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by Alan McCubbin
November 20, 2017
Photography by Ben Chen
Way back in 2009 we published an article outlining the importance of iron for cyclists. It was around that time that a bunch of new research was coming out, looking at the causes of iron deficiency in athletes, and how some simple strategies around the use of iron supplements (where appropriately prescribed) could help maximise their effectiveness and correct any deficiency.
Eight years on, it’s time to revisit the topic. What does the latest research tell us about the importance of iron for cyclists? What are the consequences of being iron-deficient? How can we ensure we’re getting enough iron in our diet (but not too much)? Dietitian Alan McCubbin has the details.
Iron is a mineral found in food and one we need for several processes in the body. In total, our bodies contain around 3-5g of iron but lose 1-2mg a day. For women, menstrual blood loss can double daily losses during that stage of a cycle.
Most people will be familiar with the role of iron in haemoglobin, the protein that carries oxygen around the blood and delivers it to our organs (including muscles). Haemoglobin is where you’ll find about two-thirds of our body’s total iron. But iron is also a crucial component of myoglobin (which binds oxygen within the muscle) as well as several processes inside cells that allow us to produce energy, and allows DNA to be replicated when cells divide.
Because of the daily losses from the body, we need to get some iron from food in order to maintain our body’s stores. Iron in food comes in two forms. When we eat foods that contain the haemoglobin and myglobin of another animal (predominately meat), the iron contained within is known as haem iron. Iron not found as part of haemoglobin or myglobin is known as non-haem iron, and includes the iron contained in plant foods.
The gut absorbs haem and non-haem iron in different ways. The absorption of haem iron is more efficient, with around 5-35% of the haem iron contained in a meal being absorbed, compared with only 2-20% for non-haem iron. In both cases that’s a fairly low percentage of what’s eaten, meaning our dietary requirements for iron are significantly greater than the 1-2mg that leaves the body each day (the Australian Recommended Dietary Intake for adult males is 8mg/day and 18mg/day for menstruating females). It’s also a very large range, as the absorption of iron from the gut is highly variable, and responds to the body’s total iron stores.
Iron deficiency can lead to fatigue, weakness and a reduction in athletic performance. Excess iron in the body can be very harmful too. As a result, the ability of our gut to adjust the absorption of iron from food is incredibly important in maintaining our health.
Although there are no specific guidelines or recommendations for the amount of iron athletes need to eat, most experts agree that people who exercise regularly probably need more iron in their diet than someone who is quite inactive.
There are multiple reasons for this. Firstly, iron losses may be greater in people who exercise a lot, due to a variety of factors, including:
– The rupturing of red blood cells (haemolysis) and the release of iron which then finds its way into urine via the kidneys. Whilst haemolysis is thought to occur primarily during running when feet strike the ground, it has also been reported in cycling and swimming. The thinking is that when muscles contract, they squash the surrounding blood vessels and rupture some red blood cells in the process.
– A small amount of iron is lost in sweat during exercise (around 0.1-0.3mg per hour depending on which study you look at).
– Small losses of blood in the urine or from the gut in some cases (particularly during and after longer rides with high-intensity efforts and in the heat).
Secondly, the absorption of iron from food depends on a hormone called hepcidin which, intriguingly, was only discovered at the beginning of this century. When total iron stores are high, hepcidin is released, which signals to reduce the amount of iron absorbed from food in the gut. However in the last 10 years research has found that exercise can also increase hepcidin levels, and may cause reduced iron absorption from food eaten in the hours after a ride.
Thirdly, for cyclists travelling to altitude or undertaking other forms of altitude training, the rapid increase in production of red blood cells in the first one to two weeks will quickly deplete the body’s iron stores, as they are used to form the additional haemoglobin. It is suggested that athletes in this position have their iron checked prior to altitude exposure, and supplement according to medical advice.
Finally, just like anyone else, a lack of iron-containing foods in the diet can reduce iron stores, regardless of how good our absorption is. Iron deficiency is the world’s most common nutrient deficiency, and athletes are certainly not immune from that.
A simple blood test can help determine whether your iron levels are adequate or not. The results, however, can be quite confusing if you don’t know what you’re looking at. While the majority of iron in the body is bound to haemoglobin and myoglobin, iron is bound to several other proteins as well, some of which are used in blood tests to understand your body’s stores.
Ironically, the actual concentration of iron in the blood is the least useful of the tests commonly done as it varies at different times of the day and does not reflect total iron stores.
Transferrin is the protein that transports iron around the body when it enters the blood from the gut. When iron enters the body’s cells, it’s taken up into various proteins, and the excess is stored as ferritin. Some ferritin then circulates in the blood, and this is the main marker used to tell us whether someone’s iron stores are adequate or not.
(Note: Ferritin levels in the blood are susceptible to inflammatory processes, which can elevate them significantly. This does not mean that iron stores are adequate. So if you had an illness, injury or infection at the time of a blood test, it is possible that ferritin will give you a false impression of your iron status.)
Because ferritin levels in the blood indicate total body iron stores (in excess of what’s needed for other processes in the body), it is this value that is generally used first to indicate whether someone is iron deficient or not. In the early stages of iron deficiency, a reduction in ferritin is all that can be detected in blood tests. However as the deficiency becomes more severe, a decline in haemoglobin concentration and in transferrin saturation becomes apparent.
The exact cut-offs are still debated amongst researchers, sports physicians and dietitians, but some suggestions have been that mild iron depletion occurs when ferritin is at less than 35 micrograms/L, with haemoglobin concentration and transferrin saturation generally normal. At this level the body’s stores of iron are low, but not low enough to prevent production of haemoglobin. Having said that, a recent study suggested that haemoglobin mass (as opposed to concentration) may already be reduced at this early stage.
It is also at this early stage of iron depletion where the potential effect on performance is the most controversial – some studies find supplementing iron at this stage has no effect on endurance performance, whereas others find an improvement. The most recent review of this research suggests that, overall, there is likely a small performance improvement to supplementing with iron at this level, either via injection or tablets.
It is suggested that once ferritin falls below 20 micrograms/L, the production of the proteins that transport iron become reduced and so the body’s cells have less iron delivered to them. Full-blown iron deficiency anaemia (significant reduction in haemoglobin production) is seen once ferritin is less than 12 micrograms/L, haemoglobin is less than 115 g/L and transferrin saturation is below 16%.
So how common is iron depletion or deficiency in athletes? Depending on the study, somewhere between 15-50%, with women more likely to have it than men. On the flipside there are some studies that found some male athletes have excessive iron stores, in part due to taking iron supplements unnecessarily in the belief they needed to or it would make them faster.
As mentioned earlier, animal foods that contain haemoglobin and myoglobin from the animal provide haem iron, as well as some non-haem iron. The main sources of haem iron are red meats and offal, although poultry and fish also contain some iron. Plant sources of iron include green vegetables such as spinach, silverbeet and broccoli, legumes (lentils and beans), nuts, seeds, grains (e.g. wheat, rice and breakfast cereals fortified with iron) and dried fruit.
One of the downsides to vegetarian diets has traditionally been thought to be the poor absorption of iron from plant foods. The green leafy vegetables contain oxalic acid, which was thought to significantly reduce the absorption of iron from spinach and the like. In a similar way, phytic acid has been thought to reduce the iron absorption from legumes, nuts, seeds and grains.
However recent research has suggested that oxalic acid may not have as much effect as first thought, and that in the medium to long-term the body adapts to a high phytic acid diet, such that it no longer prevents absorption of iron.
But, to be on the safe side, it’s suggested that foods high in Vitamin C are consumed alongside plant sources of iron in order to optimise absorption.
As mentioned, different health professionals have different opinions on how low ferritin can be before supplementation is required. But because of the risk of excessive iron intake, especially for the 1 in roughly 300 people who have a genetic condition called haemochromatosis (where the body absorbs too much iron), and the fact that iron overload has similar symptoms of fatigue and lethargy that iron deficiency can cause, it is always recommended to have your iron levels tested and checked by a suitably qualified health professional before you start taking iron tablets.
Research doesn’t just show that supplementing iron when there is no iron deficiency does not improve performance, but if iron overload results then performance is likely to be impaired.
Iron tablets can also cause constipation in some people, and for some people with poor absorption of iron (for a variety of reasons), taking tablets might be ineffective for restoring iron levels. In that case, doctors can provide iron injections or infusions in many countries. Iron injections are also often used in athletes with severe iron deficiency, who need to return iron stores to normal as quickly as possible due to upcoming heavy training or blocks of racing.
Finally, the time of the day when you take your iron supplement may also be important. Because of the increase in hepcidin that occurs after exercise (which, again, can reduce iron absorption), taking your supplement at the opposite end of the day to your ride may improve its effectiveness at being absorbed and improving your iron status more quickly.
Our bodies only contain a small amount of iron, but keeping the balance between iron deficiency and iron overload is crucial for optimal health and performance. Our body does a remarkable job of trying to control the amount of iron that is absorbed from food to maintain this balance, but a lack of iron in the diet, reduced absorption, altitude exposure or increased losses that can occur during and following exercise can increase the risk of cyclists becoming iron deficient.
Interpreting the various blood tests for iron status can be tricky, and supplementation should never be started without testing and advice from a health professional due to the risk of iron overload.
Alan McCubbin is an Accredited Sports Dietitian, Accredited Practicing Dietitian and past president of Sports Dietitians Australia. He is currently studying his PhD in sports nutrition at Monash University. He is also the founder of Next Level Nutrition, an online sports nutrition consultancy through which he works with a range of athletes from recreational to Olympians.