When I see a runner getting fatigued early on in workouts or struggling mightily in races for no good reason, there's one potential cause I always consider first: low iron.
Iron deficiency is a significantly underdiagnosed problem in distance runners. Low levels of hemoglobin in the blood, or low levels of the iron storage protein ferritin, can have a profoundly negative impact on your ability to have successful workouts and races.
Hemoglobin is the main building block for red blood cells, which carry oxygen from your lungs to your muscles. If you don't have enough hemoglobin, you can't make enough red blood cells, and as a result, your distance running performance will suffer. Furthermore, research and practical coaching experience suggests that low ferritin levels can cause poor performance, even when hemoglobin levels are normal.
We'll take a close look at the science behind low iron and distance running performance, then analyze the best ways to treat and prevent iron deficiency in runners.
The biology of iron and red blood cells
|One red blood cell contains millions of hemoglobin proteins|
Hemoglobin is an essential part of your body's oxygen delivery system. It's a protein with four iron atoms at its core, and these iron atoms are what grant red blood cells their ability to transport oxygen (as well as give them their red color).
Because red blood cells must be replaced fairly frequently, your body keeps extra iron on-hand in a storage protein called ferritin. Your body's iron reserves are mostly locked up in ferritin, which can be called upon when needed to synthesize hemoglobin for new red blood cells, or other proteins and enzymes in your body that also require iron. Low ferritin by itself is termed iron deficiency.
As you might guess, when ferritin levels in the body are inadequate, hemoglobin synthesis slows down and your body can't produce as many red blood cells. Abnormally low hemoglobin levels is a condition termed anemia, and when the cause is low iron, this is iron deficiency anemia.
The prevalence of iron deficiency and anemia in distance runners
According to research from the Centers for Disease Control and Prevention, between 9 and 11% of teenage and adult women are iron deficient, while only 1% of teenage and adult men are iron deficient.1 In this context, "iron deficient" means serum ferritin levels below the standard lab reference ranges for the general population (typically 12 ng/mL). As we'll soon see, these ranges need to be increased for endurance athletes.