabsorbed.
This accounts for the poor iron bio-availability of many iron-rich plant foods – the iron is there, but can not easily be prised away for absorption.
Even in foods whose iron is readily available, uptake can be considerably reduced by consuming other foods or drinks containing ‘iron binders’. An example of this is tea, which contains tannic acid and readily binds to iron, rendering it far less available to the body.
Iron losses present another hurdle to optimum iron nutrition. Unlike many other minerals, considerable iron losses are possible above and beyond losses that occur via natural wastage. For example, in menstruating women monthly blood losses during periods account for an average loss of around 28 mgs per month – easily doubled if periods are heavy or intrauterine devices are used.
There’s also a growing body of evidence that heavy training, particularly endurance training, is a major cause of iron loss. For example, research on trained cyclists showed that six weeks of high-intensity interval training depressed haemoglobin, haemocrit and red blood cell count (three different markers of iron status) (Wilkinson et al., 2002). Meanwhile, serum ferritin (a blood protein involved with iron storage) decreased significantly by week 5 and remained depressed even in the recovery phase.
Iron loss caused by endurance exercise has also been confirmed in other studies. For example, a large and comprehensive study found that it was a particular problem in runners (Scumacher et al., 2002), while even elite swimmers undergoing heavy training have been shown to suffer large drops in serum ferritin (Spodaryk, 2002).
Given the facts of iron nutrition, it is unsurprising that iron deficiency is surprisingly common in athletes – particularly female endurance athletes (Sinclair et al., 2005; Di Santolo et al., 2008; Merkel et al., 2009; Ahmadi et al., 2010; Gibson et al., 2011).
These results are enough to set alarm bells ringing but recently research suggests that ensuring optimum iron nutrition could be even more challenging than previously thought (Burden et al., 2014).
In a meta-study (a study that pools together data from all the previous studies on a topic), researchers set out to discover the effects of iron supplementation on endurance athletes who had had iron-status testing and were not found to be iron deficient using conventional haemoglobin tests (the sort your GP might give you). Their results showed that giving extra iron boosted levels of tissue iron storage proteins and more notably, it significantly boosted the endurance capacity of athletes. Why should this be the case?
Well, it appears that the conventional criteria used to determine iron status might not be sufficiently sensitive for athletes in training. In other words, just because your levels of haemoglobin are in the reference range, this does not necessarily mean that your body tissues have the optimum amount of iron needed to maximise your endurance performance.
The importance of these findings is to underline just how critical iron is for performance. Indeed, some scientists now believe that tissue iron deficiency without anaemia can actually blunt the training adaptations that occur following endurance training (Brownlie et al., 2002; 2004). In other words, the same volume and intensity of endurance training could produce less of a performance gain if that training is carried out when your iron stores are not optimised.
Table 1: Common Tests to Determine Iron Status
|Method||Values (Normal)||Values (Depleted)||Values (Anaemic)|
|Haemoglobin (Women)||11.5-16.5 g/dl||N/A||<11.5 g/dl|
|Haemoglobin (Men)||13.5-18.5 g/dl||N/A||<13.5 g/dl|
|Serum Ferritin||40-160 mcg/l||20 mcg/l||<20 mcg/l|
|Total Iron Binding Capacity (TIBC)||300-360 mcg/dl||360 mcg/dl||410 mcg/dl|
|Serum Transferrin Receptor (STFR) [Note 1]||2-5 mgs/l||>5 mgs/l||N/A|
- Different laboratories may have different reference ranges for STFR due to differing assay procedures.
What are the Risk Factors for Iron Deficiency?
All of the factors below can contribute to an increased risk of an iron deficiency, and those marked with an asterisk indicate a more significant risk:
- Any sport involving significant volumes of running or other forms of endurance exercise*;
- Being female;
- Being female and having regular periods*;
- Having had children;
- A history of anaemia in the family;
- A vegetarian diet;
- A vegan diet*;
- Consuming tea and coffee with meals*;
- Using bran products;
- Eating only white meat and fish (i.e. not red meat);
- Giving blood regularly*;
- Using aluminium or enamel cookware (not using stainless steel cookware or iron cookware); and/or
- Frequent use of antibiotics, aspirin or antacid (indigestion remedies).
Time for Action?
Iron is an absolutely vital nutrient for endurance performance.
Not only is it needed for oxygen transport (via red blood cells) and energy production in the muscles, even a slightly sub-optimum iron status can impair performance and blunt training adaptation.
To complicate matters further, absorbing iron from the diet is not easy – especially if little or no meat in consumed – and research shows that endurance athletes (especially female athletes) suffer iron losses during periods of heavy training, which means they definitely need more iron than the average sedentary couch potato!
At this point, you may be wondering if you should be routinely supplementing with iron. Unfortunately, unless you have a known iron deficiency as shown by a specific iron status test (see Table 1), this is not recommended:
- Self-dosage on high-strength iron supplements for long periods of time can induce toxicity; and
- Large doses of iron can reduce the uptake of other important minerals such as zinc and copper, creating possible nutrient imbalances.
A better way to address iron intake is to try and improve iron consumption on a day-to-day dietary basis. Table 2 and the ‘practical tips’ section below highlight how this can be done.
Table 2: Examples of Good Vegetable Sources of Iron
|Teff Bread (100g) (from red teff flour)||5.6|
|Fortified breakfast cereal (1 cup)||4.5|
|Pumpkin seeds (1 oz)||4.2|
|Blackstrip molasses (1 tablespoon)||3.5|
|Spinach, boiled (1/2 cup)||3.2|
|Red kidney beans, cooked (1/2 cup)||2.6|
|Prune juice (3/4 cup)||2.3|
|Lima beans, cooked (1/2 cup)||2.2|
|Tofu, firm (1/2 cup)||2.0|
|Pretzels (1 oz)||1.2|
|Whole-wheat bread (1 slice)||0.9|
|Green beans, cooked (1/2 cup)||0.8|
|White bread, with enriched flour (1 slice)||0.8|
|Egg yolk, large (1)||0.6|
|Peanut butter, chunky (2 tablespoons)||0.6|
|Apricots, dried (3)||0.6|
Iron and the Challenge for Vegetarians
Numerous studies show that compared to a meat-rich diet, following a well-balanced vegetarian diet offers a number of potential health benefits such as (Craig et al., 2009):
- Lower blood pressure;
- A reduced risk of cardiovascular disease and stroke; and
- A lower risk of some cancers.
There is also no doubt that a vegetarian diet rich in whole grain carbohydrates is ideal for those who exercise or train regularly (Craig et al., 2009).
However, when it comes to iron nutrition, care is needed because not only is there the risk that the iron content of a vegetarian diet is lower but also that the iron in that diet may be less absorbable than an equivalent meat-containing diet.
The overall result is that vegetarian athletes (and those who eat just small amounts of meat and fish) may struggle to maintain an optimum iron status – even among athletes who have good overall knowledge of nutrition.
The following study provides a good example of why caution is required. Australian researchers looked at the diets and nutritional knowledge of 107 women and what effects this had on the women’s actual iron status (Leonard et al., 2014). The first finding was that the average daily intake of iron was 11.2 mgs per day (compared with the recommended 14.8 mgs per day for women). Importantly, while those with better nutritional knowledge tended to have higher intakes of iron, they were still likely to fall short of the recommended daily intake. And when it came to the actual iron status of the women, those who avoided meat or had a low-meat diet tended to have a notably poorer iron status than women who ate meat regularly – regardless of nutritional knowledge.
Why does this matter? Well, a number of you reading this will either be vegetarians or simply eat very little meat on a regular basis. Even among elite athletes, vegetarianism is known to be popular. For example, a recent study into the dietary regimes and practices of athletes competing at the Delhi 2010 Commonwealth Games found that the majority (62%) of athletes reported following one or more dietary regimens. In particular, vegetarian athletes or those avoiding meat comprised over 20% of the athletes (Clayton et al., 2014).
- Try to include some lean cuts of red meat in your diet once or twice each week.
- Try to include more beans (especially lima beans), lentils, dark green leafy vegetables in your diet, as well as eggs and nuts, and other iron-rich foods (see Table 2).
- All athletes:
- Try to consume plenty of vitamin C-rich foods (citrus fruits, berries, new potatoes, broccoli, sprouts, tomatoes, peppers, kiwis etc with your iron-rich foods.
- Vitamin C helps convert dietary iron into its most absorbable form in the body, which can increase uptake from the gut by up to 4 times.
- Home Baking:
- Consider baking your own bread using teff flour.
- Teff is much richer in iron than other whole grains and makes a great iron-rich carbohydrate food.
- Tea & Coffee:
- Do not drink tea and coffee with your meals.
- The natural tannins present in these drinks strongly bind any iron present, making it less available to the body.
- Bran & Fibre:
- Do not use bran or other products with added fibre.
- Bran is high in compounds that bind iron and make it less absorbable.
- If you want more fibre in your diet, get it by eating whole grain breads and cereals in the first place.
- Cook using stainless steel cookware.
- It can add useful amounts of iron to cooked foods.
- Don’t take iron supplements without a proven need.
- The use of high-strength iron supplements for long periods of time can induce toxicity.
- If you think you are at a high risk of iron insufficiency (see Iron Deficiency Risk above), ask your medical professional for an iron status test and one that includes serum ferritin as well as haemoglobin.
Craig, W.J., Mangels, A.R. & American Dietetic Association. (2009) Position of the American Dietetic Association: vegetarian diets. Journal of the America Dietetic Association. 109(7), pp.1266-1282.
Leonard, A.J., Chalmers, K.A., Collins, C.E. & Patterson, A.J. (2014) The effect of nutrition knowledge and dietary iron intake on iron status in young women. Appetite. 81, pp.225-231. doi: 10.1016/j.appet.2014.06.021. Epub 2014 Jun 24.
Clayton, D.J., Evans, G.H. & James, L.J. (2014) Effect of drink carbohydrate content on postexercise gastric emptying, rehydration, and the calculation of net fluid balance. International Journal of Sport Nutrition and Exercise Metabolism. 24(1), pp.79-89. doi: 10.1123/ijsnem.2013-0024. Epub 2013 Aug 26.
Wilkinson, J.G., Martin, D.T., Adams, A.A. & Liebman, M. (2002) Iron status in cyclists during high-intensity interval training and recovery. International Journal of Sports Medicine. 23(8), pp.544-548.
Schumacher, Y.O., Schmid, A., Grathwohl, D., Bültermann, D. & Berg, A. (2002) Hematological indices and iron status in athletes of various sports and performances. Medicine and Science in Sport and Exercise. 34(5), pp.869-875.
Spodaryk, K. (2002) Iron metabolism in boys involved in intensive physical training. Physiology & Behaviour. 75(1-2), pp.201-206.
Gibson, J.C., Stuart-Hill, L., Martin, S. & Gaul, C. (2011) Nutrition status of junior elite Canadian female soccer athletes. International Journal of Sports Nutrition and Exercise Metabolism. 21(6), pp.507-514.
Ahmadi, A., Enayatizadeh, N., Akbarzadeh, M., Asadi, S. & Tabatabaee, S.H. (2010) Iron status in female athletes participating in team ball-sports. Pakistan Journal of Biological Sciences. 13(2), pp.93-96.
Di Santolo, M., Stel, G., Banfi, G., Gonano, F. & Cauci, S. (2008) Anemia and iron status in young fertile non-professional female athletes. European Journal of Applied Physiology. 102(6), pp.703-709.
Sinclair, L.M. & Hinton, P.S. (2005) Prevalence of iron deficiency with and without anemia in recreationally active men and women. Journal of the American Dietetic Association. 105(6), pp.975-978.
Merkel, D., Huerta, M., Grotto, I., Blum, D., Rachmilewitz, E., Fibach, E., Epstein, Y. & Shpilberg, O. (2009) Incidence of anemia and iron deficiency in strenuously trained adolescents: results of a longitudinal follow-up study. Journal of Adolescent Health. 45(3), pp.286-291.
Burden, R.J., Morton, K., Richards, T. Whyte, G.P. & Pedlar, C.R. (2014) Is iron treatment beneficial in, iron-deficient but non-anaemic (IDNA) endurance athletes? A systematic review and meta-analysis. British journal of Sports Medicine. 49(21), pp.1353-1353. Published Online First: 16 Oct 2015. doi: 10.1136/bjsports-2015-095507.
Brownlie 4th, T., Utermohlen, V., Hinton, P.S., Giordano, C. & Haas, J.D. (2002) Marginal iron deficiency without anemia impairs aerobic adaptation among previously untrained women. The American Journal of Clinical Nutrition. 75(4), pp.734-742.
Brownlie 4th, T., Utermohlen, V., Hinton, P.S. & Haas, J.D. (2004) Tissue iron deficiency without anemia impairs adaptation in endurance capacity after aerobic training in previously untrained women. The American Journal of Clinical Nutrition. 79(3), pp.437-443.