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Paula Findlay at the 2012 Olympic Games in London. She finished in last place and was later found to be suffering from a serious iron deficiency.Frank Gunn/The Canadian Press

Earlier this month, sports scientists at the Australian Institute of Sport in Canberra injected either a dose of iron or a placebo directly into the bloodstream of a group of long-distance runners and began to monitor their performances. The goal: to unravel the notoriously complicated links between exercise and iron levels.

As triathlete Paula Findlay's struggles at last summer's Olympics illustrate – a last-place finish followed, a few weeks later, by the discovery that her iron levels were seriously depleted – even top athletes with sophisticated support teams struggle to detect and treat this problem. Emerging research suggests that the standard tests used to assess iron levels don't always pick up signs of trouble, and fatigue may show up even at levels previously considered "normal." Female endurance athletes are among those most at risk, but the findings could apply to anyone.

Your body needs iron to make hemoglobin-rich red blood cells, which carry oxygen from your lungs to your muscles. If a blood test shows that the hemoglobin concentration in your blood is lower than normal, that means you're anemic: You may feel weak and tired, and the most common cause is low iron.

But the picture is more complicated when you look at athletes. In response to endurance training, your body adapts by increasing the total amount of blood in your vessels, including iron-rich red blood cells. As a result, the concentration of hemoglobin in your blood stays roughly the same, but you're using up more iron – and a result, you may start dipping into your iron reserves. That's where the problems start.

"Iron does a lot more for you than just red-blood-cell production," notes Dr. Laura Garvican, the University of Canberra researcher leading the Australian study. "It's also involved in energy metabolism, immune function and even brain processes."

In 2011, Swiss researchers published a study in which 90 women suffering from unexplained fatigue received injections of either iron or placebo. All of the women had normal hemoglobin levels but low levels of ferritin, which is a marker of how much iron your body has in its reserves, primarily stored in bone marrow. For women with the lowest ferritin levels, 82 per cent had reduced fatigue after receiving iron compared to just 47 per cent who reported improvement in the placebo group.

A subsequent study published last year in the Canadian Medical Association Journal, from another Swiss group, found similar results for 198 women with unexplained fatigue and low ferritin levels using oral iron supplementation.

These results offered support for the idea that people with perfectly normal hemoglobin stores might still be suffering deleterious effects from low iron if their underlying reserves are depleted. The theory, Garvican explains, is that your body preferentially uses whatever iron is available to optimize hemoglobin levels, potentially shortchanging iron's other important roles if the supply is limited.

In both Swiss studies, the subjects were sedentary women. But there are several reasons that athletes, in particular, struggle to keep their iron levels up. Endurance training stimulates the production of extra red blood cells, increasing iron demand. "Footstrike hemolysis" in sports such as running that involve repeated jarring footstrikes can physically break red blood cells. Heavy sweating, gastrointestinal bleeding and, for women, menstruation also increase iron loss.

More recently, a study published last year by researchers at Florida State University in the International Journal of Sport Nutrition and Exercise Metabolism showed that hard workouts produce a spike in levels of a hormone called hepcidin that partially blocks absorption of iron. Levels of hepcidin peak three to six hours after the workout, so Garvican suggests taking iron supplements first thing in the morning on an empty stomach, or else immediately after a workout before hepcidin levels rise.

Note that too much iron is as bad as too little, so don't start supplements without first consulting your doctor. If tests establish that your hemoglobin or ferritin is low, recheck your levels periodically.

On top of that, your diet can also affect how much iron your body can absorb: Vitamin C helps with absorption, for example; coffee and tea may hinder it. Your first line of defence is a diet that includes easily absorbed sources of iron like red meat.

All of these factors make it difficult for some athletes to maintain healthy levels of iron, even if they know their levels are low. But at least, as the results from Garvican's study and other new research are eventually published, we'll have a better idea of what warning signs to look for – before your body comes up short when it counts.

Alex Hutchinson blogs about exercise research at His latest book is Which Comes First, Cardio or Weights?

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