If you've noticed an elevated heart rate during your sessions, are struggling with power and endurance and seem to be picking up more injuries, you might have an iron deficiency. 

In fact, according to this study published in Medicine & Science in Sports & Exercise: If you’re a runner, women especially, there’s a very good chance that you are iron deficient. According to the study, deficiency is extremely common among endurance athletes like runners, impacting up to 17 percent of male and 50 percent of female endurance athletes.

WHAT IT DOES

Iron helps produce and support red blood cells, which transport oxygen throughout the body. Iron deficiency can have serious debilitating effects on a runner’s performance because – in the most simplified of terms: If you have too little, your muscles won’t get enough oxygen.

The recommended daily allowance (RDA) of iron for people between 19 and 50-years-old is 18 mg for females and 8 mg for males. As an endurance athlete you likely need slightly more.

On top of this, runners can lose as much as 70 percent more iron than sedentary people due to sweating. In addition, they have a faster breakdown of red blood cells through the mechanical force of a foot strike.

SHOUL YOU TAKE AN IRON SUPPLEMENT
Some athletes can self-diagnose their iron deficiency, take an over-the-counter supplement and feel a big improvement. Unfortunately however, this is fairly rare as certain iron supplements can cause constipation and other forms of gastrointestinal distress. In general you could take a low dose iron supplement and it could help, but it is far better to get a blood test and then follow a scripted  regimen from a doctor. 

The ideal however, is to make sure you get enough through your diet. Here are some good sources:

Some more interesting reading, here.