Andrew Hamilton looks at research into iron nutrition and explains why it matters for all fitness enthusiasts, but especially women and vegetarians.
ASK MOST PEOPLE to name a crucial mineral for health and they’ll probably mention iron. But while the role its 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 requirements – something that can seriously dent performance during activity. So why is iron nutrition such a challenge, and how can you make sure that you don’t fall into an insufficiency trap?
There are three key reasons why maintaining optimum iron nutrition can be difficult, especially for those who are very active and training regularly:
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 insufficiency than sedentary folk. Let’s take a look at these factors in more 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, it 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 passes out largely unabsorbed.
This accounts for the poor iron bioavailability of many iron-rich plant foods – the iron’s there but can’t 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”, for example tea, which contains tannic acid and readily binds to iron, rendering it far less available to the body.
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 lower blood pressure, a reduced risk of cardiovascular disease and stroke and a lower risk of some cancers(1).
There’s also no doubt that a vegetarian diet rich in whole grain carbohydrates is ideal for those who exercise or train regularly(2). When it comes to iron nutrition however, 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.
A recent 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(3). The first finding was that the average daily intake of iron was 11.2mgs per day (compared with the recommended 14.8mgs 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, many 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(4).
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 28mgs 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)(5).
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(6), while even elite swimmers undergoing heavy training have been shown to suffer large drops in serum ferritin(7).
Given the facts of iron nutrition, it’s unsurprising that iron deficiency is surprisingly common in athletes – particularly female endurance athletes(8-12). These results are enough to set alarm bells ringing but recently published research suggests that ensuring optimum iron
nutrition could be even more challenging than previously thought(13).
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 seems 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 doesn’t 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(14,15). 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.
OK, let’s summarise what we’ve learnt so far and make some practical suggestions. 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 isn’t 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. Firstly, self-dosage on high-strength iron supplements for long periods of time can induce toxicity. Secondly, large doses of iron can reduce the uptake of other important minerals such as zinc and copper, creating possible
A better way to address iron intake is to try and improve iron consumption on a day-to-day dietary basis. The practical tips shown in the box-out and foods in.
References; 1. J Am Diet Assoc. 2009 Jul;109(7):1266-82; 2. J Am Diet Assoc. 2009 Jul;109(7):1266-82; 3. Appetite. 2014 Oct;81:225-31; 4. Int J Sport Nutr Exerc Metab. 2014 Feb;24(1):28-36; 5. Int J Sports Med. 2002, 23(8): 544-8; 6. Med Sci Sports Exerc. 2002, 34(5): 869-75; 7. Physiol Behav. 2002; 75(1-2): 201-6; 8. Nutr Exerc Metab. 2011 Dec;21(6):507-14; 9. Pak J Biol Sci. 2010 Jan 15;13(2):93-6; 10. Eur J Appl Physiol. 2008 Apr;102(6):703-9; 11. J Am Diet Assoc. 2005 Jun;105(6):975-8; 12. J Adolesc Health. 2009 Sep;45(3):286-91; 13. Br J Sports Med. 2014 Oct 31. pii: bjsports-2014-093624; 14. Am J Clin Nutr. 2002 75(4): 734-42; 15. Am J Clin Nutr. 2004 79(3): 437-43;